`large_stack_frames`: unsilenceable diagnostic about compiler-generated functions

[LebedevRI]

Summary

I'm guessing it is also running on the automatic main() function for test,
which is good, but i would expect this to be silenceable
with #[allow(clippy::large_stack_frames)] on the mod test.

Lint Name

large_stack_frames

Reproducer

I tried this code:

Cargo.toml:

[package]
name = "repro"
version = "0.1.0"
edition = "2024"

[lints.clippy]
large_stack_frames = { level = "warn", priority = 0 }

[dependencies]

clippy.toml:

stack-size-threshold = 0

main.rs:

#[allow(clippy::large_stack_frames)]
fn main() {
    println!("Hello, world!");
}

#[cfg(test)]
#[allow(clippy::large_stack_frames)]
mod test {
    #[test]
    fn main_test() {}
}

I saw this happen:

$ cargo clippy --all-targets --all-features 
warning: this function may allocate 216 bytes on the stack
   |
   = note: 216 bytes is larger than Clippy's configured `stack-size-threshold` of 0
   = note: allocating large amounts of stack space can overflow the stack and cause the program to abort
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#large_stack_frames
   = note: requested on the command line with `-W clippy::large-stack-frames`

warning: `repro` (bin "repro" test) generated 1 warning
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.00s

What is "this" function?

I expected to see this happen:
No diagnostic should be outputted.

Version

rustc 1.87.0 (17067e9ac 2025-05-09)
binary: rustc
commit-hash: 17067e9ac6d7ecb70e50f92c1944e545188d2359
commit-date: 2025-05-09
host: x86_64-unknown-linux-gnu
release: 1.87.0
LLVM version: 20.1.1

Additional Labels

No response

[y21]

@LebedevRI Out of curiosity, does this compiler-generated main function that's getting warned have a much larger reported size in your actual project? I assume you're not actually running clippy with stack-size-threshold = 0 (or 216).

I'd think we could just ignore these functions entirely (and this is also what #15101 does) given that you can't allow lints there and they probably don't ever use enough stack space that it would become a problem in practice, so I'm surprised to see that you actually had to work around this in a real project.

[LebedevRI]

@LebedevRI Out of curiosity, does this compiler-generated main function that's getting warned have a much larger reported size in your actual project? I assume you're not actually running clippy with stack-size-threshold = 0 (or 216).

I've wanted to have stack-size-threshold at 4096, which should be fine for the actual code,
but tests consistently blow past that to ~16k+. I'd ideally want to have different configs for code
and more lax config for tests etc, but i'm not sure that's possible.
(No, there is no practical reason for a limit that low, at least for me.)

I'd think we could just ignore these functions entirely (and this is also what #15101 does) given that you can't allow lints there and they probably don't ever use enough stack space that it would become a problem in practice, so I'm surprised to see that you actually had to work around this in a real project.

I tend to always hit all the weird edge-cases.