regression: error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth

[cuviper]

Four crater results failed with similar error messages:

• https://crater-reports.s3.amazonaws.com/beta-1.87-3/beta-2025-04-20/reg/math_adapter-0.3.8/log.txt

[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]    --> rust/test/math/helper_test.rs:103:10
[INFO] [stdout]     |
[INFO] [stdout] 103 |         $( $Arg : tt )*
[INFO] [stdout]     |          ^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]   --> rust/test/math/macro_tools_test.rs:13:10
[INFO] [stdout]    |
[INFO] [stdout] 13 |         $( $Arg : tt )*
[INFO] [stdout]    |          ^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]    --> rust/test/math/macro_tools_test.rs:589:10
[INFO] [stdout]     |
[INFO] [stdout] 589 |         $( $Arg : tt )*
[INFO] [stdout]     |          ^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]    --> rust/test/math/macro_tools_test.rs:846:10
[INFO] [stdout]     |
[INFO] [stdout] 846 |         $( $Arg : tt )*
[INFO] [stdout]     |          ^^^^^^^^^^^^^

• https://crater-reports.s3.amazonaws.com/beta-1.87-3/beta-2025-04-20/reg/meta_tools_min-0.2.13/log.txt

[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]   --> rust/test/meta/meta/for_each_test.rs:15:10
[INFO] [stdout]    |
[INFO] [stdout] 15 |         $( $Arg : tt )*
[INFO] [stdout]    |          ^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]    --> rust/test/meta/meta/for_each_test.rs:593:10
[INFO] [stdout]     |
[INFO] [stdout] 593 |         $( $Arg : tt )*
[INFO] [stdout]     |          ^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]    --> rust/test/meta/meta/for_each_test.rs:851:10
[INFO] [stdout]     |
[INFO] [stdout] 851 |         $( $Arg : tt )*
[INFO] [stdout]     |          ^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]    --> rust/test/meta/./impls_index/impls1_test.rs:100:16
[INFO] [stdout]     |
[INFO] [stdout] 100 |             ( $( $Arg : tt )* ) => { };
[INFO] [stdout]     |                ^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]    --> rust/test/meta/./impls_index/impls2_test.rs:100:16
[INFO] [stdout]     |
[INFO] [stdout] 100 |             ( $( $Arg : tt )* ) => { };
[INFO] [stdout]     |                ^^^^^^^^^^^^^

• https://crater-reports.s3.amazonaws.com/beta-1.87-3/beta-2025-04-20/reg/non_std-0.1.4/log.txt

[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]    --> rust/test/dt/type_constructor/single_test.rs:191:10
[INFO] [stdout]     |
[INFO] [stdout] 191 |         $( $Rest : tt )*
[INFO] [stdout]     |          ^^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]    --> rust/test/dt/type_constructor/single_test.rs:278:10
[INFO] [stdout]     |
[INFO] [stdout] 278 |         $( $Rest : tt )*
[INFO] [stdout]     |          ^^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]    --> rust/test/dt/type_constructor/single_test.rs:380:10
[INFO] [stdout]     |
[INFO] [stdout] 380 |         $( $Rest : tt )*
[INFO] [stdout]     |          ^^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]    --> rust/test/dt/type_constructor/pair_test.rs:187:10
[INFO] [stdout]     |
[INFO] [stdout] 187 |         $( $Rest : tt )*
[INFO] [stdout]     |          ^^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]    --> rust/test/dt/type_constructor/homo_pair_test.rs:170:10
[INFO] [stdout]     |
[INFO] [stdout] 170 |         $( $Rest : tt )*
[INFO] [stdout]     |          ^^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]    --> rust/test/dt/type_constructor/homo_pair_test.rs:380:10
[INFO] [stdout]     |
[INFO] [stdout] 380 |         $( $Rest : tt )*
[INFO] [stdout]     |          ^^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]    --> rust/test/dt/type_constructor/homo_pair_test.rs:556:10
[INFO] [stdout]     |
[INFO] [stdout] 556 |         $( $Rest : tt )*
[INFO] [stdout]     |          ^^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]   --> rust/test/meta/./meta/for_each_test.rs:17:10
[INFO] [stdout]    |
[INFO] [stdout] 17 |         $( $Arg : tt )*
[INFO] [stdout]    |          ^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]    --> rust/test/meta/./impls_index/impls1_test.rs:100:16
[INFO] [stdout]     |
[INFO] [stdout] 100 |             ( $( $Arg : tt )* ) => { };
[INFO] [stdout]     |                ^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]    --> rust/test/meta/./impls_index/impls2_test.rs:100:16
[INFO] [stdout]     |
[INFO] [stdout] 100 |             ( $( $Arg : tt )* ) => { };
[INFO] [stdout]     |                ^^^^^^^^^^^^^

• https://crater-reports.s3.amazonaws.com/beta-1.87-3/beta-2025-04-20/reg/type_constructor-0.3.0/log.txt

[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]    --> tests/inc/single/single_parametrized_test.rs:189:10
[INFO] [stdout]     |
[INFO] [stdout] 189 |         $( $Rest : tt )*
[INFO] [stdout]     |          ^^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]    --> tests/inc/single/single_parametrized_test.rs:276:10
[INFO] [stdout]     |
[INFO] [stdout] 276 |         $( $Rest : tt )*
[INFO] [stdout]     |          ^^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]   --> tests/inc/pair/./pair_parametrized_main_test_only.rs:13:10
[INFO] [stdout]    |
[INFO] [stdout] 13 |         $( $Rest : tt )*
[INFO] [stdout]    |          ^^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]   --> tests/inc/pair/./homo_pair_parametrized_main_test_only.rs:20:10
[INFO] [stdout]    |
[INFO] [stdout] 20 |         $( $Rest : tt )*
[INFO] [stdout]    |          ^^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
[INFO] [stdout]    --> tests/inc/many/many_parameter_test.rs:125:10
[INFO] [stdout]     |
[INFO] [stdout] 125 |         $( $Rest : tt )*
[INFO] [stdout]     |          ^^^^^^^^^^^^^^

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

[jieyouxu]

searched nightlies: from nightly-2025-02-15 to nightly-2025-04-24
regressed nightly: nightly-2025-03-13
searched commit range: 6650252...249cb84
regressed commit: aaa2d47

bisected with cargo-bisect-rustc v0.6.9

Host triple: aarch64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --regress=error -- test -- --skip=macro_tools_test::for_each

[jieyouxu]

For me locally math_adapter@0.3.8 already doesn't pass all tests on stable (cloned Wandalen/wTools@9ae6281) because...

thread 'macro_tools_test::for_each' panicked at module/rust/math_adapter/rust/test/math/adapter/macro_tools_test.rs:918:1:
assertion failed: `(left == right)`

Diff < left / right > :
<std::collections::HashMap+std::collections::BTreeMap+
>std :: collections :: HashMap+std :: collections :: BTreeMap+

[jieyouxu]

Bisects to #138083 cc @nnethercote 🤔 Should we also include compat relnotes for #138083?

Initially I thought it was also #137517, but this instead bisects to #138083.

[jieyouxu]

EDIT: ah, I see, this is discussed in crater run analysis for #124141. Might be considered "accepted breakage" but Nicholas might want to double-check.

[moxian]

from the hackmd in the above comment, the mcve looks like:

// Produces a macro `produce_item` that, when called, produces `$item`.
macro_rules! tests_impls {
    ($item: item) => {
        macro_rules! produce_item {
            () => { $item };
        }
    };
}

// Produce a macro `produce_item` that, when called, produces a macro `_m`.
tests_impls! {
    macro_rules! _m {
        ($( $arg:tt )*) => { $( $arg )* };
    }
}

produce_item!();

@rustbot label: -E-needs-mcve +S-has-mcve

[apiraino]

Assigning priority (discussion on Zulip), until we don't have a feedback about this being an accepted breaking change.

@rustbot label -I-prioritize +P-high

[wesleywiser]

Closing as intended change in behavior per @jieyouxu's comment. Relnote is in #140241.