Can't `#[bench]` a function named `b`

[theemathas]

#![feature(test)]

extern crate test;

#[bench]
fn b(_: &mut test::bench::Bencher) {}

I should be able to run this as a benchmark, but I got the following error:

error[E0308]: mismatched types
 --> src/lib.rs:6:1
  |
5 | #[bench]
  | -------- in this attribute macro expansion
6 | fn b(_: &mut test::bench::Bencher) {}
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  | |
  | expected `&mut Bencher`, found `TestDescAndFn`
  | expected due to this
  | constant defined here
  | `b` is interpreted as a constant, not a new binding
  | help: introduce a new binding instead: `other_b`

error[E0308]: mismatched types
 --> src/lib.rs:6:1
  |
5 | #[bench]
  | -------- in this attribute macro expansion
6 | fn b(_: &mut test::bench::Bencher) {}
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  | |
  | expected `&mut Bencher`, found `TestDescAndFn`
  | arguments to this function are incorrect
  |
note: function defined here
 --> src/lib.rs:6:4
  |
6 | fn b(_: &mut test::bench::Bencher) {}
  |    ^ ----------------------------

For more information about this error, try `rustc --explain E0308`.

It seems that the macro expansion of #[bench] has a name collision with a variable named b.

rust/compiler/rustc_builtin_macros/src/test.rs

Lines 210 to 218 in 292be5c

	// A simple ident for a lambda
	let b = Ident::from_str_and_span("b", attr_sp);
	cx.expr_call(
	sp,
	cx.expr_path(test_path("StaticBenchFn")),
	thin_vec![
	// #[coverage(off)]
	// |b| self::test::assert_test_result(

Meta

Reproducible on the playground with version 1.93.0-nightly (2025-10-28 278a90913daf77077910) (click on the three dots on top left -> "test")

[IntegralPilot]

Seems like a simple case of changing

let b = Ident::from_str_and_span("b", attr_sp);

to

let b = Ident::from_str_and_span("b", cx.with_def_site_ctxt(attr_sp));

(or similar) in order to fix.

Edit: Actually no, attr_sp is already set as using the definition context at line 165, so this is strange. I'll investigate.

Edit 2: The full expanded form is

#![feature(prelude_import)]
#![feature(test)]
#[macro_use]
extern crate std;
#[prelude_import]
use std::prelude::rust_2024::*;

extern crate test;

extern crate test;
#[rustc_test_marker = "b"]
#[doc(hidden)]
pub const b: test::TestDescAndFn =
    test::TestDescAndFn {
        desc: test::TestDesc {
            name: test::StaticTestName("b"),
            ignore: false,
            ignore_message: ::core::option::Option::None,
            source_file: "blah/src/lib.rs",
            start_line: 6usize,
            start_col: 4usize,
            end_line: 6usize,
            end_col: 5usize,
            compile_fail: false,
            no_run: false,
            should_panic: test::ShouldPanic::No,
            test_type: test::TestType::UnitTest,
        },
        testfn: test::StaticBenchFn(#[coverage(off)] |b|
                test::assert_test_result(b(b))),
    };
fn b(_: &mut test::bench::Bencher) {}
#[rustc_main]
#[coverage(off)]
#[doc(hidden)]
pub fn main() -> () {
    extern crate test;
    test::test_main_static(&[&b])
}

it looks to me like the solution is to change the function call to self::b (and declare the function first) to remove the ambiguity, which I'm trying now.

[theemathas]

Using self::b() won't work if the #[bench] function is declared inside another function.

[IntegralPilot]

You're right, I caught that when testing. I have another idea I'm testing now that's looking like it's working.

Edit: it's working! opened a PR :)

[IntegralPilot]

@rustbot claim