Rust: rename target_dir to cargo_target_dir, add to extraction options

Also removed the now unused `extract_dependencies` one.

rust/codeql-extractor.yml

@@ -23,14 +23,15 @@ options:
         title: Controls compression for the TRAP files written by the extractor.
         description: >
           This option is only intended for use in debugging the extractor. Accepted
-          values are 'gzip' (the default, to write gzip-compressed TRAP) and 'none'
-          (to write uncompressed TRAP).
+          values are 'gzip' (to write gzip-compressed TRAP) and 'none'
+          (currently the default, to write uncompressed TRAP).
         type: string
         pattern: "^(none|gzip)$"
-  extract_dependencies:
-    title: Whether to extract dependencies.
+  cargo_target_dir:
+    title: Directory to use for cargo output files.
     description: >
-      Extract the source code of dependencies and the standard libraries in addition to 
-      normal source code.
+      This value is an optional path to use as `CARGO_TARGET_DIR` for the internal
+      cargo commands the extractor uses. Pointing it to a persistent directory may
+      reduce execution time of consecutive extractor runs. By default, a new scratch
+      directory is used for each run.
     type: string
-    pattern: "^(false|true)$"

rust/extractor/src/config.rs

@@ -37,8 +37,7 @@ pub struct Config {
     pub scratch_dir: PathBuf,
     pub trap_dir: PathBuf,
     pub source_archive_dir: PathBuf,
-    pub target_dir: Option<PathBuf>,
-    pub extract_dependencies: bool,
+    pub cargo_target_dir: Option<PathBuf>,
     pub verbose: u8,
     pub compression: Compression,
     pub inputs: Vec<PathBuf>,

rust/extractor/src/main.rs

@@ -131,7 +131,7 @@ fn main() -> anyhow::Result<()> {
         extractor.extract_without_semantics(file, "no manifest found");
     }
     let target_dir = &cfg
-        .target_dir
+        .cargo_target_dir
         .unwrap_or_else(|| cfg.scratch_dir.join("target"));
     for (manifest, files) in map.values().filter(|(_, files)| !files.is_empty()) {
         if let Some((ref db, ref vfs)) = RustAnalyzer::load_workspace(manifest, target_dir) {