`cfg_select` should check for `unexpected_cfgs` on other arms

[tgross35]

Code:

#![feature(cfg_select)]

fn main() {
    let demo1 = cfg_select! {
        true => "valid",
        invalid_cfg1 => "invalid",
        _ => "fallback"
    };
    let demo2 = cfg_select! {
        invalid_cfg2 => "invalid",
        true => "valid",
        _ => "fallback"
    };
    dbg!(demo1, demo2);
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=170a24120aafa2dcf4d941fd1b189a56

Output:

   Compiling playground v0.0.1 (/playground)
warning: unexpected `cfg` condition name: `invalid_cfg2`
  --> src/main.rs:10:9
   |
10 |         invalid_cfg2 => "invalid",
   |         ^^^^^^^^^^^^
   |
   = help: expected names are: `docsrs`, `feature`, and `test` and 31 more
   = help: consider using a Cargo feature instead
   = help: or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint:
            [lints.rust]
            unexpected_cfgs = { level = "warn", check-cfg = ['cfg(invalid_cfg2)'] }
   = help: or consider adding `println!("cargo::rustc-check-cfg=cfg(invalid_cfg2)");` to the top of the `build.rs`
   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
   = note: `#[warn(unexpected_cfgs)]` on by default

warning: `playground` (bin "playground") generated 1 warning
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.60s
     Running `target/debug/playground`
[src/main.rs:14:5] demo1 = "valid"
[src/main.rs:14:5] demo2 = "valid"

Here we get a warning on invalid_cfg2 since the invalid arm for demo2 comes before the matched arm (true). Ideally we should get the same warning for invalid_cfg1 because it also isn't a valid configuration: however, since true is hit first in the demo2 case, the rest of the arms don't currently get checked.

Version: 1.93.0-nightly (2025-11-18 3d461af)

[JonathanBrouwer]

I think I got really close to accidentally fixing this in #148712
I'll see how easy this is to fix
@rustbot claim

[tgross35]

Cc @rust-lang/lang to make sure this makes sense. I believe it's what users would expect following the approximate mental model of cfg_select!{ pred1 => res1, pred2 => res2 } expanding to #[cfg(pred1)] res1; #[cfg(pred2)] res2;.

[Urgau]

If we think of them as arms, we should examine all the arms and lint on them, regardless of whether they have been evaluated.

We already do lint in a similar situation for configs that are "not evaluated" in predicates: cfg(any(true, invalid_cfg)).