Deduplicate and document helper types

ql/src/semmle/go/Expr.qll

@@ -593,6 +593,29 @@ class SelectorExpr extends @selectorexpr, Expr {
   override string getAPrimaryQlClass() { result = "SelectorExpr" }
 }
 
+/**
+ * A selector expression that refers to a promoted field. These selectors may implicitly
+ * address an embedded struct of their base type (e.g. the selector `x.field` may implicitly
+ * address `x.Embedded.field`). Note they may also explicitly address `field`; being a
+ * `PromotedFieldSelector` only indicates the addressed field may be promoted, not that it
+ * is promoted in this particular context.
+ */
+class PromotedFieldSelector extends SelectorExpr {
+  PromotedFieldSelector() { this.refersTo(any(PromotedField f)) }
+
+  /**
+   * Gets the underlying struct type of this selector's base. Note because this selector
+   * addresses a promoted field, the addressed field may not directly occur in the returned
+   * struct type.
+   */
+  StructType getSelectedStructType() {
+    exists(Type baseType | baseType = this.getBase().getType().getUnderlyingType() |
+      pragma[only_bind_into](result) =
+        [baseType, baseType.(PointerType).getBaseType().getUnderlyingType()]
+    )
+  }
+}
+
 /**
  * An index expression, that is, a base expression followed by an index.
  *

ql/src/semmle/go/Scopes.qll

@@ -353,6 +353,15 @@ class Field extends Variable {
   }
 }
 
+/**
+ * A field that belongs to a struct that may be embedded within another struct.
+ *
+ * When a selector addresses such a field, it is possible it is implicitly addressing a nested struct.
+ */
+class PromotedField extends Field {
+  PromotedField() { this = any(StructType t).getFieldOfEmbedded(_, _, _, _) }
+}
+
 /** A built-in or declared function. */
 class Function extends ValueEntity, @functionobject {
   /** Gets a call to this function. */

ql/src/semmle/go/controlflow/ControlFlowGraphImpl.qll

@@ -48,16 +48,9 @@ private predicate isCond(Expr e) {
   e = any(ParenExpr par | isCond(par)).getExpr()
 }
 
-private StructType getSelectedStructType(PromotedFieldSelector e) {
-  exists(Type baseType | baseType = e.getBase().getType().getUnderlyingType() |
-    pragma[only_bind_into](result) =
-      [baseType, baseType.(PointerType).getBaseType().getUnderlyingType()]
-  )
-}
-
 private predicate implicitFieldSelection(PromotedFieldSelector e, int i, Field implicitField) {
   exists(StructType baseType, PromotedField child |
-    baseType = getSelectedStructType(e) and
+    baseType = e.getSelectedStructType() and
     (
       e.refersTo(child)
       or
@@ -68,14 +61,6 @@ private predicate implicitFieldSelection(PromotedFieldSelector e, int i, Field i
   )
 }
 
-private class PromotedFieldSelector extends SelectorExpr {
-  PromotedFieldSelector() { this.refersTo(any(PromotedField f)) }
-}
-
-private class PromotedField extends Field {
-  PromotedField() { this = any(StructType t).getFieldOfEmbedded(_, _, _, _) }
-}
-
 /**
  * A node in the intra-procedural control-flow graph of a Go function or file.
  *

ql/src/semmle/go/controlflow/IR.qll

@@ -271,23 +271,8 @@ module IR {
     result = simpleSelectorBase(e)
   }
 
-  private class PromotedField extends Field {
-    PromotedField() { this = any(StructType t).getFieldOfEmbedded(_, _, _, _) }
-  }
-
-  private class PromotedFieldSelector extends SelectorExpr {
-    PromotedFieldSelector() { this.refersTo(any(PromotedField f)) }
-  }
-
-  private StructType getSelectedStructType(PromotedFieldSelector e) {
-    exists(Type baseType | baseType = e.getBase().getType().getUnderlyingType() |
-      pragma[only_bind_into](result) =
-        [baseType, baseType.(PointerType).getBaseType().getUnderlyingType()]
-    )
-  }
-
   private Instruction promotedFieldSelectorBase(PromotedFieldSelector se, Field field) {
-    exists(StructType baseStructType | baseStructType = getSelectedStructType(se) |
+    exists(StructType baseStructType | baseStructType = se.getSelectedStructType() |
       if field = baseStructType.getOwnField(_, _)
       then
         result = MkImplicitDeref(se.getBase())