A dependency in one of my projects was constantly being rebuilt on CI when using only sccache and no target folder caching. One part of that issue looked like non-determinism of .rmeta files.

Here is a minimal repro: https://github.com/jprochazk/rustc-rmeta-repro

client depends on bridge, and transitively depends on proc-macro foo. There is also a foo_extra crate in the same workspace, which does not participate in the dependency graph of client. When libfoo_extra is in /target/debug/deps, it produces a different libclient-<hash>.rmeta than when it's missing.

Here is what I believe is happening:

When client is built, it depends on bridge, which depends on foo, but rustc is not given a path to foo, so it must locate it as well. It searches through the deps dir for libfoo but only finds the .so, because it's a proc-macro. It then attempts to scan every library whose name starts with libfoo. In our case, the directory it scans also contains libfoo_extra. rustc opens and decodes it, and discards it after checking the crate name doesn't match foo. As a side effect, it also interns the foo_extra symbol.

Symbol hashes the interner index. Doc links are resolved after all the crates are loaded, so when doc_link_resolutions is populated, the indices are all shifted by one. The serialization of doc_link_resolutions depends on the iteration order of the map, which is now different due to there being an extra symbol in the interner. The resulting .rmeta file is no longer identical to the one built previously.

Meta

Repros on 1.97.1 and latest nightly as of posting

rustc --version --verbose:

rustc 1.97.1 (8bab26f4f 2026-07-14)
binary: rustc
commit-hash: 8bab26f4f68e0e26f0bb7960be334d5b520ea452
commit-date: 2026-07-14
host: x86_64-unknown-linux-gnu
release: 1.97.1
LLVM version: 22.1.6

rustc 1.99.0-nightly (87e5904f5 2026-07-20)
binary: rustc
commit-hash: 87e5904f5eb6398af6b22eac2802c78934260c48
commit-date: 2026-07-20
host: x86_64-unknown-linux-gnu
release: 1.99.0-nightly
LLVM version: 22.1.8

Previously reported as:

• Nondeterminism encountered in metadata for doc_link_resolutions #124357

That was closed because it didn't seem like a bug in rustc