debug assertion ice: bad spans: -Wunused-lifetimes

[matthiaskrgr]

Code

fn main() {
    let _: extern fn<'a: 'static>();
}

The problem is this first garbage span:

warning: lifetime parameter `'a` never used
 --> recover-fn-ptr-with-generics.rs:2:22
  |
2 |     let _: extern fn<'a: 'static>();
  |            -         ^^
  |            |
  |            help: elide the unused lifetime
  |
  = note: requested on the command line with `-W unused-lifetimes`

Meta

rustc --version --verbose:

eebdfb55fce148676c24555505aebf648123b2de

Error output

<output>

unused_lifetime.txt

[KittyBorgX]

@matthiaskrgr I think I can take this up.
Before I do, just some questions:

• What's the expected output which I can use to verify?
• I can't reproduce the help: elide the unused lifetime part:
  
  and there's no ICE for me.

rustc --version --verbose for me:

rustc 1.69.0-nightly (7aa413d59 2023-02-19)
binary: rustc
commit-hash: 7aa413d59206fd511137728df3d9e0fd377429bd
commit-date: 2023-02-19
host: aarch64-apple-darwin
release: 1.69.0-nightly
LLVM version: 15.0.7

Edit : I'm on the latest nightly

[clubby789]

You need to build with debug assertions, and invoke with -Wunused-lifetimes. I believe the bad span is created around

rust/compiler/rustc_resolve/src/late.rs

Line 739 in e7eaed2

let span = ty.span.shrink_to_lo().to(bare_fn.decl_span.shrink_to_lo());

I tried modifying that span to instead contain the generics, which fixes this case but regresses a few other cases

[layerkugou]

very good

[cyrgani]

@rustbot label S-bug-has-test