Vec<T> implements PartialEq<[T; N]> but the other way round doesn't work

[Ablesius]

Hello,

it seems that there is an oversight (or possibly it's wanted behaviour, but it felt odd to me) in how vectors and arrays can be compared, based on PartialEq implementation.

I tried this code:

fn main() {
    let a = vec![1, 2, 3];
    let b = [1, 2, 3];
    
    assert_eq!(a, b);
    assert_eq!(b, a);
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=3c95d140909d5ac62d769b8f663bcd47

I expected to see this happen: No errors because assert_eq should consider these two equal (or shouldn't it?)

Instead, this happened:

   Compiling playground v0.0.1 (/playground)
error[E0277]: can't compare `[{integer}; 3]` with `Vec<{integer}>`
 --> src/main.rs:6:5
  |
6 |     assert_eq!(b, a);
  |     ^^^^^^^^^^^^^^^^ no implementation for `[{integer}; 3] == Vec<{integer}>`
  |
  = help: the trait `PartialEq<Vec<{integer}>>` is not implemented for `[{integer}; 3]`
  = note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider dereferencing here
 --> /playground/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/macros/mod.rs:40:35
  |
40|                 if !(*left_val == **right_val) {
  |                                   +

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` (bin "playground") due to 1 previous error

Meta

I also checked beta and nightly (on the playground, not on my own machine; please tell me if I should repeat it here), and the results are the same over all three of these.

rustc --version --verbose:

rustc 1.77.2 (25ef9e3d8 2024-04-09)
binary: rustc
commit-hash: 25ef9e3d85d934b27d9dada2f9dd52b1dc63bb04
commit-date: 2024-04-09
host: x86_64-unknown-linux-gnu
release: 1.77.2
LLVM version: 17.0.6

Backtrace

(I did activate RUST_BACKTRACE=1 but the output was the same as before with the other runs)

[Ablesius]

Update:

let slice: &[i32] = &[1, 2, 3];
let vec = vec![1, 2, 3];

assert_eq!(slice, vec);
assert_eq!(vec, slice);

this works, which indicates further that it might just be an oversight that it doesn't work for comparing an array with a vec.