regression: error: expected one of !, ., ;, ?, {, }, or an operator, found ::

[cuviper]

• https://crater-reports.s3.amazonaws.com/beta-1.87-3/beta-2025-04-20/gh/KSXGitHub.assert-cmp/log.txt

[INFO] [stdout] error: expected one of `!`, `.`, `;`, `?`, `{`, `}`, or an operator, found `::`
[INFO] [stdout]  --> test.rs:8:5
[INFO] [stdout]   |
[INFO] [stdout] 8 |     ::assert_cmp::assert_op!(123 < 456);
[INFO] [stdout]   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected one of 7 possible tokens
[INFO] [stdout]   |
[INFO] [stdout]   = note: this error originates in the macro `::assert_cmp::assert_op` (in Nightly builds, run with -Z macro-backtrace for more info)

(and many more of the same)

With the suggested macro-backtrace, that particular instance reports:

error: expected one of `!`, `.`, `;`, `?`, `{`, `}`, or an operator, found `::`
  --> [...]/assert-cmp/lib.rs:48:24
   |
37 | /         macro_rules! $name_simple {
38 | |             ($left:ident $op:tt $right:ident) => {
39 | |                 $module::$name_expr!($left, $op, $right)
40 | |             };
...  |
48 | |                 $module::$name_expr!($left, $op, $right)
   | |                        ^^ expected one of 7 possible tokens
49 | |             };
50 | |         }
   | |_________- in this expansion of `::assert_cmp::assert_op!`
   |
  ::: test.rs:8:5
   |
8  |       ::assert_cmp::assert_op!(123 < 456);
   |       ----------------------------------- in this macro invocation

Version it worked on

It most recently worked on: 1.86.0

Version with regression

rustc 1.87.0-beta.5 (386abeb93 2025-04-19) in crater #139827.

@rustbot modify labels: +regression-from-stable-to-beta -regression-untriaged

[fmease]

Same regression range as #140219

• PASS: rustc 1.87.0-nightly (2874876 2025-02-28)
• FAIL: rustc 1.87.0-nightly (8c39296 2025-03-01)

[fmease]

Therefore it has to be #137517, cc @nnethercote

[mejrs]

MCVE is similar to #140219

Other crate (let's call it some_crate):

macro_rules! make_macro {
    ( $module:path, $function:path) => {
        #[macro_export]
        macro_rules! some_macro {
            () => {
                $module::$function();
            };
        }
    };
}

make_macro!(my_module, my_function);

pub mod my_module{
    pub fn my_function() {}
}

Main crate:

use some_crate::{my_module, some_macro};

fn main() {
    some_macro!();
}

If inlined into a single crate, then it fails on any rust version with the same expected one of 7 possible tokens error.

[nnethercote]

Yes, the crater run analysis for #124141 discusses assert-cmp. In other words, this is expected breakage and any fix should be done on the crate side.

[jieyouxu]

Tagged #137517 w/ compat relnotes (#140241), since I can't seem to find any relnotes entries for it.