rust: support SCCACHE_BASEDIRS across checkout roots

docs/Rust.md

@@ -9,5 +9,9 @@ sccache includes support for caching Rust compilation. This includes many caveat
 * Procedural macros that read files from the filesystem may not be cached properly.
 * `rustc`'s incremental compilation needs to be disabled. See [The Cargo Book](https://doc.rust-lang.org/cargo/reference/profiles.html#incremental)
 * Crates that invoke the system linker cannot be cached. Examples are `bin`, `dylib`, `cdylib`, and `proc-macro` crates.
+* `SCCACHE_BASEDIRS` normalizes paths in Rust cache keys when rustc's `--remap-path-prefix` covers the working directory with the default or `all` remap scope. Cached dep-info is rebuilt with output targets and dependency records from the current invocation.
+* Path normalization remains location-sensitive when explicit external crates or dynamic libraries on crate search paths may invoke procedural macros, because they can observe physical paths without reporting a dependency to rustc.
+* Rust path normalization is limited to ASCII paths on Windows.
+* Distributed Rust compilation falls back to local compilation when `--remap-path-prefix` is used with a non-identity path transformer.
 
 If you are using Rust 1.18 or later, you can ask cargo to wrap all compilation with sccache by setting `RUSTC_WRAPPER=sccache` in your build environment.

src/compiler/compiler.rs

@@ -638,7 +638,11 @@ where
                 };
 
                 let hit = CompileResult::CacheHit(duration);
-                match entry.extract_objects(filtered_outputs, &pool).await {
+                let extraction = entry
+                    .extract_objects(filtered_outputs, &pool)
+                    .await
+                    .and_then(|()| compilation.postprocess_cache_hit(&cwd));
+                match extraction {
                     Ok(()) => Ok(CacheLookupResult::Success(hit, output)),
                     Err(e) => {
                         if e.downcast_ref::<DecompressionFailure>().is_some() {
@@ -1112,6 +1116,11 @@ where
         true
     }
 
+    /// Adjust extracted outputs for the current invocation after a cache hit.
+    fn postprocess_cache_hit(&self, _cwd: &Path) -> Result<()> {
+        Ok(())
+    }
+
     /// Returns an iterator over the results of this compilation.
     ///
     /// Each item is a descriptive (and unique) name of the output paired with