Different `RUSTFLAGS` between `cargo build` and `cargo clippy` invalidate "core" crate when using build-std

[schultetwin1]

Problem

Normally, I pass the flag -W missing_docs to cargo clippy but not cargo build this does not affect the other's build. However, when using -Z build-std, I've noticed that adding -W missing_docs to my cargo clippy command will cause my the cargo build command to re-build the core crate, which in turn re-builds most dependent crates.

Steps

This was reproduced on with the following environment:

rustc --version --verbose
rustc 1.81.0 (eeb90cda1 2024-09-04)
binary: rustc
commit-hash: eeb90cda1969383f56a2637cbd3037bdf598841c
commit-date: 2024-09-04
host: x86_64-unknown-linux-gnu
release: 1.81.0
LLVM version: 18.1.7

1. cargo new --bin buildstdtest
2. cd buildstdtest
3. cargo clippy with -Z build-std and set RUSTFLAGS="-W missing_docs"

   RUSTFLAGS="-W missing_docs" cargo +nightly clippy -Z build-std --target x86_64-unknown-linux-gnu
   

4. cargo build with -Z build-stdand noRUSTFLAGS` set

   cargo +nightly build -Z build-std --target x86_64-unknown-linux-gnu
   

5. Run cargo clippy again. I expect nothing rebuilds since no inputs have changed.

   RUSTFLAGS="-W missing_docs" cargo +nightly clippy -Z build-std --target x86_64-unknown-linux-gnu
   

   However, the core crate re-builds due to a change in rustflags.

   cargo +nightly build -vv -Z build-std --target x86_64-unknown-linux-gnu
      Dirty compiler_builtins v0.1.125: the rustflags changed
   Compiling compiler_builtins v0.1.125
      Dirty core v0.0.0 (/home/matsch/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core): the rustflags changed
   ...
   

Possible Solution(s)

No response

Notes

I enabled tracing on fingerprinting via the following command:

CARGO_LOG=cargo::core::compiler::fingerprint=trace cargo +nightly build -vv -Z build-std --target x86_64-unknown-linux-gnu

The output shows that the core crate did have a change in rustflags.

   0.066914712s  INFO prepare_target{force=false package_id=core v0.0.0 (/home/matsch/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core) target="core"}: cargo::core::compiler::fingerprint:     dirty: RustflagsChanged { old: ["-W", "missing_docs"], new: [] }

So the question to figure out is, why do the build-std crates pick up on the RUSTFLAGS change, but when I remove -Z build-std cargo build and cargo clippy can run side by side without affecting eachother's build fingerprints.

Version

cargo version --verbose
cargo 1.81.0 (2dbb1af80 2024-08-20)
release: 1.81.0
commit-hash: 2dbb1af80a2914475ba76827a312e29cedfa6b2f
commit-date: 2024-08-20
host: x86_64-unknown-linux-gnu
libgit2: 1.8.1 (sys:0.19.0 vendored)
libcurl: 8.8.0-DEV (sys:0.4.73+curl-8.8.0 vendored ssl:OpenSSL/1.1.1w)
ssl: OpenSSL 1.1.1w  11 Sep 2023
os: Debian n/a (rodete) [64-bit]

[weihanglo]

This is probably similar to #9280, and essentially #8716. Doesn't need -Zbuild-std to be involved.