missing-const-for-fn: println is not const-(st)able yet

Summary

fn a() {
    let x = 1 else { return }; //~ WARN irrefutable `let...else` pattern

    // Multiline else blocks should not get printed
    let x = 1 else { //~ WARN irrefutable `let...else` pattern
        eprintln!("problem case encountered");
        return
    };
}

this cannot be a const fn yet although clippy suggets it:

error[E0015]: cannot call non-const fn `std::io::_eprint` in constant functions
 --> a.rs:6:9
  |
6 |         eprintln!("problem case encountered");
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
  = note: this error originates in the macro `eprintln` (in Nightly builds, run with -Z macro-backtrace for more info)

error: `std::fmt::Arguments::<'a>::new_const` is not yet stable as a const fn
 --> a.rs:6:9
  |
6 |         eprintln!("problem case encountered");
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: add `#![feature(const_fmt_arguments_new)]` to the crate attributes to enable
  = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `eprintln` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 2 previous errors; 4 warnings emitted

Lint Name

missing-const-for-fn:

Reproducer

I tried this code:

<code>

I saw this happen:

<output>

I expected to see this happen:

Version

rustc 1.81.0-nightly (9c3bc805d 2024-06-27)
binary: rustc
commit-hash: 9c3bc805dd9cb84019c124b9a50fdff1e62a7ec9
commit-date: 2024-06-27
host: x86_64-unknown-linux-gnu
release: 1.81.0-nightly
LLVM version: 18.1.7

Additional Labels

No response

Read the missing-const-for-fn lint implementation and its existing tests, using the shown let-else example with eprintln! as the starting case. Confirm that the lint does not suggest const fn when the body calls non-const printing machinery, and add or update a regression test for the intended diagnostic.