FP missing-const-for-fn: `the destructor for this type cannot be evaluated in constant functions

[matthiaskrgr]

Summary

.

Reproducer

I tried this code:

use std::cell::Cell;

const NONE_CELL_STRING: Option<Cell<String>> = None;

fn a() { // clippy suggests this to be const
    let _: &'static Option<Cell<String>> = &NONE_CELL_STRING;
}

I expected to see this happen:

Instead, this happened:

error[E0493]: destructor of `std::option::Option<std::cell::Cell<std::string::String>>` cannot be evaluated at compile-time
 --> a.rs:6:45
  |
6 |     let _: &'static Option<Cell<String>> = &NONE_CELL_STRING;
  |                                             ^^^^^^^^^^^^^^^^ the destructor for this type cannot be evaluated in constant functions
7 | }
  | - value is dropped here

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

[J-ZhengLi]

@rustbot claim

[blyxyas]

Not related to the lint, but I just an idea I had: Wouldn't this type of FP be solved by re-evaluating the expression recursively after the change?

[J-ZhengLi]

Not related to the lint, but I just an idea I had: Wouldn't this type of FP be solved by re-evaluating the expression recursively after the change?

Wouldn't that be the key to all suggestion-causes-error?

[blyxyas]

Yes, it would be the solution. But it's waaaaay faster to do some extra processing before emitting a suggestion than to reevaluate the whole expression (and then evaluate its parent, and its parent's parent, all the way to the root)