`transmute_int_to_char` incorrectly reported in `const` contexts

[SabrinaJewson]

Summary

Neither from_u32 nor from_u32_unchecked are stably const, so this in a const context Clippy should not warn on calling std::mem::transmute::<u32, char>.

Lint Name

transmute_int_to_char

Reproducer

I tried this code:

const fn foo() -> char {
    unsafe { std::mem::transmute(3486) }
}

I saw this happen:

warning: transmute from a `i32` to a `char`
 --> src/lib.rs:2:14
  |
2 |     unsafe { std::mem::transmute(3486) }
  |              ^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::char::from_u32(3486 as u32).unwrap()`
  |
  = note: `#[warn(clippy::transmute_int_to_char)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#transmute_int_to_char

I expected to see this happen: No error.

Version

rustc 1.58.0 (02072b482 2022-01-11)
binary: rustc
commit-hash: 02072b482a8b5357f7fb5e5637444ae30e423c40
commit-date: 2022-01-11
host: x86_64-unknown-linux-gnu
release: 1.58.0
LLVM version: 13.0.0

Additional Labels

@rustbot label +I-suggestion-causes-error

[rossmacarthur]

Got a similar error when transmuting bytes to a &str

const VAR: &str = unsafe { core::mem::transmute::<&[u8], &str>(&[]) };

warning: transmute from a `&[u8]` to a `&str`
 --> src/main.rs:2:5
  |
2 |     core::mem::transmute::<&[u8], &str>(&[])
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::str::from_utf8(&[]).unwrap()`
  |
  = note: `#[warn(clippy::transmute_bytes_to_str)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#transmute_bytes_to_str

It should suggest the const std::str::from_utf8_unchecked