unreachable code is not detected if it depends on never type fallback

I tried this code:

fn main() {
    (|| panic!())();
    ();
}

I expected to see this happen: (); is marked as unreachable code

Instead, this happened: there is no warning.

This happens because we emit unreachable code lint in rustc_hir_typeck, while checking expressions (we track it via FnCtxt::diverges), which means that we can have unresolved inference variables.

In this case, when checking code reachibility we see that the type of (|| panic!())() is a (diverging) inference variable and not ! so we don't set FnCtxt::diverges to "always". Later this inference variable is set to ! (NB: currently only on edition = 2024), but at this point we don't check for unreachable code.

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rustc --version --verbose:

1.91.0-nightly (2025-08-31 07d246fc6dc227903da2)