no-op remap_path_prefix change invalidates incremental cache

I'm building rust code in a CI environment, setting -Cincremental to a shared location. CI allocates a new working directory for each build. To prevent leaking that info to the debug info, we set --remap_path_prefix=${PWD}= to eliminate the unwanted prefix from the debug info.

rustc src/lib.rs \
  --crate-name=xxx \
  --crate-type=rlib \
  --error-format=human \
  --out-dir=${OUTDIR} \
  --remap-path-prefix=${PWD}= \
  --emit=dep-info,link \
  -Ccodegen-units=256 \
  -Zincremental_info \
  -Cincremental=${CACHEDIR}

I expected this will reuse the incremental build cache, but it's not:

[incremental] session directory: 259 files hard-linked
[incremental] session directory: 0 files copied
[incremental] completely ignoring cache because of differing commandline arguments

Further inspection of how the hash being generated, I found:

https://github.com/rust-lang/rust/blob/017ae1b21f7be6dcdcfc95631e54bde806653a8a/compiler/rustc_session/src/options.rs#L195-L222

for our case, working_dir always uses remapped_path "", which is stable over build. but the remap_path_prefix is different everytime. and the logic for populating working_dir is at:

https://github.com/rust-lang/rust/blob/017ae1b21f7be6dcdcfc95631e54bde806653a8a/compiler/rustc_session/src/config.rs#L2690-L2695

So to get correct remapped working_dir, I must set --remap-path-prefix, but setting it will change remap_path_prefix and causing hash mismatch:

https://github.com/rust-lang/rust/blob/017ae1b21f7be6dcdcfc95631e54bde806653a8a/compiler/rustc_incremental/src/persist/load.rs#L159-L164

Meta

rustc --version --verbose:

rustc 1.82.0-nightly (f6e511eec 2024-10-15)
binary: rustc
commit-hash: f6e511eec7342f59a25f7c0534f1dbea00d01b14
commit-date: 2024-10-15
host: aarch64-apple-darwin
release: 1.82.0-nightly
LLVM version: 19.1.1

Start with the option hashing in compiler/rustc_session/src/options.rs, the remapped working-directory setup in compiler/rustc_session/src/config.rs, and cache validation in compiler/rustc_incremental/src/persist/load.rs. Reproduce the shared incremental-cache build using changing --remap-path-prefix values, then trace which command-line inputs affect the hash. Done means a no-op remap change no longer prevents valid cache reuse, with regression coverage for the reported scenario.