`[missing_const_for_fn]` is triggered when a function uses a `Drop` parameter

[botahamec]

Summary

The lint says that the function below could be made into a const fn. Following this suggestion results in a compiler error, which points at x and says, "destructors cannot be evaluated at compile-time"

Lint Name

[`missing_const_for_fn`]

Reproducer

I tried this code:

#![allow(clippy::all)]
#![allow(dead_code)]
#![warn(clippy::missing_const_for_fn)]

fn foo(x: Box<usize>) -> usize {
    *x + 5
}

I saw this happen:

    Checking playground v0.0.1 (/playground)
warning: this could be a `const fn`
 --> src/lib.rs:5:1
  |
5 | / fn foo(x: Box<usize>) -> usize {
6 | |     *x + 5
7 | | }
  | |_^
  |
note: the lint level is defined here
 --> src/lib.rs:3:9
  |
3 | #![warn(clippy::missing_const_for_fn)]
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#missing_const_for_fn

warning: `playground` (lib) generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 0.36s

I expected to see this happen: No output

Version

rustc 1.64.0-nightly (f8588549c 2022-07-18)
PS C:\Users\epice\Projects\lunars> rustc -Vv      
rustc 1.64.0-nightly (f8588549c 2022-07-18)
binary: rustc
commit-hash: f8588549c3c3d45c32b404210cada01e2a45def3
commit-date: 2022-07-18
host: x86_64-pc-windows-msvc
release: 1.64.0-nightly
LLVM version: 14.0.6

Additional Labels

@rustbot label +I-suggestion-causes-error

[satvikpendem]

I get this often for some basic String conversion methods I have, so I have to do this each time:

#[allow(clippy::missing_const_for_fn)]
pub fn into_inner(self) -> String {
    self.0
}

Looks like this is related to this issue on the rust repository: rust-lang/rust#60964

[JarredAllen]

Are there any plans to fix this false positive? Or are we just waiting for rust-lang/rust#73255 to let these functions be const? I've just hit this issue with something similar to @satvikpendem's example.

[Centri3]

It already has been. #10891

[JarredAllen]

Neat! Looking forward to when it hits stable!