regression: cfg_match resolution ambiguity

[Mark-Simulacrum]

In:

• https://crater-reports.s3.amazonaws.com/beta-1.74-4/beta-2023-10-21/reg/maybe-dangling-0.1.0/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.74-4/beta-2023-10-21/reg/cfg-match-0.2.1/log.txt

[INFO] [stdout] error[E0658]: use of unstable library feature 'cfg_match'
[INFO] [stdout]   --> src/maybe_dangling.rs:65:1
[INFO] [stdout]    |
[INFO] [stdout] 65 | crate::cfg_match! {
[INFO] [stdout]    | ^^^^^^^^^^^^^^^^
[INFO] [stdout]    |
[INFO] [stdout]    = note: see issue #115585 <https://github.com/rust-lang/rust/issues/115585> for more information
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: no rules expected the token `feature`
[INFO] [stdout]   --> src/maybe_dangling.rs:66:5
[INFO] [stdout]    |
[INFO] [stdout] 66 |     feature = "nightly-dropck_eyepatch" => {
[INFO] [stdout]    |     ^^^^^^^ no rules expected this token in macro call
[INFO] [stdout]    |
[INFO] [stdout] note: while trying to match `cfg`
[INFO] [stdout]   --> /rustc/489647f984b2b3a5ee6b2a0d46a527c8d926ceae/library/core/src/macros/mod.rs:358:11
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error[E0659]: `cfg_match` is ambiguous
[INFO] [stdout]   --> src/lib.rs:42:5
[INFO] [stdout]    |
[INFO] [stdout] 42 | use cfg_match;
[INFO] [stdout]    |     ^^^^^^^^^ ambiguous name
[INFO] [stdout]    |
[INFO] [stdout]    = note: ambiguous because of a conflict between a macro-expanded name and a less macro-expanded name from outer scope during import or macro resolution
[INFO] [stdout]    = note: `cfg_match` could refer to a macro from prelude

[Noratrieb]

@crlf0710

[Noratrieb]

mean to ping @c410-f3r instead 😅

[c410-f3r]

The macro was originally named match_cfg to avoid this situation (#115416).

I don't know if it is possible to solve this issue in the compiler so I will just revert the changes.

[Noratrieb]

cc @petrochenkov, maybe you have ideas

[Mark-Simulacrum]

This may just be acceptable breakage - I don't know that we need to revert things. I would wait on libs-api weighing in, impact is pretty minimal in crater.

[Noratrieb]

the problem is that this adds it to the prelude. maybe there's a way to add it somewhere where it doesn't get added to the prelude? for example by making it a macro 2.0. @c410-f3r

[c410-f3r]

Looks like a good short-term solution. Preferably, it would be nice to have prelude support with a different name or an acceptable breakage.

[petrochenkov]

cc @petrochenkov, maybe you have ideas

#82775 (comment)

[danielhenrymantilla]

FWIW, I can take care of fixing maybe_dangling too, if that makes things simpler. I don't know about the eponymous ::cfg_match crate, however.

• The funny thing is that I am working around the current issue by renaming cfg_match as match_cfg 🙃, so if this were to become the decision here please let me know so that I pick some third name 😄