transmute_undefined_repr is wrong, transmute does not require a defined layout

[dtolnay]

Summary

The description of transmute_undefined_repr says:

What it does
Checks for transmutes either to or from a type which does not have a defined representation.

Why is this bad?
The results of such a transmute are not defined.

I think this is not correct. Transmute does not require either side to have a defined representation. It only requires that both sides have the same representation, which can be guaranteed without defining what that representation is.

Lint Name

transmute_undefined_repr

Reproducer

#[repr(C)]
struct Wrapper(String);

fn main() {
    let string = String::new();
    let _ = unsafe { std::mem::transmute::<String, Wrapper>(string) };
}

$ cargo clippy

error: transmute from `std::string::String` which has an undefined layout
 --> src/main.rs:6:22
  |
6 |     let _ = unsafe { std::mem::transmute::<String, Wrapper>(string) };
  |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[deny(clippy::transmute_undefined_repr)]` on by default
  = note: the contained type `std::vec::Vec<u8>` has an undefined layout
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#transmute_undefined_repr

Version

rustc 1.60.0-nightly (e789f3a3a 2022-02-11)
binary: rustc
commit-hash: e789f3a3a3d96ebf99b7bbd95011527e5be32a11
commit-date: 2022-02-11
host: x86_64-unknown-linux-gnu
release: 1.60.0-nightly
LLVM version: 13.0.0

Additional Labels

No response

[Jarcho]

Hilariously it works if you remove #[repr(C)]. I can fix it later today.