Transmute does not see the size of generic-over-constants types

[jonasmalacofilho]

I tried this code:

pub fn noop<const N: usize>(src: [i32; N]) -> [i32; N] {
    unsafe { std::mem::transmute::<_, _>(src) }
}

I expected a successful compilation.

Instead, this happened:

error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
 --> src/lib.rs:2:14
  |
2 |     unsafe { std::mem::transmute::<_, _>(src) }
  |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `[i32; N]` does not have a fixed size

Meta

rustc --version --verbose:

rustc 1.52.0-nightly (a143517d4 2021-02-16)
binary: rustc
commit-hash: a143517d44cac50b20cbd3a0b579addab40dd399
commit-date: 2021-02-16
host: x86_64-unknown-linux-gnu
release: 1.52.0-nightly
LLVM version: 11.0.1

Note: this also reproduces with the current beta (1.51.0-beta.2) on the Rust Playground [link].

Backtrace

$ RUST_BACKTRACE=1 cargo build
   Compiling transmute_no_fixed_size_with_min_const_generics v0.1.0 (/home/jonas/Code/issues/rust/transmute_no_fixed_size_with_min_const_generics)
error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
 --> src/lib.rs:2:14
  |
2 |     unsafe { std::mem::transmute::<_, _>(src) }
  |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `[i32; N]` does not have a fixed size

error: aborting due to previous error

For more information about this error, try `rustc --explain E0512`.
error: could not compile `transmute_no_fixed_size_with_min_const_generics`

To learn more, run the command again with --verbose.

[the8472]

Have you tried transmute_copy?

[Mark-Simulacrum]

I think this is expected - transmute doesn't work with generics of any kind AFAIK. That said, the error could be better.

I think this is a duplicate of #61956, though. Thanks!

[jonasmalacofilho]

I think this is expected - transmute doesn't work with generics of any kind AFAIK. That said, the error could be better.

Oh, I hadn't come across this yet.

I think this is a duplicate of [...]

I also missed the other issue; apparently I made a typo somewhere when searching for existing issues.

Sorry about for noise, and thanks!

---

Have you tried transmute_copy?

I did and it compiles, since it doesn't check the sizes.

Still, thanks! : )