rustfmt does not discover modules through `cfg_select!`

[mkroening]

In case this issue is a better fit for the rustfmt repo, feel free to move it there. Also, if this is not a good fit for the regression tag, please change it.

Given this lib.rs and corresponding module files:

cfg_match! {
    target_family = "unix" => {
        mod format_me_please_1;
    }
    _ => {
        mod format_me_please_2;
    }
}

When formatting this, everything is fine:

$ rustfmt src/lib.rs --verbose
Formatting src/format_me_please_1.rs
Formatting src/format_me_please_2.rs
Formatting src/lib.rs
Spent 0.004 secs in the parsing phase, and 0.002 secs in the formatting phase

Support for detecting module files through cfg_match! was addded in rust-lang/rustfmt#6522. But cfg_match! was renamed to cfg_select! in #137198, and if you change the code above to say cfg_select!, it no longer detects the module files:

$ rustfmt src/lib.rs --verbose
Formatting src/lib.rs
Spent 0.002 secs in the parsing phase, and 0.001 secs in the formatting phase

So anyone who has used cfg_select! to define modules in other files has possibly unformatted files. This is especially relevant due to the recent stabilization of cfg_select! (#149783) the migration from cfg_if! to cfg_select! in the ecosystem (e.g., rust-lang/compiler-builtins#1224).

#154202 does not appear to fix this issue. Renaming the corresponding occurrences in rustfmt fixes the issue for me, so I have opened #158372.

[tgross35]

Started a Zulip discussion #t-rustfmt > cfg_select module discovery

[rustbot]

Assigning P-medium (discussion on Zulip).