Lint suppressions via RUSTFLAGS don't apply to proc-macro crates

[Veetaha]

I tried this code:

A proc-macro crate:

[package]
edition = "2021"
name = "my-lovely-macro"
version = "0.1.0"

[lib]
proc-macro = true

#[proc_macro]
pub fn my_lovely_macro(stream: proc_macro::TokenStream) -> proc_macro::TokenStream {
    let unused = 1;

    stream
}

And a runtime crate:

[package]
edition = "2021"
name    = "sandbox"
version = "0.1.0"

[dependencies]
my-lovely-macro = { path = "../my-lovely-macro" }

fn main() {}

I invoked the following command:

RUSTFLAGS="--allow unused_variables" cargo +nightly miri run

The following output was generated:

   Compiling my-lovely-macro v0.1.0 (/home/veetaha/sandbox/rust/crates/my-lovely-macro)
warning: unused variable: `unused`
 --> crates/my-lovely-macro/src/lib.rs:3:9
  |
3 |     let unused = 1;
  |         ^^^^^^ help: if this is intentional, prefix it with an underscore: `_unused`
  |
  = note: `#[warn(unused_variables)]` on by default

warning: `my-lovely-macro` (lib) generated 1 warning
   Compiling sandbox v0.1.0 (/home/veetaha/sandbox/rust/crates/sandbox)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.25s
     Running `/home/veetaha/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo-miri runner target/miri/x86_64-unknown-linux-gnu/debug/sandbox`

With --verbose:

 cargo +nightly miri run --verbose
   Compiling my-lovely-macro v0.1.0 (/home/veetaha/sandbox/rust/crates/my-lovely-macro)
     Running `/home/veetaha/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo-miri /home/veetaha/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/miri --crate-name my_lovely_macro --edition=2021 crates/my-lovely-macro/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=133 --crate-type proc-macro --emit=dep-info,link -C prefer-dynamic -C embed-bitcode=no -C debuginfo=2 --check-cfg 'cfg(docsrs)' --check-cfg 'cfg(feature, values())' -C metadata=aae35d6b91939280 -C extra-filename=-aae35d6b91939280 --out-dir /home/veetaha/sandbox/rust/target/miri/debug/deps -C linker=clang -C incremental=/home/veetaha/sandbox/rust/target/miri/debug/incremental -L dependency=/home/veetaha/sandbox/rust/target/miri/debug/deps --extern proc_macro`
warning: unused variable: `unused`
 --> crates/my-lovely-macro/src/lib.rs:3:9
  |
3 |     let unused = 1;
  |         ^^^^^^ help: if this is intentional, prefix it with an underscore: `_unused`
  |
  = note: `#[warn(unused_variables)]` on by default

warning: `my-lovely-macro` (lib) generated 1 warning
   Compiling sandbox v0.1.0 (/home/veetaha/sandbox/rust/crates/sandbox)
     Running `/home/veetaha/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo-miri /home/veetaha/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/miri --crate-name sandbox --edition=2021 crates/sandbox/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=133 --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debuginfo=2 --check-cfg 'cfg(docsrs)' --check-cfg 'cfg(feature, values())' -C metadata=a65f98a81a567a5d -C extra-filename=-a65f98a81a567a5d --out-dir /home/veetaha/sandbox/rust/target/miri/x86_64-unknown-linux-gnu/debug/deps --target x86_64-unknown-linux-gnu -C linker=clang -C incremental=/home/veetaha/sandbox/rust/target/miri/x86_64-unknown-linux-gnu/debug/incremental -L dependency=/home/veetaha/sandbox/rust/target/miri/x86_64-unknown-linux-gnu/debug/deps -L dependency=/home/veetaha/sandbox/rust/target/miri/debug/deps --extern my_lovely_macro=/home/veetaha/sandbox/rust/target/miri/debug/deps/libmy_lovely_macro-aae35d6b91939280.so --allow unused_variables`
     Running `/home/veetaha/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo-miri /home/veetaha/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/miri --crate-name sandbox --edition=2021 crates/sandbox/src/main.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=133 --crate-type bin --emit=dep-info,link -C embed-bitcode=no -C debuginfo=2 --check-cfg 'cfg(docsrs)' --check-cfg 'cfg(feature, values())' -C metadata=5bd7ab0fcb5408a4 -C extra-filename=-5bd7ab0fcb5408a4 --out-dir /home/veetaha/sandbox/rust/target/miri/x86_64-unknown-linux-gnu/debug/deps --target x86_64-unknown-linux-gnu -C linker=clang -C incremental=/home/veetaha/sandbox/rust/target/miri/x86_64-unknown-linux-gnu/debug/incremental -L dependency=/home/veetaha/sandbox/rust/target/miri/x86_64-unknown-linux-gnu/debug/deps -L dependency=/home/veetaha/sandbox/rust/target/miri/debug/deps --extern my_lovely_macro=/home/veetaha/sandbox/rust/target/miri/debug/deps/libmy_lovely_macro-aae35d6b91939280.so --extern sandbox=/home/veetaha/sandbox/rust/target/miri/x86_64-unknown-linux-gnu/debug/deps/libsandbox-a65f98a81a567a5d.rlib --allow unused_variables`
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.23s
     Running `/home/veetaha/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo-miri runner target/miri/x86_64-unknown-linux-gnu/debug/sandbox`

Notice that the compilation of the proc macro didn't receive the --allow unused_variables parameter.

Version

$ cargo +nightly miri --version
miri 0.1.0 (b8c8287a22 2024-11-03)

[RalfJung]

Thanks for the report!

That's a cargo issue, not a Miri issue. Miri always invokes cargo with --target <tuple>, meaning cargo separately builds host crates and target crates. RUSTFLAGS only applies to target crates. So closing as a duplicate of rust-lang/cargo#4423.