Add yeast crate: AST desugaring framework with proc-macro DSL

YEAST (Yet another Elaborator for Abstract Syntax Trees) is a framework
for transforming tree-sitter parse trees before CodeQL extraction.

Core components:
- shared/yeast/ — Ast, Node, Schema, query matching engine, captures,
  FreshScope, BuildCtx
- shared/yeast-macros/ — proc macros: query!, tree!, trees!, rule!

The query language is inspired by tree-sitter queries:
  (assignment left: (_) @lhs right: (_) @rhs)

Templates support embedded Rust ({expr}), splicing ({..expr}),
computed literals (#{expr}), fresh identifiers ($name), and captures (@name).

The rule! macro combines query and transform:
  rule!((for pattern: (_) @pat ...) => (call receiver: {val} ...))

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Cargo.lock

@@ -416,6 +416,7 @@ dependencies = [
  "tree-sitter",
  "tree-sitter-json",
  "tree-sitter-ql",
+ "yeast",
  "zstd",
 ]
 
@@ -2470,7 +2471,6 @@ version = "1.0.145"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "402a6f66d8c709116cf22f558eab210f5a50187f702eb4d7e5ef38d9a7f1c79c"
 dependencies = [
- "indexmap 2.11.4",
  "itoa",
  "memchr",
  "ryu",
@@ -2853,14 +2853,13 @@ dependencies = [
 
 [[package]]
 name = "tree-sitter"
-version = "0.25.9"
+version = "0.24.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ccd2a058a86cfece0bf96f7cce1021efef9c8ed0e892ab74639173e5ed7a34fa"
+checksum = "a5387dffa7ffc7d2dae12b50c6f7aab8ff79d6210147c6613561fc3d474c6f75"
 dependencies = [
  "cc",
  "regex",
  "regex-syntax",
- "serde_json",
  "streaming-iterator",
  "tree-sitter-language",
 ]
@@ -2891,6 +2890,16 @@ version = "0.1.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c4013970217383f67b18aef68f6fb2e8d409bc5755227092d32efb0422ba24b8"
 
+[[package]]
+name = "tree-sitter-python"
+version = "0.23.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3d065aaa27f3aaceaf60c1f0e0ac09e1cb9eb8ed28e7bcdaa52129cffc7f4b04"
+dependencies = [
+ "cc",
+ "tree-sitter-language",
+]
+
 [[package]]
 name = "tree-sitter-ql"
 version = "0.23.1"
@@ -3367,6 +3376,29 @@ version = "1.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "cfe53a6657fd280eaa890a3bc59152892ffa3e30101319d168b781ed6529b049"
 
+[[package]]
+name = "yeast"
+version = "0.1.0"
+dependencies = [
+ "clap",
+ "serde",
+ "serde_json",
+ "serde_yaml",
+ "tree-sitter",
+ "tree-sitter-python",
+ "tree-sitter-ruby",
+ "yeast-macros",
+]
+
+[[package]]
+name = "yeast-macros"
+version = "0.1.0"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "yoke"
 version = "0.8.0"

Cargo.toml

@@ -4,6 +4,8 @@
 resolver = "2"
 members = [
     "shared/tree-sitter-extractor",
+    "shared/yeast",
+    "shared/yeast-macros",
     "ruby/extractor",
     "rust/extractor",
     "rust/extractor/macros",

shared/yeast-macros/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "yeast-macros"
+version = "0.1.0"
+edition = "2021"
+
+[lib]
+proc-macro = true
+
+[dependencies]
+proc-macro2 = "1.0"
+quote = "1.0"
+syn = "2.0"

shared/yeast-macros/src/lib.rs

@@ -0,0 +1,105 @@
+use proc_macro::TokenStream;
+use proc_macro2::TokenStream as TokenStream2;
+
+mod parse;
+
+/// Proc macro for constructing a `QueryNode` from a tree-sitter-inspired pattern.
+///
+/// # Syntax
+///
+/// ```text
+/// (_)                          - match any named node (skips unnamed tokens)
+/// (kind)                       - match a named node of the given kind
+/// ("literal")                  - match an unnamed token by its text
+/// (kind field: (pattern))      - match with named field
+/// (kind (pat) (pat)...)        - match unnamed children (after all fields)
+/// (pattern) @capture           - capture the matched node
+/// (pattern)* @capture          - capture each repeated match
+/// (pattern)?                   - zero or one
+/// ```
+#[proc_macro]
+pub fn query(input: TokenStream) -> TokenStream {
+    let input2: TokenStream2 = input.into();
+    match parse::parse_query_top(input2) {
+        Ok(output) => output.into(),
+        Err(err) => err.to_compile_error().into(),
+    }
+}
+
+/// Build a single AST node from a template, returning its `Id`.
+///
+/// # Template syntax
+///
+/// ```text
+/// (kind "literal")             - leaf with static content
+/// (kind #{expr})               - leaf with computed content (expr.to_string())
+/// (kind $fresh)                - leaf with auto-generated unique name
+/// {expr}                       - embed a Rust expression returning Id
+/// {..expr}                     - splice an iterable of Id (in child/field position)
+/// field: {..expr}              - splice into a named field
+/// ```
+///
+/// Can be called with an explicit context or using the implicit context
+/// from an enclosing `rule!`:
+///
+/// ```text
+/// tree!(ctx, (kind ...))     // explicit BuildCtx
+/// tree!((kind ...))          // implicit context from rule!
+/// ```
+#[proc_macro]
+pub fn tree(input: TokenStream) -> TokenStream {
+    let input2: TokenStream2 = input.into();
+    match parse::parse_tree_top(input2) {
+        Ok(output) => output.into(),
+        Err(err) => err.to_compile_error().into(),
+    }
+}
+
+/// Build a list of AST nodes from a template, returning `Vec<Id>`.
+///
+/// Like `tree!` but returns `Vec<Id>` and supports multiple top-level
+/// elements. All syntax from `tree!` is available.
+///
+/// Can be called with an explicit context or using the implicit context
+/// from an enclosing `rule!`:
+///
+/// ```text
+/// trees!(ctx, (node1 ...) (node2 ...))   // explicit BuildCtx
+/// trees!((node1 ...) (node2 ...))        // implicit context from rule!
+/// ```
+#[proc_macro]
+pub fn trees(input: TokenStream) -> TokenStream {
+    let input2: TokenStream2 = input.into();
+    match parse::parse_trees_top(input2) {
+        Ok(output) => output.into(),
+        Err(err) => err.to_compile_error().into(),
+    }
+}
+
+/// Define a desugaring rule with query and transform in one declaration.
+///
+/// ```text
+/// rule!(
+///     (query_pattern field: (_) @name (kind)* @repeated (_)? @optional)
+///     =>
+///     (output_template field: {name} {..repeated})
+/// )
+///
+/// // Shorthand: captures become fields on the output node
+/// rule!((query ...) => output_kind)
+/// ```
+///
+/// Captures become Rust variables automatically:
+/// - `@name` (no quantifier) → `name: Id`
+/// - `@name` (after `*`/`+`) → `name: Vec<Id>`
+/// - `@name` (after `?`) → `name: Option<Id>`
+///
+/// `tree!` and `trees!` can be used without explicit context inside `{...}`.
+#[proc_macro]
+pub fn rule(input: TokenStream) -> TokenStream {
+    let input2: TokenStream2 = input.into();
+    match parse::parse_rule_top(input2) {
+        Ok(output) => output.into(),
+        Err(err) => err.to_compile_error().into(),
+    }
+}

shared/yeast-macros/src/parse.rs

@@ -0,0 +1,772 @@
+use proc_macro2::{Delimiter, Ident, Literal, Span, TokenStream, TokenTree};
+use quote::quote;
+use std::iter::Peekable;
+
+type Tokens = Peekable<proc_macro2::token_stream::IntoIter>;
+type Result<T> = std::result::Result<T, syn::Error>;
+
+// ---------------------------------------------------------------------------
+// Query parsing
+// ---------------------------------------------------------------------------
+
+/// Top-level entry: parse a single query node from the full input.
+pub fn parse_query_top(input: TokenStream) -> Result<TokenStream> {
+    let mut tokens = input.into_iter().peekable();
+    let result = parse_query_node(&mut tokens)?;
+    if let Some(tok) = tokens.next() {
+        return Err(syn::Error::new_spanned(tok, "unexpected token after query"));
+    }
+    Ok(result)
+}
+
+/// Parse a single query node (possibly with a trailing `@capture`).
+fn parse_query_node(tokens: &mut Tokens) -> Result<TokenStream> {
+    let base = parse_query_atom(tokens)?;
+    // Check for trailing @capture
+    if peek_is_at(tokens) {
+        tokens.next(); // consume @
+        let capture_name = expect_ident(tokens, "expected capture name after @")?;
+        let name_str = capture_name.to_string();
+        Ok(quote! {
+            yeast::query::QueryNode::Capture {
+                capture: #name_str,
+                node: Box::new(#base),
+            }
+        })
+    } else {
+        Ok(base)
+    }
+}
+
+/// Parse a query atom: `(kind fields...)` or `(kind fields... bare_children...)`.
+/// Does not handle `@capture` — that's handled by the caller as a postfix.
+fn parse_query_atom(tokens: &mut Tokens) -> Result<TokenStream> {
+    match tokens.peek() {
+        None => Err(syn::Error::new(Span::call_site(), "unexpected end of query")),
+        Some(TokenTree::Group(g)) if g.delimiter() == Delimiter::Parenthesis => {
+            let group = expect_group(tokens, Delimiter::Parenthesis)?;
+            let mut inner = group.stream().into_iter().peekable();
+            let result = parse_query_node_inner(&mut inner)?;
+            if let Some(tok) = inner.next() {
+                return Err(syn::Error::new_spanned(tok, "unexpected token in query node"));
+            }
+            Ok(result)
+        }
+        Some(tok) => Err(syn::Error::new_spanned(
+            tok.clone(),
+            "expected `(` in query; use `(_) @name` to capture a wildcard",
+        )),
+    }
+}
+
+/// Parse the inside of a parenthesized query node: `kind fields...` or `_` or `"lit"`.
+fn parse_query_node_inner(tokens: &mut Tokens) -> Result<TokenStream> {
+    match tokens.peek() {
+        None => Err(syn::Error::new(Span::call_site(), "empty parenthesized group in query")),
+        Some(TokenTree::Ident(id)) if id.to_string() == "_" => {
+            tokens.next();
+            Ok(quote! { yeast::query::QueryNode::Any() })
+        }
+        Some(TokenTree::Literal(_)) => {
+            let lit = expect_literal(tokens)?;
+            Ok(quote! { yeast::query::QueryNode::UnnamedNode { kind: #lit } })
+        }
+        Some(TokenTree::Ident(_)) => {
+            let kind = expect_ident(tokens, "expected node kind")?;
+            let kind_str = kind.to_string();
+            let fields = parse_query_fields(tokens)?;
+            Ok(quote! {
+                yeast::query::QueryNode::Node {
+                    kind: #kind_str,
+                    children: vec![#(#fields),*],
+                }
+            })
+        }
+        Some(tok) => Err(syn::Error::new_spanned(
+            tok.clone(),
+            "expected node kind, `_`, or string literal",
+        )),
+    }
+}
+
+/// Parse zero or more field specifications and trailing bare patterns.
+/// Named fields: `name: pattern` or `name*: (list...)`.
+/// Bare patterns (no field name) become implicit `child` field entries.
+fn parse_query_fields(tokens: &mut Tokens) -> Result<Vec<TokenStream>> {
+    let mut fields = Vec::new();
+    while tokens.peek().is_some() {
+        if peek_is_field(tokens) {
+            let field_name = expect_ident(tokens, "expected field name")?;
+            let field_str = field_name.to_string();
+
+            expect_punct(tokens, ':', "expected `:` after field name")?;
+
+            let child = parse_query_node(tokens)?;
+            fields.push(quote! {
+                (#field_str, vec![yeast::query::QueryListElem::SingleNode(#child)])
+            });
+        } else {
+            // Bare patterns — collect as implicit `child` field
+            let elems = parse_query_list(tokens)?;
+            if !elems.is_empty() {
+                fields.push(quote! {
+                    ("child", vec![#(#elems),*])
+                });
+            }
+            break;
+        }
+    }
+    Ok(fields)
+}
+
+/// Parse a list of query elements (bare children).
+/// Each element is a node pattern, possibly followed by `*`, `+`, `?`.
+fn parse_query_list(tokens: &mut Tokens) -> Result<Vec<TokenStream>> {
+    let mut elems = Vec::new();
+    while tokens.peek().is_some() {
+        // Check for parenthesized group
+        if peek_is_group(tokens, Delimiter::Parenthesis) {
+            let group = expect_group(tokens, Delimiter::Parenthesis)?;
+            let mut inner = group.stream().into_iter().peekable();
+
+            // Check for repetition after the group
+            if peek_is_repetition(tokens) {
+                let rep = expect_repetition(tokens)?;
+                // Determine if the group is a single node pattern or a list
+                // of patterns. If it starts with an identifier (node kind) or
+                // `_`, treat it as a single repeated node. Otherwise, parse
+                // as a repeated list of sub-patterns.
+                let is_single_node = matches!(inner.peek(), Some(TokenTree::Ident(_)));
+                if is_single_node {
+                    let node = parse_query_node_inner(&mut inner)?;
+                    let elem = quote! {
+                        yeast::query::QueryListElem::Repeated {
+                            children: vec![yeast::query::QueryListElem::SingleNode(#node)],
+                            rep: #rep,
+                        }
+                    };
+                    let elem = maybe_wrap_list_capture(tokens, elem)?;
+                    elems.push(elem);
+                } else {
+                    let sub_elems = parse_query_list(&mut inner)?;
+                    let elem = quote! {
+                        yeast::query::QueryListElem::Repeated {
+                            children: vec![#(#sub_elems),*],
+                            rep: #rep,
+                        }
+                    };
+                    let elem = maybe_wrap_list_capture(tokens, elem)?;
+                    elems.push(elem);
+                }
+            } else {
+                // Single parenthesized node, possibly followed by @capture
+                let node = parse_query_node_inner(&mut inner)?;
+                let node = maybe_wrap_capture(tokens, node)?;
+                elems.push(quote! {
+                    yeast::query::QueryListElem::SingleNode(#node)
+                });
+            }
+            continue;
+        }
+
+        // Check for string literal (unnamed node)
+        if peek_is_literal(tokens) {
+            let lit = expect_literal(tokens)?;
+            let node = quote! { yeast::query::QueryNode::UnnamedNode { kind: #lit } };
+            let elem = maybe_wrap_repetition(tokens, quote! {
+                yeast::query::QueryListElem::SingleNode(#node)
+            })?;
+            elems.push(elem);
+            continue;
+        }
+
+        // Check for bare _ (wildcard), possibly followed by @capture
+        if peek_is_underscore(tokens) {
+            tokens.next();
+            let node = quote! { yeast::query::QueryNode::Any() };
+            let node = maybe_wrap_capture(tokens, node)?;
+            let elem = maybe_wrap_repetition(tokens, quote! {
+                yeast::query::QueryListElem::SingleNode(#node)
+            })?;
+            elems.push(elem);
+            continue;
+        }
+
+        break;
+    }
+    Ok(elems)
+}
+
+// ---------------------------------------------------------------------------
+// tree! / trees! parsing — direct code generation against BuildCtx
+// ---------------------------------------------------------------------------
+
+const IMPLICIT_CTX: &str = "__yeast_ctx";
+
+/// Determine the context identifier: either explicit `ctx,` or the implicit
+/// `__yeast_ctx` from an enclosing `rule!`.
+fn parse_ctx_or_implicit(tokens: &mut Tokens) -> Ident {
+    // Check if first token is an ident followed by a comma
+    let mut lookahead = tokens.clone();
+    let is_explicit = matches!(lookahead.next(), Some(TokenTree::Ident(_)))
+        && matches!(lookahead.next(), Some(TokenTree::Punct(p)) if p.as_char() == ',');
+
+    if is_explicit {
+        let ctx = expect_ident(tokens, "").unwrap();
+        let _ = tokens.next(); // consume comma
+        ctx
+    } else {
+        Ident::new(IMPLICIT_CTX, Span::call_site())
+    }
+}
+
+/// Parse `tree!(ctx, (template))` or `tree!((template))` — returns single `Id`.
+pub fn parse_tree_top(input: TokenStream) -> Result<TokenStream> {
+    let mut tokens = input.into_iter().peekable();
+    let ctx = parse_ctx_or_implicit(&mut tokens);
+
+    let first = parse_direct_node(&mut tokens, &ctx)?;
+
+    if let Some(tok) = tokens.next() {
+        return Err(syn::Error::new_spanned(tok, "unexpected tokens after tree! template; use trees! for multiple nodes"));
+    }
+
+    Ok(quote! { { #first } })
+}
+
+/// Parse `trees!(ctx, ...)` or `trees!(...)` — returns `Vec<Id>`.
+pub fn parse_trees_top(input: TokenStream) -> Result<TokenStream> {
+    let mut tokens = input.into_iter().peekable();
+    let ctx = parse_ctx_or_implicit(&mut tokens);
+    let items = parse_direct_list(&mut tokens, &ctx)?;
+    if let Some(tok) = tokens.next() {
+        return Err(syn::Error::new_spanned(tok, "unexpected token after trees! template"));
+    }
+    Ok(quote! {
+        {
+            let mut __nodes: Vec<usize> = Vec::new();
+            #(#items)*
+            __nodes
+        }
+    })
+}
+
+/// Parse a single node template and generate code that returns an `Id`.
+/// Handles: `(kind fields... children...)` and `{expr}`.
+fn parse_direct_node(tokens: &mut Tokens, ctx: &Ident) -> Result<TokenStream> {
+    match tokens.peek() {
+        Some(TokenTree::Group(g)) if g.delimiter() == Delimiter::Brace => {
+            let group = expect_group(tokens, Delimiter::Brace)?;
+            let expr = group.stream();
+            Ok(quote! { #expr })
+        }
+        Some(TokenTree::Group(g)) if g.delimiter() == Delimiter::Parenthesis => {
+            let group = expect_group(tokens, Delimiter::Parenthesis)?;
+            let mut inner = group.stream().into_iter().peekable();
+            parse_direct_node_inner(&mut inner, ctx)
+        }
+        Some(tok) => Err(syn::Error::new_spanned(tok.clone(), "expected `(` or `{` in tree template")),
+        None => Err(syn::Error::new(Span::call_site(), "unexpected end of tree template")),
+    }
+}
+
+/// Parse the inside of a parenthesized node: `kind fields... children...`
+/// or `kind "literal"` or `kind $fresh`.
+fn parse_direct_node_inner(tokens: &mut Tokens, ctx: &Ident) -> Result<TokenStream> {
+    let kind = expect_ident(tokens, "expected node kind")?;
+    let kind_str = kind.to_string();
+
+    // Check for (kind "literal")
+    if peek_is_literal(tokens) {
+        let lit = expect_literal(tokens)?;
+        return Ok(quote! { #ctx.literal(#kind_str, #lit) });
+    }
+
+    // Check for (kind #{expr}) — computed literal, expr converted via .to_string()
+    if peek_is_hash(tokens) {
+        tokens.next(); // consume #
+        let group = expect_group(tokens, Delimiter::Brace)?;
+        let expr = group.stream();
+        return Ok(quote! { #ctx.literal(#kind_str, &(#expr).to_string()) });
+    }
+
+    // Check for (kind $fresh)
+    if peek_is_dollar(tokens) {
+        tokens.next();
+        let name = expect_ident(tokens, "expected fresh variable name after $")?;
+        let name_str = name.to_string();
+        return Ok(quote! { #ctx.fresh(#kind_str, #name_str) });
+    }
+
+    // Parse named fields
+    let mut stmts = Vec::new();
+    let mut field_args = Vec::new();
+
+    // Named fields — compute each value into a temp, then reference it
+    while peek_is_field(tokens) {
+        let field_name = expect_ident(tokens, "expected field name")?;
+        let field_str = field_name.to_string();
+        expect_punct(tokens, ':', "expected `:` after field name")?;
+
+        // Check for field: {..expr} — splice a Vec<Id> into the field
+        if peek_is_group(tokens, Delimiter::Brace) {
+            let group_clone = tokens.clone().next().unwrap();
+            if let TokenTree::Group(g) = &group_clone {
+                let mut inner_check = g.stream().into_iter();
+                let is_splice = matches!(inner_check.next(), Some(TokenTree::Punct(p)) if p.as_char() == '.')
+                    && matches!(inner_check.next(), Some(TokenTree::Punct(p)) if p.as_char() == '.');
+                if is_splice {
+                    let group = expect_group(tokens, Delimiter::Brace)?;
+                    let mut inner = group.stream().into_iter().peekable();
+                    inner.next(); // consume first .
+                    inner.next(); // consume second .
+                    let expr: proc_macro2::TokenStream = inner.collect();
+                    let temp = Ident::new(&format!("__field_{field_str}"), Span::call_site());
+                    stmts.push(quote! { let #temp: Vec<usize> = #expr; });
+                    field_args.push(quote! { (#field_str, #temp) });
+                    continue;
+                }
+            }
+        }
+
+        let value = parse_direct_node(tokens, ctx)?;
+        let temp = Ident::new(&format!("__field_{field_str}"), Span::call_site());
+        stmts.push(quote! { let #temp = #value; });
+        field_args.push(quote! { (#field_str, vec![#temp]) });
+    }
+
+    // After all named fields, no other tokens are allowed.
+    // Output templates require all children to be in named fields.
+    if let Some(tok) = tokens.peek() {
+        return Err(syn::Error::new_spanned(
+            tok.clone(),
+            "expected named field (`name:`) or end of node template; \
+             output templates do not support unnamed children",
+        ));
+    }
+
+    Ok(quote! {
+        {
+            #(#stmts)*
+            #ctx.node(#kind_str, vec![#(#field_args),*])
+        }
+    })
+}
+
+/// Parse the top-level list of a `trees!` template.
+/// Each item is a node template or `{expr}` splice.
+fn parse_direct_list(tokens: &mut Tokens, ctx: &Ident) -> Result<Vec<TokenStream>> {
+    let mut items = Vec::new();
+    while tokens.peek().is_some() {
+        if peek_is_group(tokens, Delimiter::Parenthesis) {
+            let group = expect_group(tokens, Delimiter::Parenthesis)?;
+            let mut inner = group.stream().into_iter().peekable();
+
+            // Regular node
+            let node = parse_direct_node_inner(&mut inner, ctx)?;
+            items.push(quote! { __nodes.push(#node); });
+            continue;
+        }
+
+        // {expr} or {..expr} — single node or splice
+        if peek_is_group(tokens, Delimiter::Brace) {
+            let group = expect_group(tokens, Delimiter::Brace)?;
+            let mut inner = group.stream().into_iter().peekable();
+            if peek_is_dotdot(&mut inner) {
+                inner.next(); // consume first .
+                inner.next(); // consume second .
+                let expr: TokenStream = inner.collect();
+                items.push(quote! { __nodes.extend(#expr); });
+            } else {
+                let expr = group.stream();
+                items.push(quote! { __nodes.push(#expr); });
+            }
+            continue;
+        }
+
+        break;
+    }
+    Ok(items)
+}
+
+// ---------------------------------------------------------------------------
+// rule! parsing
+// ---------------------------------------------------------------------------
+
+/// A captured variable from a query pattern.
+struct CaptureInfo {
+    name: String,
+    multiplicity: CaptureMultiplicity,
+}
+
+#[derive(Clone, Copy, PartialEq)]
+enum CaptureMultiplicity {
+    /// Exactly one match (bare pattern or after no quantifier)
+    Single,
+    /// Zero or one match (after `?`)
+    Optional,
+    /// Zero or more matches (after `*` or `+`, or inside a repeated group)
+    Repeated,
+}
+
+/// Walk a token stream and extract all `@name` captures, noting whether
+/// they appear after `*` or `+` (repeated) or not.
+fn extract_captures(stream: &TokenStream) -> Vec<CaptureInfo> {
+    let mut captures = Vec::new();
+    extract_captures_inner(&mut stream.clone().into_iter().peekable(), &mut captures, CaptureMultiplicity::Single);
+    captures
+}
+
+fn extract_captures_inner(tokens: &mut Tokens, captures: &mut Vec<CaptureInfo>, parent_mult: CaptureMultiplicity) {
+    let mut last_mult = CaptureMultiplicity::Single;
+    while let Some(tok) = tokens.next() {
+        match tok {
+            TokenTree::Group(g) => {
+                let mut inner = g.stream().into_iter().peekable();
+                let group_mult = match tokens.peek() {
+                    Some(TokenTree::Punct(p)) if p.as_char() == '*' || p.as_char() == '+' => CaptureMultiplicity::Repeated,
+                    Some(TokenTree::Punct(p)) if p.as_char() == '?' => CaptureMultiplicity::Optional,
+                    _ => CaptureMultiplicity::Single,
+                };
+                last_mult = group_mult;
+                let child_mult = if parent_mult == CaptureMultiplicity::Repeated || group_mult == CaptureMultiplicity::Repeated {
+                    CaptureMultiplicity::Repeated
+                } else if parent_mult == CaptureMultiplicity::Optional || group_mult == CaptureMultiplicity::Optional {
+                    CaptureMultiplicity::Optional
+                } else {
+                    CaptureMultiplicity::Single
+                };
+                extract_captures_inner(&mut inner, captures, child_mult);
+            }
+            TokenTree::Punct(p) if p.as_char() == '@' => {
+                if let Some(TokenTree::Ident(name)) = tokens.next() {
+                    let mult = if parent_mult == CaptureMultiplicity::Repeated || last_mult == CaptureMultiplicity::Repeated {
+                        CaptureMultiplicity::Repeated
+                    } else if parent_mult == CaptureMultiplicity::Optional || last_mult == CaptureMultiplicity::Optional {
+                        CaptureMultiplicity::Optional
+                    } else {
+                        CaptureMultiplicity::Single
+                    };
+                    captures.push(CaptureInfo {
+                        name: name.to_string(),
+                        multiplicity: mult,
+                    });
+                }
+                last_mult = CaptureMultiplicity::Single;
+            }
+            TokenTree::Punct(p) if matches!(p.as_char(), '*' | '+' | '?') => {
+                // Keep last_mult — the @capture follows
+            }
+            _ => {
+                last_mult = CaptureMultiplicity::Single;
+            }
+        }
+    }
+}
+
+/// Parse `rule!( query => transform )`.
+pub fn parse_rule_top(input: TokenStream) -> Result<TokenStream> {
+    let mut tokens = input.into_iter().peekable();
+
+    // Collect query tokens up to `=>`
+    let mut query_tokens = Vec::new();
+    loop {
+        match tokens.peek() {
+            None => return Err(syn::Error::new(Span::call_site(), "expected `=>` in rule!")),
+            Some(TokenTree::Punct(p)) if p.as_char() == '=' => {
+                let eq = tokens.next().unwrap();
+                match tokens.peek() {
+                    Some(TokenTree::Punct(p)) if p.as_char() == '>' => {
+                        tokens.next(); // consume >
+                        break;
+                    }
+                    _ => {
+                        query_tokens.push(eq);
+                        continue;
+                    }
+                }
+            }
+            _ => {
+                query_tokens.push(tokens.next().unwrap());
+            }
+        }
+    }
+
+    let query_stream: TokenStream = query_tokens.into_iter().collect();
+
+    // Extract captures from query
+    let captures = extract_captures(&query_stream);
+
+    // Parse query
+    let query_code = parse_query_top(query_stream.clone())?;
+
+    // Generate capture bindings
+    let ctx_ident = Ident::new(IMPLICIT_CTX, Span::call_site());
+    let bindings: Vec<TokenStream> = captures.iter().map(|cap| {
+        let name = Ident::new(&cap.name, Span::call_site());
+        let name_str = &cap.name;
+        match cap.multiplicity {
+            CaptureMultiplicity::Repeated => {
+                quote! { let #name: Vec<usize> = __captures.get_all(#name_str); }
+            }
+            CaptureMultiplicity::Optional => {
+                quote! { let #name: Option<usize> = __captures.get_opt(#name_str); }
+            }
+            CaptureMultiplicity::Single => {
+                quote! { let #name: usize = __captures.get_var(#name_str).unwrap(); }
+            }
+        }
+    }).collect();
+
+    // Parse transform: either shorthand `=> kind_name` or full `=> (template ...)`
+    let transform_body = if peek_is_field(&mut tokens) && {
+        // Shorthand form: bare identifier = output node kind.
+        // Auto-generate template from captures.
+        let mut lookahead = tokens.clone();
+        lookahead.next(); // skip ident
+        lookahead.peek().is_none() // nothing after = shorthand
+    } {
+        let output_kind = expect_ident(&mut tokens, "expected output node kind")?;
+        let output_kind_str = output_kind.to_string();
+
+        // Generate field assignments from captures
+        let field_stmts: Vec<TokenStream> = captures.iter().map(|cap| {
+            let name = Ident::new(&cap.name, Span::call_site());
+            let name_str = &cap.name;
+            match cap.multiplicity {
+                CaptureMultiplicity::Repeated => quote! {
+                    let __field_id = #ctx_ident.ast.field_id_for_name(#name_str)
+                        .unwrap_or_else(|| panic!("field '{}' not found", #name_str));
+                    __fields.insert(__field_id, #name);
+                },
+                CaptureMultiplicity::Optional => quote! {
+                    let __field_id = #ctx_ident.ast.field_id_for_name(#name_str)
+                        .unwrap_or_else(|| panic!("field '{}' not found", #name_str));
+                    if let Some(__id) = #name {
+                        __fields.entry(__field_id).or_insert_with(Vec::new).push(__id);
+                    }
+                },
+                CaptureMultiplicity::Single => quote! {
+                    let __field_id = #ctx_ident.ast.field_id_for_name(#name_str)
+                        .unwrap_or_else(|| panic!("field '{}' not found", #name_str));
+                    __fields.entry(__field_id).or_insert_with(Vec::new).push(#name);
+                },
+            }
+        }).collect();
+
+        quote! {
+            let __kind = #ctx_ident.ast.id_for_node_kind(#output_kind_str)
+                .unwrap_or_else(|| panic!("node kind '{}' not found", #output_kind_str));
+            let mut __fields = std::collections::BTreeMap::new();
+            #(#field_stmts)*
+            let __id = #ctx_ident.ast.create_node_with_range(
+                __kind,
+                yeast::NodeContent::DynamicString(String::new()),
+                __fields,
+                true,
+                __source_range,
+            );
+            vec![__id]
+        }
+    } else {
+        // Full template form
+        let transform_items = parse_direct_list(&mut tokens, &ctx_ident)?;
+
+        if let Some(tok) = tokens.next() {
+            return Err(syn::Error::new_spanned(tok, "unexpected token after rule! transform"));
+        }
+
+        quote! {
+            let mut __nodes: Vec<usize> = Vec::new();
+            #(#transform_items)*
+            __nodes
+        }
+    };
+
+    Ok(quote! {
+        {
+            let __query = #query_code;
+            yeast::Rule::new(__query, Box::new(|__ast: &mut yeast::Ast, __captures: yeast::captures::Captures, __fresh: &yeast::tree_builder::FreshScope, __source_range: Option<tree_sitter::Range>| {
+                #(#bindings)*
+                let mut #ctx_ident = yeast::build::BuildCtx::with_source_range(__ast, &__captures, __fresh, __source_range);
+                #transform_body
+            }))
+        }
+    })
+}
+
+// ---------------------------------------------------------------------------
+// Token utilities
+// ---------------------------------------------------------------------------
+
+fn peek_is_at(tokens: &mut Tokens) -> bool {
+    matches!(tokens.peek(), Some(TokenTree::Punct(p)) if p.as_char() == '@')
+}
+
+fn peek_is_literal(tokens: &mut Tokens) -> bool {
+    matches!(tokens.peek(), Some(TokenTree::Literal(_)))
+}
+
+fn peek_is_dollar(tokens: &mut Tokens) -> bool {
+    matches!(tokens.peek(), Some(TokenTree::Punct(p)) if p.as_char() == '$')
+}
+
+fn peek_is_hash(tokens: &mut Tokens) -> bool {
+    matches!(tokens.peek(), Some(TokenTree::Punct(p)) if p.as_char() == '#')
+}
+
+/// Check for `..` (two consecutive dot punctuation tokens).
+fn peek_is_dotdot(tokens: &Tokens) -> bool {
+    let mut lookahead = tokens.clone();
+    matches!(lookahead.next(), Some(TokenTree::Punct(p)) if p.as_char() == '.')
+        && matches!(lookahead.next(), Some(TokenTree::Punct(p)) if p.as_char() == '.')
+}
+
+fn peek_is_underscore(tokens: &mut Tokens) -> bool {
+    matches!(tokens.peek(), Some(TokenTree::Ident(id)) if id.to_string() == "_")
+}
+
+/// Check if the next tokens form a field specification (ident followed by `:` or `*:`).
+/// A bare identifier (other than `_`) at this position is always a field name, since
+/// bare child patterns must start with `(`, `@`, `"literal"`, or `_`.
+fn peek_is_field(tokens: &mut Tokens) -> bool {
+    match tokens.peek() {
+        Some(TokenTree::Ident(id)) if id.to_string() != "_" => true,
+        _ => false,
+    }
+}
+
+fn peek_is_group(tokens: &mut Tokens, delim: Delimiter) -> bool {
+    matches!(tokens.peek(), Some(TokenTree::Group(g)) if g.delimiter() == delim)
+}
+
+fn peek_is_repetition(tokens: &mut Tokens) -> bool {
+    matches!(tokens.peek(), Some(TokenTree::Punct(p)) if matches!(p.as_char(), '*' | '+' | '?'))
+}
+
+fn expect_ident(tokens: &mut Tokens, msg: &str) -> Result<Ident> {
+    match tokens.next() {
+        Some(TokenTree::Ident(id)) => Ok(id),
+        Some(tok) => Err(syn::Error::new_spanned(tok, msg)),
+        None => Err(syn::Error::new(Span::call_site(), msg)),
+    }
+}
+
+fn expect_literal(tokens: &mut Tokens) -> Result<Literal> {
+    match tokens.next() {
+        Some(TokenTree::Literal(lit)) => Ok(lit),
+        Some(tok) => Err(syn::Error::new_spanned(tok, "expected string literal")),
+        None => Err(syn::Error::new(Span::call_site(), "expected string literal")),
+    }
+}
+
+fn expect_punct(tokens: &mut Tokens, ch: char, msg: &str) -> Result<()> {
+    match tokens.next() {
+        Some(TokenTree::Punct(p)) if p.as_char() == ch => Ok(()),
+        Some(tok) => Err(syn::Error::new_spanned(tok, msg)),
+        None => Err(syn::Error::new(Span::call_site(), msg)),
+    }
+}
+
+fn expect_group(tokens: &mut Tokens, delim: Delimiter) -> Result<proc_macro2::Group> {
+    match tokens.next() {
+        Some(TokenTree::Group(g)) if g.delimiter() == delim => Ok(g),
+        Some(tok) => Err(syn::Error::new_spanned(
+            tok,
+            format!("expected {:?} group", delim),
+        )),
+        None => Err(syn::Error::new(
+            Span::call_site(),
+            format!("expected {:?} group", delim),
+        )),
+    }
+}
+
+fn expect_repetition(tokens: &mut Tokens) -> Result<TokenStream> {
+    match tokens.next() {
+        Some(TokenTree::Punct(p)) => match p.as_char() {
+            '*' => Ok(quote! { yeast::query::Rep::ZeroOrMore }),
+            '+' => Ok(quote! { yeast::query::Rep::OneOrMore }),
+            '?' => Ok(quote! { yeast::query::Rep::ZeroOrOne }),
+            _ => Err(syn::Error::new(p.span(), "expected `*`, `+`, or `?`")),
+        },
+        Some(tok) => Err(syn::Error::new_spanned(tok, "expected repetition quantifier")),
+        None => Err(syn::Error::new(Span::call_site(), "expected repetition quantifier")),
+    }
+}
+
+fn maybe_wrap_capture(tokens: &mut Tokens, base: TokenStream) -> Result<TokenStream> {
+    if peek_is_at(tokens) {
+        tokens.next(); // consume @
+        let name = expect_ident(tokens, "expected capture name after @")?;
+        let name_str = name.to_string();
+        Ok(quote! {
+            yeast::query::QueryNode::Capture {
+                capture: #name_str,
+                node: Box::new(#base),
+            }
+        })
+    } else {
+        Ok(base)
+    }
+}
+
+fn maybe_wrap_repetition(tokens: &mut Tokens, single: TokenStream) -> Result<TokenStream> {
+    if peek_is_repetition(tokens) {
+        let rep = expect_repetition(tokens)?;
+        Ok(quote! {
+            yeast::query::QueryListElem::Repeated {
+                children: vec![#single],
+                rep: #rep,
+            }
+        })
+    } else {
+        Ok(single)
+    }
+}
+
+/// If `@name` follows a Repeated list element, wrap each child SingleNode
+/// inside the repetition with a Capture. This matches tree-sitter semantics
+/// where `(_)* @name` captures each matched node.
+fn maybe_wrap_list_capture(tokens: &mut Tokens, elem: TokenStream) -> Result<TokenStream> {
+    if peek_is_at(tokens) {
+        tokens.next();
+        let name = expect_ident(tokens, "expected capture name after @")?;
+        let name_str = name.to_string();
+        // Re-parse the element isn't practical, so we generate a wrapper
+        // that creates a new Repeated with each child wrapped in a capture.
+        // The simplest approach: generate code that the runtime can interpret.
+        // Actually, the capture annotation on repeated elements is best handled
+        // by re-generating the Repeated with captures injected.
+        // For now, assume the common case: the repetition contains a single
+        // SingleNode child, and we wrap that node in a capture.
+        Ok(quote! {
+            {
+                let __rep = #elem;
+                match __rep {
+                    yeast::query::QueryListElem::Repeated { children, rep } => {
+                        yeast::query::QueryListElem::Repeated {
+                            children: children.into_iter().map(|child| {
+                                match child {
+                                    yeast::query::QueryListElem::SingleNode(node) => {
+                                        yeast::query::QueryListElem::SingleNode(
+                                            yeast::query::QueryNode::Capture {
+                                                capture: #name_str,
+                                                node: Box::new(node),
+                                            }
+                                        )
+                                    }
+                                    other => other,
+                                }
+                            }).collect(),
+                            rep,
+                        }
+                    }
+                    other => other,
+                }
+            }
+        })
+    } else {
+        Ok(elem)
+    }
+}

shared/yeast/.envrc

@@ -0,0 +1 @@
+use flake

shared/yeast/.gitignore

@@ -0,0 +1 @@
+/target

shared/yeast/Cargo.lock

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+checksum = "83fc039473c5595ace860d8c4fafa220ff474b3fc6bfdb4293327f1a37e94d86"
+dependencies = [
+ "itoa",
+ "memchr",
+ "serde",
+ "serde_core",
+ "zmij",
+]
+
+[[package]]
+name = "shlex"
+version = "1.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0fda2ff0d084019ba4d7c6f371c95d8fd75ce3524c3cb8fb653a3023f6323e64"
+
+[[package]]
+name = "streaming-iterator"
+version = "0.1.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2b2231b7c3057d5e4ad0156fb3dc807d900806020c5ffa3ee6ff2c8c76fb8520"
+
+[[package]]
+name = "strsim"
+version = "0.11.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7da8b5736845d9f2fcb837ea5d9e2628564b3b043a70948a3f0b778838c5fb4f"
+
+[[package]]
+name = "syn"
+version = "2.0.117"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e665b8803e7b1d2a727f4023456bbbbe74da67099c585258af0ad9c5013b9b99"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "unicode-ident",
+]
+
+[[package]]
+name = "tree-sitter"
+version = "0.24.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a5387dffa7ffc7d2dae12b50c6f7aab8ff79d6210147c6613561fc3d474c6f75"
+dependencies = [
+ "cc",
+ "regex",
+ "regex-syntax",
+ "streaming-iterator",
+ "tree-sitter-language",
+]
+
+[[package]]
+name = "tree-sitter-language"
+version = "0.1.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "009994f150cc0cd50ff54917d5bc8bffe8cad10ca10d81c34da2ec421ae61782"
+
+[[package]]
+name = "tree-sitter-python"
+version = "0.23.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3d065aaa27f3aaceaf60c1f0e0ac09e1cb9eb8ed28e7bcdaa52129cffc7f4b04"
+dependencies = [
+ "cc",
+ "tree-sitter-language",
+]
+
+[[package]]
+name = "tree-sitter-ruby"
+version = "0.23.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "be0484ea4ef6bb9c575b4fdabde7e31340a8d2dbc7d52b321ac83da703249f95"
+dependencies = [
+ "cc",
+ "tree-sitter-language",
+]
+
+[[package]]
+name = "unicode-ident"
+version = "1.0.24"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e6e4313cd5fcd3dad5cafa179702e2b244f760991f45397d14d4ebf38247da75"
+
+[[package]]
+name = "utf8parse"
+version = "0.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "06abde3611657adf66d383f00b093d7faecc7fa57071cce2578660c9f1010821"
+
+[[package]]
+name = "windows-link"
+version = "0.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f0805222e57f7521d6a62e36fa9163bc891acd422f971defe97d64e70d0a4fe5"
+
+[[package]]
+name = "windows-sys"
+version = "0.61.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ae137229bcbd6cdf0f7b80a31df61766145077ddf49416a728b02cb3921ff3fc"
+dependencies = [
+ "windows-link",
+]
+
+[[package]]
+name = "yeast"
+version = "0.1.0"
+dependencies = [
+ "clap",
+ "serde",
+ "serde_json",
+ "tree-sitter",
+ "tree-sitter-python",
+ "tree-sitter-ruby",
+]
+
+[[package]]
+name = "zmij"
+version = "1.0.21"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b8848ee67ecc8aedbaf3e4122217aff892639231befc6a1b58d29fff4c2cabaa"

shared/yeast/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "yeast"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+clap = { version = "4.4.10", features = ["derive"] }
+serde = { version = "1.0.193", features = ["derive"] }
+serde_json = "1.0.108"
+serde_yaml = "0.9"
+tree-sitter = "0.24"
+yeast-macros = { path = "../yeast-macros" }
+
+tree-sitter-ruby = "0.23"
+tree-sitter-python = "0.23"

shared/yeast/src/bin/main.rs

@@ -0,0 +1,26 @@
+use clap::Parser;
+
+#[derive(Parser)]
+#[clap(name = "yeast", about = "yeast elaborates abstract syntax trees")]
+struct Cli {
+    file: String,
+    #[clap(default_value = "ruby")]
+    language: String,
+}
+
+fn get_language(language: &str) -> tree_sitter::Language {
+    match language {
+        "ruby" => tree_sitter_ruby::LANGUAGE.into(),
+        "python" => tree_sitter_python::LANGUAGE.into(),
+        _ => panic!("Unsupported language: {}", language),
+    }
+}
+
+fn main() {
+    let args = Cli::parse();
+    let language = get_language(&args.language);
+    let source = std::fs::read_to_string(&args.file).unwrap();
+    let runner = yeast::Runner::new(language, vec![]);
+    let ast = runner.run(&source).unwrap();
+    println!("{}", ast.print(&source, ast.get_root()));
+}

shared/yeast/src/build.rs

@@ -0,0 +1,79 @@
+use std::collections::BTreeMap;
+
+use crate::captures::Captures;
+use crate::tree_builder::FreshScope;
+use crate::{Ast, FieldId, Id, NodeContent};
+
+/// Context for building new AST nodes during a transformation.
+///
+/// Used by the `tree!` and `trees!` macros. Holds a mutable reference to the
+/// AST, a reference to the captures from a query match, and a `FreshScope` for
+/// generating unique identifiers.
+pub struct BuildCtx<'a> {
+    pub ast: &'a mut Ast,
+    pub captures: &'a Captures,
+    pub fresh: &'a FreshScope,
+    /// Source range of the matched node, inherited by synthetic nodes.
+    pub source_range: Option<tree_sitter::Range>,
+}
+
+impl<'a> BuildCtx<'a> {
+    pub fn new(ast: &'a mut Ast, captures: &'a Captures, fresh: &'a FreshScope) -> Self {
+        Self {
+            ast,
+            captures,
+            fresh,
+            source_range: None,
+        }
+    }
+
+    pub fn with_source_range(ast: &'a mut Ast, captures: &'a Captures, fresh: &'a FreshScope, source_range: Option<tree_sitter::Range>) -> Self {
+        Self {
+            ast,
+            captures,
+            fresh,
+            source_range,
+        }
+    }
+
+    /// Look up a capture variable, returning its node Id.
+    pub fn capture(&self, name: &str) -> Id {
+        self.captures
+            .get_var(name)
+            .unwrap_or_else(|e| panic!("build: {e}"))
+    }
+
+    /// Get all values of a repeated capture variable.
+    pub fn capture_all(&self, name: &str) -> Vec<Id> {
+        self.captures.get_all(name)
+    }
+
+    /// Create a named AST node with the given kind and fields.
+    pub fn node(&mut self, kind: &str, fields: Vec<(&str, Vec<Id>)>) -> Id {
+        let kind_id = self
+            .ast
+            .id_for_node_kind(kind)
+            .unwrap_or_else(|| panic!("build: node kind '{kind}' not found"));
+        let mut field_map: BTreeMap<FieldId, Vec<Id>> = BTreeMap::new();
+        for (name, ids) in fields {
+            let field_id = self
+                .ast
+                .field_id_for_name(name)
+                .unwrap_or_else(|| panic!("build: field '{name}' not found"));
+            field_map.entry(field_id).or_default().extend(ids);
+        }
+        self.ast
+            .create_node_with_range(kind_id, NodeContent::DynamicString(String::new()), field_map, true, self.source_range)
+    }
+
+    /// Create a leaf node with a fixed string content.
+    pub fn literal(&mut self, kind: &'static str, value: &str) -> Id {
+        self.ast.create_named_token_with_range(kind, value.to_string(), self.source_range)
+    }
+
+    /// Create a leaf node with an auto-generated unique name.
+    pub fn fresh(&mut self, kind: &'static str, name: &str) -> Id {
+        let generated = self.fresh.resolve(name);
+        self.ast.create_named_token_with_range(kind, generated, self.source_range)
+    }
+}

shared/yeast/src/captures.rs

@@ -0,0 +1,105 @@
+use std::collections::{BTreeMap, BTreeSet};
+
+use crate::Id;
+
+#[derive(Debug, Clone)]
+pub struct Captures {
+    captures: BTreeMap<&'static str, Vec<Id>>,
+}
+
+impl Default for Captures {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl Captures {
+    pub fn new() -> Self {
+        Captures {
+            captures: BTreeMap::new(),
+        }
+    }
+
+    pub fn get_var(&self, key: &str) -> Result<Id, String> {
+        let ids = self.captures.get(key);
+        if let Some(ids) = ids {
+            if ids.len() == 1 {
+                Ok(ids[0])
+            } else {
+                Err(format!(
+                    "Variable {} has {} matches, use * to allow repetition",
+                    key,
+                    ids.len()
+                ))
+            }
+        } else {
+            Err(format!("No variable named {}", key))
+        }
+    }
+
+    /// Get all values of a capture variable (for repeated captures).
+    pub fn get_all(&self, key: &str) -> Vec<Id> {
+        self.captures.get(key).cloned().unwrap_or_default()
+    }
+
+    /// Get an optional capture variable. Returns None if unmatched,
+    /// Some(id) if matched exactly once.
+    pub fn get_opt(&self, key: &str) -> Option<Id> {
+        self.captures.get(key).and_then(|ids| {
+            if ids.len() == 1 { Some(ids[0]) } else { None }
+        })
+    }
+
+    pub fn insert(&mut self, key: &'static str, id: Id) {
+        self.captures.entry(key).or_default().push(id);
+    }
+
+    pub fn map_captures(&mut self, kind: &str, f: &mut impl FnMut(Id) -> Id) {
+        if let Some(ids) = self.captures.get_mut(kind) {
+            for id in ids {
+                *id = f(*id);
+            }
+        }
+    }
+    pub fn map_captures_to(&mut self, from: &str, to: &'static str, f: &mut impl FnMut(Id) -> Id) {
+        if let Some(from_ids) = self.captures.get(from) {
+            let new_values = from_ids.iter().copied().map(f).collect();
+            self.captures.insert(to, new_values);
+        }
+    }
+
+    pub fn merge(&mut self, other: &Captures) {
+        for (key, ids) in &other.captures {
+            self.captures.entry(key).or_default().extend(ids);
+        }
+    }
+
+    pub fn un_star<'a>(
+        &'a self,
+        children: &'a BTreeSet<&'static str>,
+    ) -> Result<impl Iterator<Item = Captures> + 'a, String> {
+        let mut id_iter = children.iter();
+
+        if let Some(fst) = id_iter.next() {
+            let repeats = self
+                .captures
+                .get(fst)
+                .ok_or_else(|| format!("No variable named {}", fst))?
+                .len();
+            // TODO: better error on missing capture
+            if id_iter.any(|id| self.captures.get(id).map(Vec::len).unwrap_or(0) != repeats) {
+                return Err("Repeated captures must have the same number of matches".to_string());
+            }
+            Ok((0..repeats).map(move |iter| {
+                let mut new_vars: Captures = Captures::new();
+                for id in children {
+                    let child_capture = self.captures.get(id).unwrap()[iter];
+                    new_vars.captures.insert(id, vec![child_capture]);
+                }
+                new_vars
+            }))
+        } else {
+            Err("Repeated captures must have at least one capture".to_string())
+        }
+    }
+}

shared/yeast/src/cursor.rs

@@ -0,0 +1,8 @@
+pub trait Cursor<'a, T, N, F> {
+    fn node(&self) -> &'a N;
+    fn field_id(&self) -> Option<F>;
+    fn field_name(&self) -> Option<&'static str>;
+    fn goto_first_child(&mut self) -> bool;
+    fn goto_next_sibling(&mut self) -> bool;
+    fn goto_parent(&mut self) -> bool;
+}

shared/yeast/src/lib.rs

@@ -0,0 +1,691 @@
+use std::collections::BTreeMap;
+
+extern crate self as yeast;
+
+use serde::Serialize;
+use serde_json::{json, Value};
+
+pub mod build;
+pub mod captures;
+pub mod cursor;
+pub mod dump;
+pub mod node_types_yaml;
+pub mod print;
+pub mod query;
+mod range;
+pub mod schema;
+pub mod tree_builder;
+mod visitor;
+
+pub use yeast_macros::{query, rule, tree, trees};
+
+use captures::Captures;
+pub use cursor::Cursor;
+use query::QueryNode;
+
+/// Node ids are indexes into the arena
+type Id = usize;
+
+/// Field and Kind ids are provided by tree-sitter
+type FieldId = u16;
+type KindId = u16;
+
+pub const CHILD_FIELD: u16 = u16::MAX;
+
+#[derive(Debug)]
+pub struct AstCursor<'a> {
+    ast: &'a Ast,
+    /// A stack of parents, along with iterators for their children
+    parents: Vec<(&'a Node, ChildrenIter<'a>)>,
+    node: &'a Node,
+}
+
+impl<'a> AstCursor<'a> {
+    pub fn new(ast: &'a Ast) -> Self {
+        // TODO: handle non-zero root
+        let node = ast.get_node(ast.root).unwrap();
+        Self {
+            ast,
+            parents: vec![],
+            node,
+        }
+    }
+
+    fn goto_next_sibling_opt(&mut self) -> Option<()> {
+        self.node = self.parents.last_mut()?.1.next()?;
+        Some(())
+    }
+
+    fn goto_first_child_opt(&mut self) -> Option<()> {
+        let parent = self.node;
+        let mut children = ChildrenIter::new(self.ast, parent);
+        let first_child = children.next()?;
+        self.node = first_child;
+        self.parents.push((parent, children));
+        Some(())
+    }
+
+    fn goto_parent_opt(&mut self) -> Option<()> {
+        self.node = self.parents.pop()?.0;
+        Some(())
+    }
+}
+impl<'a> Cursor<'a, Ast, Node, FieldId> for AstCursor<'a> {
+    fn node(&self) -> &'a Node {
+        self.node
+    }
+
+    fn field_id(&self) -> Option<FieldId> {
+        let (_, children) = self.parents.last()?;
+        children.current_field()
+    }
+
+    fn field_name(&self) -> Option<&'static str> {
+        if self.field_id() == Some(CHILD_FIELD) {
+            None
+        } else {
+            self.field_id()
+                .and_then(|id| self.ast.field_name_for_id(id))
+        }
+    }
+
+    fn goto_first_child(&mut self) -> bool {
+        self.goto_first_child_opt().is_some()
+    }
+
+    fn goto_next_sibling(&mut self) -> bool {
+        self.goto_next_sibling_opt().is_some()
+    }
+
+    fn goto_parent(&mut self) -> bool {
+        self.goto_parent_opt().is_some()
+    }
+}
+
+/// An iterator over all the child nodes of a node.
+#[derive(Debug)]
+struct ChildrenIter<'a> {
+    ast: &'a Ast,
+    current_field: Option<FieldId>,
+    fields: std::collections::btree_map::Iter<'a, FieldId, Vec<Id>>,
+    field_children: Option<std::slice::Iter<'a, Id>>,
+}
+
+impl<'a> ChildrenIter<'a> {
+    fn new(ast: &'a Ast, node: &'a Node) -> Self {
+        Self {
+            ast,
+            current_field: None,
+            fields: node.fields.iter(),
+            field_children: None,
+        }
+    }
+
+    fn get_node(&self, id: Id) -> &'a Node {
+        self.ast.get_node(id).unwrap()
+    }
+
+    fn current_field(&self) -> Option<FieldId> {
+        self.current_field
+    }
+}
+
+impl<'a> Iterator for ChildrenIter<'a> {
+    type Item = &'a Node;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match self.field_children.as_mut() {
+            None => match self.fields.next() {
+                Some((field, children)) => {
+                    self.current_field = Some(*field);
+                    self.field_children = Some(children.iter());
+                    self.next()
+                }
+                None => None,
+            },
+            Some(children) => match children.next() {
+                None => match self.fields.next() {
+                    None => None,
+                    Some((field, children)) => {
+                        self.current_field = Some(*field);
+                        self.field_children = Some(children.iter());
+                        self.next()
+                    }
+                },
+                Some(child_id) => Some(self.get_node(*child_id)),
+            },
+        }
+    }
+}
+
+/// Our AST
+pub struct Ast {
+    root: Id,
+    nodes: Vec<Node>,
+    schema: schema::Schema,
+}
+
+impl std::fmt::Debug for Ast {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("Ast")
+            .field("root", &self.root)
+            .field("nodes", &self.nodes.len())
+            .finish()
+    }
+}
+
+impl Ast {
+    /// Construct an AST from a TS tree
+    pub fn from_tree(language: tree_sitter::Language, tree: &tree_sitter::Tree) -> Self {
+        let schema = schema::Schema::from_language(&language);
+        Self::from_tree_with_schema(schema, tree, &language)
+    }
+
+    pub fn from_tree_with_schema(schema: schema::Schema, tree: &tree_sitter::Tree, language: &tree_sitter::Language) -> Self {
+        let mut visitor = visitor::Visitor::new(language.clone());
+        visitor.visit(tree);
+        let ast = visitor.build_with_schema(schema);
+        ast
+    }
+
+    pub fn walk(&self) -> AstCursor {
+        AstCursor::new(self)
+    }
+
+    pub fn nodes(&self) -> &[Node] {
+        &self.nodes
+    }
+
+    pub fn get_root(&self) -> Id {
+        self.root
+    }
+
+    pub fn set_root(&mut self, root: Id) {
+        self.root = root;
+    }
+
+    pub fn get_node(&self, id: Id) -> Option<&Node> {
+        self.nodes.get(id)
+    }
+
+    pub fn print(&self, source: &str, root_id: Id) -> Value {
+        let root = &self.nodes()[root_id];
+        self.print_node(root, source)
+    }
+
+    pub fn create_node(
+        &mut self,
+        kind: KindId,
+        content: NodeContent,
+        fields: BTreeMap<FieldId, Vec<Id>>,
+        is_named: bool,
+    ) -> Id {
+        self.create_node_with_range(kind, content, fields, is_named, None)
+    }
+
+    pub fn create_node_with_range(
+        &mut self,
+        kind: KindId,
+        content: NodeContent,
+        fields: BTreeMap<FieldId, Vec<Id>>,
+        is_named: bool,
+        source_range: Option<tree_sitter::Range>,
+    ) -> Id {
+        let id = self.nodes.len();
+        self.nodes.push(Node {
+            id,
+            kind,
+            kind_name: self.schema.node_kind_for_id(kind).unwrap(),
+            fields,
+            content,
+            is_missing: false,
+            is_error: false,
+            is_extra: false,
+            is_named,
+            source_range,
+        });
+        id
+    }
+
+    pub fn create_named_token(&mut self, kind: &'static str, content: String) -> Id {
+        self.create_named_token_with_range(kind, content, None)
+    }
+
+    pub fn create_named_token_with_range(&mut self, kind: &'static str, content: String, source_range: Option<tree_sitter::Range>) -> Id {
+        let kind_id = self.schema.id_for_node_kind(kind)
+            .unwrap_or_else(|| panic!("create_named_token: node kind '{kind}' not found in schema"));
+        let id = self.nodes.len();
+        self.nodes.push(Node {
+            id,
+            kind: kind_id,
+            kind_name: kind,
+            is_named: true,
+            is_missing: false,
+            is_error: false,
+            source_range,
+            is_extra: false,
+            fields: BTreeMap::new(),
+            content: NodeContent::DynamicString(content),
+        });
+        id
+    }
+
+    pub fn field_name_for_id(&self, id: FieldId) -> Option<&'static str> {
+        self.schema.field_name_for_id(id)
+    }
+
+    pub fn field_id_for_name(&self, name: &str) -> Option<FieldId> {
+        self.schema.field_id_for_name(name)
+    }
+
+    /// Print a node for debugging
+    fn print_node(&self, node: &Node, source: &str) -> Value {
+        let fields: BTreeMap<&'static str, Vec<Value>> = node
+            .fields
+            .iter()
+            .map(|(field_id, nodes)| {
+                let field_name = if field_id == &CHILD_FIELD {
+                    "rest"
+                } else {
+                    self.field_name_for_id(*field_id).unwrap()
+                };
+                let nodes: Vec<Value> = nodes
+                    .iter()
+                    .map(|id| self.print_node(self.get_node(*id).unwrap(), source))
+                    .collect();
+                (field_name, nodes)
+            })
+            .collect();
+        let mut value = BTreeMap::new();
+        let kind = self.schema.node_kind_for_id(node.kind).unwrap();
+        let content = match &node.content {
+            NodeContent::Range(range) => {
+                let end = range.end_byte;
+                std::str::from_utf8(&source.as_bytes()[range.start_byte..end])
+                    .unwrap_or("<invalid utf-8>")
+                    .to_string()
+            }
+            NodeContent::String(s) => s.to_string(),
+            NodeContent::DynamicString(s) => s.clone(),
+        };
+        if fields.is_empty() {
+            value.insert(kind, json!(content));
+        } else {
+            let mut fields: BTreeMap<_, _> =
+                fields.into_iter().map(|(k, v)| (k, json!(v))).collect();
+            fields.insert("content", json!(content));
+            value.insert(kind, json!(fields));
+        }
+        json!(value)
+    }
+
+    /// Return an example AST, for testing and to fill implementation gaps
+    pub fn example(language: tree_sitter::Language) -> Self {
+        // x = 1
+        Self {
+            root: 0,
+            schema: schema::Schema::from_language(&language),
+            nodes: vec![
+                // assignment
+                Node {
+                    id: 0,
+                    kind: 276,
+                    kind_name: "assignment",
+                    fields: {
+                        let mut map = BTreeMap::new();
+                        map.insert(18, vec![1]);
+                        map.insert(28, vec![3]);
+                        map
+                    },
+                    content: NodeContent::String("x = 1"),
+                    is_missing: false,
+                    is_error: false,
+            source_range: None,
+                    is_extra: false,
+                    is_named: true,
+                },
+                // identifier
+                Node {
+                    id: 1,
+                    kind: 1,
+                    kind_name: "identifier",
+                    fields: BTreeMap::new(),
+                    content: NodeContent::String("x"),
+                    is_missing: false,
+                    is_error: false,
+            source_range: None,
+                    is_extra: false,
+                    is_named: true,
+                },
+                // "="
+                Node {
+                    id: 2,
+                    kind: 17,
+                    kind_name: "=",
+                    fields: BTreeMap::new(),
+                    content: NodeContent::String("="),
+                    is_missing: false,
+                    is_error: false,
+            source_range: None,
+                    is_extra: false,
+                    is_named: false,
+                },
+                // integer
+                Node {
+                    id: 3,
+                    kind: 110,
+                    kind_name: "integer",
+                    fields: BTreeMap::new(),
+                    content: NodeContent::String("1"),
+                    is_missing: false,
+                    is_error: false,
+            source_range: None,
+                    is_extra: false,
+                    is_named: true,
+                },
+            ],
+        }
+    }
+
+    pub fn id_for_node_kind(&self, kind: &str) -> Option<KindId> {
+        let id = self.schema.id_for_node_kind(kind).unwrap_or(0);
+        if id == 0 {
+            None
+        } else {
+            Some(id)
+        }
+    }
+
+    fn id_for_unnamed_node_kind(&self, kind: &str) -> Option<KindId> {
+        let id = self.schema.id_for_unnamed_node_kind(kind).unwrap_or(0);
+        if id == 0 {
+            None
+        } else {
+            Some(id)
+        }
+    }
+}
+
+/// A node in our AST
+#[derive(PartialEq, Eq, Debug, Clone, Serialize)]
+pub struct Node {
+    id: Id,
+    kind: KindId,
+    kind_name: &'static str,
+    pub(crate) fields: BTreeMap<FieldId, Vec<Id>>,
+    pub(crate) content: NodeContent,
+    /// For synthetic nodes, the source range of the original node they
+    /// were desugared from. Used for location information in TRAP output.
+    #[serde(skip)]
+    source_range: Option<tree_sitter::Range>,
+    is_named: bool,
+    is_missing: bool,
+    is_extra: bool,
+    is_error: bool,
+}
+
+impl Node {
+    pub fn id(&self) -> Id {
+        self.id
+    }
+
+    pub fn kind(&self) -> &'static str {
+        self.kind_name
+    }
+
+    pub fn kind_name(&self) -> &'static str {
+        self.kind_name
+    }
+
+    pub fn is_named(&self) -> bool {
+        self.is_named
+    }
+
+    pub fn is_missing(&self) -> bool {
+        self.is_missing
+    }
+
+    pub fn is_extra(&self) -> bool {
+        self.is_extra
+    }
+
+    pub fn is_error(&self) -> bool {
+        self.is_error
+    }
+
+    fn fake_point(&self) -> tree_sitter::Point {
+        tree_sitter::Point { row: 0, column: 0 }
+    }
+
+    pub fn start_position(&self) -> tree_sitter::Point {
+        match self.content {
+            NodeContent::Range(range) => range.start_point,
+            _ => self.source_range.map_or_else(
+                || self.fake_point(),
+                |r| r.start_point,
+            ),
+        }
+    }
+
+    pub fn end_position(&self) -> tree_sitter::Point {
+        match self.content {
+            NodeContent::Range(range) => range.end_point,
+            _ => self.source_range.map_or_else(
+                || self.fake_point(),
+                |r| r.end_point,
+            ),
+        }
+    }
+
+    pub fn start_byte(&self) -> usize {
+        match self.content {
+            NodeContent::Range(range) => range.start_byte,
+            _ => self.source_range.map_or(0, |r| r.start_byte),
+        }
+    }
+
+    pub fn end_byte(&self) -> usize {
+        match self.content {
+            NodeContent::Range(range) => range.end_byte,
+            _ => self.source_range.map_or(0, |r| r.end_byte),
+        }
+    }
+
+    pub fn byte_range(&self) -> std::ops::Range<usize> {
+        self.start_byte()..self.end_byte()
+    }
+
+    pub fn opt_string_content(&self) -> Option<String> {
+        match &self.content {
+            NodeContent::Range(_range) => None,
+            NodeContent::String(s) => Some(s.to_string()),
+            NodeContent::DynamicString(s) => Some(s.to_string()),
+        }
+    }
+}
+
+/// The contents of a node is either a range in the original source file,
+/// or a new string if the node is synthesized.
+#[derive(PartialEq, Eq, Debug, Clone, Serialize)]
+pub enum NodeContent {
+    Range(#[serde(with = "range::Range")] tree_sitter::Range),
+    String(&'static str),
+    DynamicString(String),
+}
+
+impl From<&'static str> for NodeContent {
+    fn from(value: &'static str) -> Self {
+        NodeContent::String(value)
+    }
+}
+
+impl From<tree_sitter::Range> for NodeContent {
+    fn from(value: tree_sitter::Range) -> Self {
+        NodeContent::Range(value)
+    }
+}
+
+pub struct Rule {
+    query: QueryNode,
+    transform: Box<dyn Fn(&mut Ast, Captures, &tree_builder::FreshScope, Option<tree_sitter::Range>) -> Vec<Id>>,
+}
+
+impl Rule {
+    pub fn new(query: QueryNode, transform: Box<dyn Fn(&mut Ast, Captures, &tree_builder::FreshScope, Option<tree_sitter::Range>) -> Vec<Id>>) -> Self {
+        Self { query, transform }
+    }
+
+    fn try_rule(&self, ast: &mut Ast, node: Id, fresh: &tree_builder::FreshScope) -> Result<Option<Vec<Id>>, String> {
+        let mut captures = Captures::new();
+        if self.query.do_match(ast, node, &mut captures)? {
+            fresh.next_scope();
+            let source_range = ast.get_node(node).and_then(|n| {
+                match n.content {
+                    NodeContent::Range(r) => Some(r),
+                    _ => n.source_range,
+                }
+            });
+            Ok(Some((self.transform)(ast, captures, fresh, source_range)))
+        } else {
+            Ok(None)
+        }
+    }
+}
+
+const MAX_REWRITE_DEPTH: usize = 100;
+
+/// Index of rules by their root query kind for fast lookup.
+struct RuleIndex<'a> {
+    /// Rules indexed by root node kind name.
+    by_kind: BTreeMap<&'static str, Vec<&'a Rule>>,
+    /// Rules with wildcard queries (Any) that apply to all nodes.
+    wildcard: Vec<&'a Rule>,
+}
+
+impl<'a> RuleIndex<'a> {
+    fn new(rules: &'a [Rule]) -> Self {
+        let mut by_kind: BTreeMap<&'static str, Vec<&'a Rule>> = BTreeMap::new();
+        let mut wildcard = Vec::new();
+        for rule in rules {
+            match rule.query.root_kind() {
+                Some(kind) => by_kind.entry(kind).or_default().push(rule),
+                None => wildcard.push(rule),
+            }
+        }
+        Self { by_kind, wildcard }
+    }
+
+    fn rules_for_kind(&self, kind: &str) -> impl Iterator<Item = &&'a Rule> {
+        self.by_kind
+            .get(kind)
+            .into_iter()
+            .flat_map(|v| v.iter())
+            .chain(self.wildcard.iter())
+    }
+}
+
+fn apply_rules(rules: &[Rule], ast: &mut Ast, id: Id, fresh: &tree_builder::FreshScope) -> Result<Vec<Id>, String> {
+    let index = RuleIndex::new(rules);
+    apply_rules_inner(&index, ast, id, fresh, 0)
+}
+
+fn apply_rules_inner(index: &RuleIndex, ast: &mut Ast, id: Id, fresh: &tree_builder::FreshScope, rewrite_depth: usize) -> Result<Vec<Id>, String> {
+    if rewrite_depth > MAX_REWRITE_DEPTH {
+        return Err(format!(
+            "Desugaring exceeded maximum rewrite depth ({MAX_REWRITE_DEPTH}). \
+             This likely indicates a non-terminating rule cycle."
+        ));
+    }
+
+    let node_kind = ast.get_node(id).map(|n| n.kind()).unwrap_or("");
+    for rule in index.rules_for_kind(node_kind) {
+        if let Some(result_node) = rule.try_rule(ast, id, fresh)? {
+            let mut results = Vec::new();
+            for node in result_node {
+                results.extend(apply_rules_inner(index, ast, node, fresh, rewrite_depth + 1)?);
+            }
+            return Ok(results);
+        }
+    }
+
+    // Collect fields before recursing (avoids borrowing ast immutably during mutation)
+    let field_entries: Vec<(FieldId, Vec<Id>)> = ast.nodes[id]
+        .fields
+        .iter()
+        .map(|(&fid, children)| (fid, children.clone()))
+        .collect();
+
+    // recursively descend into all the fields
+    // Child traversal does not increment rewrite depth
+    let mut changed = false;
+    let mut new_fields = BTreeMap::new();
+    for (field_id, children) in field_entries {
+        let mut new_children = Vec::new();
+        for child_id in children {
+            let result = apply_rules_inner(index, ast, child_id, fresh, rewrite_depth)?;
+            if result.len() != 1 || result[0] != child_id {
+                changed = true;
+            }
+            new_children.extend(result);
+        }
+        new_fields.insert(field_id, new_children);
+    }
+
+    if !changed {
+        return Ok(vec![id]);
+    }
+
+    let mut node = ast.nodes[id].clone();
+    node.fields = new_fields;
+    node.id = ast.nodes.len();
+    ast.nodes.push(node);
+    Ok(vec![ast.nodes.len() - 1])
+}
+
+pub struct Runner {
+    language: tree_sitter::Language,
+    schema: schema::Schema,
+    rules: Vec<Rule>,
+}
+
+impl Runner {
+    /// Create a runner using the input grammar's schema for output.
+    pub fn new(language: tree_sitter::Language, rules: Vec<Rule>) -> Self {
+        let schema = schema::Schema::from_language(&language);
+        Self { language, schema, rules }
+    }
+
+    /// Create a runner with separate input language and output schema.
+    pub fn with_schema(language: tree_sitter::Language, schema: schema::Schema, rules: Vec<Rule>) -> Self {
+        Self { language, schema, rules }
+    }
+
+    pub fn run_from_tree(&self, tree: &tree_sitter::Tree) -> Result<Ast, String> {
+        let fresh = tree_builder::FreshScope::new();
+        let mut ast = Ast::from_tree_with_schema(self.schema.clone(), tree, &self.language);
+        let root = ast.get_root();
+        let res = apply_rules(&self.rules, &mut ast, root, &fresh)?;
+        if res.len() != 1 {
+            return Err(format!("Expected exactly one result node, got {}", res.len()));
+        }
+        ast.set_root(res[0]);
+        Ok(ast)
+    }
+
+    pub fn run(&self, input: &str) -> Result<Ast, String> {
+        let fresh = tree_builder::FreshScope::new();
+        let mut parser = tree_sitter::Parser::new();
+        parser.set_language(&self.language)
+            .map_err(|e| format!("Failed to set language: {e}"))?;
+        let tree = parser.parse(input, None)
+            .ok_or_else(|| "Failed to parse input".to_string())?;
+        let mut ast = Ast::from_tree_with_schema(self.schema.clone(), &tree, &self.language);
+        let root = ast.get_root();
+        let res = apply_rules(&self.rules, &mut ast, root, &fresh)?;
+        if res.len() != 1 {
+            return Err(format!("Expected exactly one result node, got {}", res.len()));
+        }
+        ast.set_root(res[0]);
+        Ok(ast)
+    }
+}

shared/yeast/src/print.rs

@@ -0,0 +1,34 @@
+use crate::{cursor::Cursor, AstCursor, Node};
+
+pub struct Printer {}
+
+impl Printer {
+    pub fn visit(&mut self, mut cursor: AstCursor<'_>) {
+        self.enter_node(cursor.node());
+        let mut recurse = true;
+        loop {
+            if recurse && cursor.goto_first_child() {
+                recurse = self.enter_node(cursor.node());
+            } else {
+                self.leave_node(cursor.node());
+
+                if cursor.goto_next_sibling() {
+                    recurse = self.enter_node(cursor.node());
+                } else if cursor.goto_parent() {
+                    recurse = false;
+                } else {
+                    break;
+                }
+            }
+        }
+    }
+
+    pub fn enter_node(&mut self, node: &Node) -> bool {
+        println!("enter_node: {:?}", node);
+        true
+    }
+    pub fn leave_node(&mut self, node: &Node) -> bool {
+        println!("leave_node: {:?}", node);
+        true
+    }
+}

shared/yeast/src/query.rs

@@ -0,0 +1,230 @@
+
+use crate::{captures::Captures, Ast, Id};
+
+#[derive(Debug, Clone)]
+pub enum QueryNode {
+    Any(),
+    Node {
+        kind: &'static str,
+        children: Vec<(&'static str, Vec<QueryListElem>)>,
+    },
+    UnnamedNode {
+        kind: &'static str,
+    },
+    Capture {
+        capture: &'static str,
+        node: Box<QueryNode>,
+    },
+}
+
+impl QueryNode {
+    /// Returns the root node kind this query matches, if it's specific.
+    /// Returns None for wildcards (Any) and captures wrapping wildcards.
+    pub fn root_kind(&self) -> Option<&'static str> {
+        match self {
+            QueryNode::Node { kind, .. } => Some(kind),
+            QueryNode::UnnamedNode { kind } => Some(kind),
+            QueryNode::Capture { node, .. } => node.root_kind(),
+            QueryNode::Any() => None,
+        }
+    }
+}
+
+#[derive(Debug, Clone)]
+pub enum QueryListElem {
+    Repeated { children: Vec<QueryListElem>, rep: Rep },
+    SingleNode(QueryNode),
+}
+
+#[derive(Debug, PartialEq, Eq, Copy, Clone)]
+pub enum Rep {
+    ZeroOrMore,
+    OneOrMore,
+    ZeroOrOne,
+}
+
+impl QueryNode {
+    /// Returns true if this query only matches named nodes (not unnamed tokens).
+    /// Used to skip unnamed children in positional matching, matching tree-sitter
+    /// semantics where `(_)` only matches named nodes.
+    fn matches_named_only(&self) -> bool {
+        match self {
+            QueryNode::Any() => true,
+            QueryNode::Node { .. } => true,
+            QueryNode::UnnamedNode { .. } => false,
+            QueryNode::Capture { node, .. } => node.matches_named_only(),
+        }
+    }
+
+    pub fn do_match(&self, ast: &Ast, node: Id, matches: &mut Captures) -> Result<bool, String> {
+        match self {
+            QueryNode::Any() => Ok(true),
+            QueryNode::Node { kind, children } => {
+                let node = ast.get_node(node).unwrap();
+                let target_kind = ast.id_for_node_kind(kind).ok_or_else(|| {
+                    format!("Node kind {} not found in language", kind)
+                })?;
+                if node.kind != target_kind {
+                    return Ok(false);
+                }
+                for (field, field_children) in children {
+                    let field_id = ast
+                        .field_id_for_name(field)
+                        .ok_or_else(|| format!("Field {} not found in language", field))?;
+                    let empty = Vec::new();
+                    let mut child_iter = node
+                        .fields
+                        .get(&field_id)
+                        .unwrap_or(&empty)
+                        .iter()
+                        .cloned();
+                    if !match_children(field_children.iter(), ast, &mut child_iter, matches)? {
+                        return Ok(false);
+                    }
+                }
+                Ok(true)
+            }
+            QueryNode::UnnamedNode { kind } => {
+                let node = ast.get_node(node).unwrap();
+                let target_kind = ast.id_for_unnamed_node_kind(kind).ok_or_else(|| {
+                    format!("unnamed Node kind {} not found in language", kind)
+                })?;
+                Ok(node.kind == target_kind)
+            }
+            QueryNode::Capture {
+                capture,
+                node: sub_query,
+            } => {
+                let matched = sub_query.do_match(ast, node, matches)?;
+                if matched {
+                    matches.insert(capture, node);
+                }
+                Ok(matched)
+            }
+        }
+    }
+}
+
+fn match_children<'a>(
+    child_matchers: impl Iterator<Item = &'a QueryListElem>,
+    ast: &Ast,
+    remaining_children: &mut (impl Iterator<Item = Id> + Clone),
+    matches: &mut Captures,
+) -> Result<bool, String> {
+    for child in child_matchers {
+        if !child.do_match(ast, remaining_children, matches)? {
+            return Ok(false);
+        }
+    }
+    Ok(true)
+}
+
+impl QueryListElem {
+    fn do_match(
+        &self,
+        ast: &Ast,
+        remaining_children: &mut (impl Iterator<Item = Id> + Clone),
+        matches: &mut Captures,
+    ) -> Result<bool, String> {
+        match self {
+            QueryListElem::Repeated { children, rep } => {
+                if children.is_empty() {
+                    // Empty repetition always succeeds without consuming
+                    return Ok(*rep != Rep::OneOrMore);
+                }
+
+                let mut iters = 0;
+
+                loop {
+                    let matches_initial = matches.clone();
+                    let start = remaining_children.clone();
+                    let start_next = start.clone().next();
+                    if !match_children(children.iter(), ast, remaining_children, matches)? {
+                        *remaining_children = start;
+                        *matches = matches_initial;
+                        break;
+                    }
+                    // Guard against zero-width matches: if the iterator
+                    // didn't advance, break to avoid infinite looping.
+                    let current_next = remaining_children.clone().next();
+                    if start_next == current_next {
+                        break;
+                    }
+                    iters += 1;
+                    if *rep == Rep::ZeroOrOne {
+                        break;
+                    }
+                }
+                if *rep == Rep::OneOrMore && iters == 0 {
+                    // We didn't match any children but we were supposed to
+                    Ok(false)
+                } else {
+                    Ok(true)
+                }
+            }
+            QueryListElem::SingleNode(sub_query) => {
+                if sub_query.matches_named_only() {
+                    // Skip unnamed children, matching tree-sitter semantics
+                    // where (_) only matches named nodes.
+                    loop {
+                        match remaining_children.next() {
+                            Some(child) => {
+                                let node = ast.get_node(child).unwrap();
+                                if node.is_named() {
+                                    return sub_query.do_match(ast, child, matches);
+                                }
+                                // Skip unnamed child, continue to next
+                            }
+                            None => return Ok(false),
+                        }
+                    }
+                } else if let Some(child) = remaining_children.next() {
+                    sub_query.do_match(ast, child, matches)
+                } else {
+                    Ok(false)
+                }
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::query::*;
+    #[test]
+    fn it_works() {
+        let query1: QueryNode = yeast::query!((_));
+        println!("{:?}", query1);
+        let query2 = yeast::query!((foo));
+        println!("{:?}", query2);
+        let query3 = yeast::query!((foo child: (_)));
+        println!("{:?}", query3);
+        let query4 = yeast::query!((foo (_)*));
+        println!("{:?}", query4);
+        let query5: QueryNode = yeast::query!((foo (_)*));
+        println!("{:?}", query5);
+        let query6: QueryNode = yeast::query!((_) @bar);
+        println!("{:?}", query6);
+        let query7: QueryNode = yeast::query!((foo child: (_) @bar));
+        println!("{:?}", query7);
+        let query8: QueryNode = yeast::query!(
+            (assignment
+                left: (element_reference
+                    object: (_) @obj
+                    (_) @index
+                )
+                right: (_) @rhs
+            )
+        );
+        println!("{:?}", query8);
+        let query9 = yeast::query!(
+            (program
+                child: (assignment
+                    left: (_) @left
+                    right: (_) @right
+                )
+            )
+        );
+        println!("{:?}", query9);
+    }
+}

shared/yeast/src/range.rs

@@ -0,0 +1,21 @@
+//! (de)-serialize helpers for tree_sitter::Range
+
+use serde::{Deserialize, Serialize};
+
+#[derive(Serialize, Deserialize)]
+#[serde(remote = "tree_sitter::Point")]
+pub struct Point {
+    pub row: usize,
+    pub column: usize,
+}
+
+#[derive(Serialize, Deserialize)]
+#[serde(remote = "tree_sitter::Range")]
+pub struct Range {
+    pub start_byte: usize,
+    pub end_byte: usize,
+    #[serde(with = "Point")]
+    pub start_point: tree_sitter::Point,
+    #[serde(with = "Point")]
+    pub end_point: tree_sitter::Point,
+}

shared/yeast/src/schema.rs

@@ -0,0 +1,132 @@
+use std::collections::BTreeMap;
+
+use crate::{FieldId, KindId, CHILD_FIELD};
+
+/// A schema defining node kinds and field names for the output AST.
+/// Built from a node-types.yml file, independent of any tree-sitter grammar.
+#[derive(Clone)]
+pub struct Schema {
+    field_ids: BTreeMap<String, FieldId>,
+    field_names: BTreeMap<FieldId, &'static str>,
+    next_field_id: FieldId,
+    kind_ids: BTreeMap<String, KindId>,
+    unnamed_kind_ids: BTreeMap<String, KindId>,
+    kind_names: BTreeMap<KindId, &'static str>,
+    next_kind_id: KindId,
+}
+
+impl Schema {
+    pub fn new() -> Self {
+        Self {
+            field_ids: BTreeMap::new(),
+            field_names: BTreeMap::new(),
+            next_field_id: 1, // 0 is reserved
+            kind_ids: BTreeMap::new(),
+            unnamed_kind_ids: BTreeMap::new(),
+            kind_names: BTreeMap::new(),
+            next_kind_id: 1, // 0 is reserved
+        }
+    }
+
+    /// Create a schema from a tree-sitter language, importing all its
+    /// known field and kind names.
+    pub fn from_language(language: &tree_sitter::Language) -> Self {
+        let mut schema = Self::new();
+        // Import all field names, preserving tree-sitter's IDs
+        for id in 1..=language.field_count() as u16 {
+            if let Some(name) = language.field_name_for_id(id) {
+                schema.field_ids.insert(name.to_string(), id);
+                schema.field_names.insert(id, name);
+                if id >= schema.next_field_id {
+                    schema.next_field_id = id + 1;
+                }
+            }
+        }
+        // Import all node kind names, preserving tree-sitter's IDs.
+        // Track named and unnamed variants separately.
+        // For named kinds, use the canonical ID from id_for_node_kind(name, true)
+        // since some languages have multiple IDs for the same named kind.
+        for id in 0..language.node_kind_count() as u16 {
+            if let Some(name) = language.node_kind_for_id(id) {
+                if !name.is_empty() {
+                    let is_named = language.node_kind_is_named(id);
+                    if is_named {
+                        let canonical_id = language.id_for_node_kind(name, true);
+                        if canonical_id != 0 && !schema.kind_ids.contains_key(name) {
+                            schema.kind_ids.insert(name.to_string(), canonical_id);
+                            schema.kind_names.insert(canonical_id, name);
+                        }
+                    } else {
+                        // For unnamed kinds, only insert if we don't already have one
+                        // (some languages have multiple unnamed IDs for the same text)
+                        schema.unnamed_kind_ids.entry(name.to_string()).or_insert(id);
+                    }
+                    // Always track the name for any ID we encounter
+                    schema.kind_names.entry(id).or_insert(name);
+                    if id >= schema.next_kind_id {
+                        schema.next_kind_id = id + 1;
+                    }
+                }
+            }
+        }
+        schema
+    }
+
+    /// Register a field name, returning its ID.
+    /// If already registered, returns the existing ID.
+    pub fn register_field(&mut self, name: &str) -> FieldId {
+        if name == "child" {
+            return CHILD_FIELD;
+        }
+        if let Some(&id) = self.field_ids.get(name) {
+            return id;
+        }
+        let id = self.next_field_id;
+        assert!(id < CHILD_FIELD, "too many fields");
+        self.next_field_id += 1;
+        let leaked: &'static str = Box::leak(name.to_string().into_boxed_str());
+        self.field_ids.insert(name.to_string(), id);
+        self.field_names.insert(id, leaked);
+        id
+    }
+
+    /// Register a node kind name, returning its ID.
+    /// If already registered, returns the existing ID.
+    pub fn register_kind(&mut self, name: &str) -> KindId {
+        if let Some(&id) = self.kind_ids.get(name) {
+            return id;
+        }
+        let id = self.next_kind_id;
+        self.next_kind_id += 1;
+        let leaked: &'static str = Box::leak(name.to_string().into_boxed_str());
+        self.kind_ids.insert(name.to_string(), id);
+        self.kind_names.insert(id, leaked);
+        id
+    }
+
+    pub fn field_id_for_name(&self, name: &str) -> Option<FieldId> {
+        if name == "child" {
+            return Some(CHILD_FIELD);
+        }
+        self.field_ids.get(name).copied()
+    }
+
+    pub fn field_name_for_id(&self, id: FieldId) -> Option<&'static str> {
+        if id == CHILD_FIELD {
+            return Some("child");
+        }
+        self.field_names.get(&id).copied()
+    }
+
+    pub fn id_for_node_kind(&self, kind: &str) -> Option<KindId> {
+        self.kind_ids.get(kind).copied()
+    }
+
+    pub fn id_for_unnamed_node_kind(&self, kind: &str) -> Option<KindId> {
+        self.unnamed_kind_ids.get(kind).copied()
+    }
+
+    pub fn node_kind_for_id(&self, id: KindId) -> Option<&'static str> {
+        self.kind_names.get(&id).copied()
+    }
+}

shared/yeast/src/tree_builder.rs

@@ -0,0 +1,37 @@
+use std::collections::BTreeMap;
+use std::cell::Cell;
+
+/// Tracks fresh identifier generation during a single tree-building operation.
+/// All occurrences of the same `$name` within one build share the same generated value.
+pub struct FreshScope {
+    counter: Cell<u32>,
+    resolved: std::cell::RefCell<BTreeMap<String, String>>,
+}
+
+impl FreshScope {
+    pub fn new() -> Self {
+        Self {
+            counter: Cell::new(0),
+            resolved: std::cell::RefCell::new(BTreeMap::new()),
+        }
+    }
+
+    pub fn resolve(&self, name: &str) -> String {
+        self.resolved
+            .borrow_mut()
+            .entry(name.to_string())
+            .or_insert_with(|| {
+                let id = self.counter.get();
+                self.counter.set(id + 1);
+                format!("${name}-{id}")
+            })
+            .clone()
+    }
+
+    /// Clear resolved names but keep the counter. Called between rule
+    /// applications so that `$tmp` in different rules gets different values
+    /// while the counter increases monotonically.
+    pub fn next_scope(&self) {
+        self.resolved.borrow_mut().clear();
+    }
+}

shared/yeast/src/visitor.rs

@@ -0,0 +1,111 @@
+use std::collections::BTreeMap;
+use tree_sitter::{Language, Tree};
+
+use crate::{Ast, Id, Node, NodeContent, CHILD_FIELD};
+
+#[derive(Debug)]
+struct VisitorNode {
+    inner: Node,
+    parent: Option<Id>,
+}
+
+/// A type that can walk a TS tree and produce an `Ast`.
+#[derive(Debug)]
+pub(crate) struct Visitor {
+    nodes: Vec<VisitorNode>,
+    current: Option<Id>,
+    language: Language,
+}
+
+impl Visitor {
+    pub fn new(language: Language) -> Self {
+        Self {
+            nodes: Vec::new(),
+            current: None,
+            language,
+        }
+    }
+
+    pub fn visit(&mut self, tree: &Tree) {
+        let cursor = &mut tree.walk();
+        self.enter_node(cursor.node());
+        let mut recurse = true;
+        loop {
+            if recurse && cursor.goto_first_child() {
+                recurse = self.enter_node(cursor.node());
+            } else {
+                self.leave_node(cursor.field_name(), cursor.node());
+
+                if cursor.goto_next_sibling() {
+                    recurse = self.enter_node(cursor.node());
+                } else if cursor.goto_parent() {
+                    recurse = false;
+                } else {
+                    break;
+                }
+            }
+        }
+    }
+
+    pub fn build_with_schema(self, schema: crate::schema::Schema) -> Ast {
+        Ast {
+            root: self.nodes[0].inner.id,
+            schema,
+            nodes: self.nodes.into_iter().map(|n| n.inner).collect(),
+        }
+    }
+
+    fn add_node(&mut self, n: tree_sitter::Node<'_>, content: NodeContent, is_named: bool) -> Id {
+        let id = self.nodes.len();
+        self.nodes.push(VisitorNode {
+            inner: Node {
+                id,
+                kind: self.language.id_for_node_kind(n.kind(), is_named),
+                kind_name: n.kind(),
+                content,
+                fields: BTreeMap::new(),
+                is_missing: n.is_missing(),
+                is_named: n.is_named(),
+                is_extra: n.is_extra(),
+                is_error: n.is_error(),
+                source_range: None,
+            },
+            parent: self.current,
+        });
+        id
+    }
+
+    fn enter_node(&mut self, node: tree_sitter::Node<'_>) -> bool {
+        let id = self.add_node(node, node.range().into(), node.is_named());
+        self.current = Some(id);
+        true
+    }
+
+    fn leave_node(&mut self, field_name: Option<&'static str>, _node: tree_sitter::Node<'_>) {
+        let node = self.current.map(|i| &self.nodes[i]).unwrap();
+        let node_id = node.inner.id;
+        let node_parent = node.parent;
+
+        if let Some(parent_id) = node.parent {
+            let parent = self.nodes.get_mut(parent_id).unwrap();
+            if let Some(field) = field_name {
+                let field_id = self.language.field_id_for_name(field).unwrap().get();
+                parent
+                    .inner
+                    .fields
+                    .entry(field_id)
+                    .or_default()
+                    .push(node_id);
+            } else {
+                parent
+                    .inner
+                    .fields
+                    .entry(CHILD_FIELD)
+                    .or_default()
+                    .push(node_id);
+            }
+        }
+
+        self.current = node_parent;
+    }
+}