[sorinirimies]

Summary

With the default relative target_dir_path ("spirv-builder"), invoke_rustc runs cargo metadata to find the target dir on every call — so every rebuild of a SpirvWatcher.

 SpirvWatcher::recv_inner
+  pin_target_dir()                 // once: relative -> absolute via cargo metadata;
+                                   // on failure: debug log, retried next build
   builder.invoke_rustc()
-    cargo metadata                 // every rebuild
+    resolve_target_dir()           // absolute now -> no subprocess

SpirvBuilder::resolve_target_dir is extracted from invoke_rustc unchanged.

Evidence

cargo metadata cost saved per rebuild (sky-shader, warm):

hyperfine -N 'cargo metadata --format-version 1'
  Time (mean ± σ):  228.4 ms ± 5.4 ms   [15 runs]

New tests::target_dir:

absolute_target_dir_is_kept                       ok   // no cargo metadata needed
relative_target_dir_is_inside_cargo_target_dir    ok

cargo test -p spirv-builder --features watch,clap: all pass. Clippy -D warnings clean.

Merge Danger

Door: two-way

Blast Radius: small — watcher only. If a user changes the cargo target dir (e.g. .cargo/config.toml build.target-dir) while watching, the watcher keeps the old one until restarted.