redundant-closure-for-method-calls suggests <dyn trait>::fn which does not compile

Summary

.

Lint Name

redundant-closure-for-method-calls

Reproducer

I tried this code:

#![feature(error_generic_member_access, provide_any)]

use std::backtrace::{Backtrace};
use std::error::Error;

const _: fn(&dyn Error) -> Option<&Backtrace> = |err| err.request_ref::<Backtrace>();

pub fn main() {}

I saw this happen:

warning: redundant closure
 --> src/main.rs:6:49
  |
6 | const _: fn(&dyn Error) -> Option<&Backtrace> = |err| err.request_ref::<Backtrace>();
  |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace the closure with the method itself: `<dyn std::error::Error>::request_ref`
  |

Suggested code does not compile

The following errors were reported:
error[E0308]: mismatched types
 --> src/main.rs:6:49
  |
6 | const _: fn(&dyn Error) -> Option<&Backtrace> = <dyn std::error::Error>::request_ref;
  |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
  |
  = note: expected fn pointer `for<'a> fn(&'a (dyn std::error::Error + 'a)) -> std::option::Option<&'a std::backtrace::Backtrace>`
                found fn item `fn(&dyn std::error::Error) -> std::option::Option<&_> {<dyn std::error::Error>::request_ref::<_>}`

Version

rustc 1.69.0-nightly (b1719530f 2023-03-04)
binary: rustc
commit-hash: b1719530f44e3c8ec903f76020a52bd8764d5d10
commit-date: 2023-03-04
host: x86_64-unknown-linux-gnu
release: 1.69.0-nightly
LLVM version: 15.0.7

Additional Labels

No response

Start by running the Rust reproducer in the issue and comparing the redundant-closure-for-method-calls warning with the suggested method expression. Trace where this lint constructs its replacement, then verify the suggested code compiles for the shown higher-ranked function-pointer case. Done means the warning no longer suggests code that fails to compile.