PartialEq implementations are missing for symmetric comparisons between certain collections

[is57primenumber]

While a == b works, b == a often fails to compile for many standard collection combinations (e.g., VecDeque vs Vec, Vec vs Array). This violates the intuitive expectation of symmetry in equality comparisons.

Related issue is #149017

I tried this code:

#![crate_type = "lib"]

use std::{borrow::Cow, collections::VecDeque};

fn test_deq_vec() {
    let mut a = VecDeque::new();
    a.push_back(1);

    let b = vec![1];

    let _ = a == b;
    // compile error
    let _ = b == a;
}

fn test_cowvec_vec() {
    let a = Cow::Owned(vec![1]);
    let b = vec![1];

    let _ = a == b;
    // compile error
    let _ = b == a;
}

fn test_deq_slice() {
    let mut a = VecDeque::new();
    a.push_back(1);
    let b = &[1][..];

    let _ = a == b;
    // compile error
    let _ = b == a;
}

fn test_deq_array() {
    let mut a = VecDeque::new();
    a.push_back(1);
    let b = [1];

    let _ = a == b;
    // compile error
    let _ = b == a;
}

fn test_deq_mutslice() {
    let mut a = VecDeque::new();
    a.push_back(1);
    let b = &mut [1][..];

    let _ = a == b;
    // compile error
    let _ = b == a;
}

fn test_deq_array_ref() {
    let mut a = VecDeque::new();
    a.push_back(1);

    let b = &[1];

    let _ = a == b;
    // compile error
    let _ = b == a;
}

fn test_deq_array_mut_ref() {
    let mut a = VecDeque::new();
    a.push_back(1);

    let b = &mut [1];

    let _ = a == b;
    // compile error
    let _ = b == a;
}

fn test_cow_mutslice() {
    let a = Cow::Owned(vec![1]);
    let b = &mut [1][..];

    let _ = a == b;

    // compile error
    let _ = b == a;
}

I expected that this code successfully compiled

Instead, compile error happened.

Meta

rustc --version --verbose:

rustc 1.95.0-nightly (838709580 2026-02-17)
binary: rustc
commit-hash: 8387095803f21a256a9a772ac1f9b41ed4d5aa0a
commit-date: 2026-02-17
host: x86_64-unknown-linux-gnu
release: 1.95.0-nightly
LLVM version: 22.1.0

Backtrace

error[E0277]: can't compare `Vec<{integer}>` with `VecDeque<{integer}>`
  --> src/main.rs:14:15
   |
14 |     let _ = b == a;
   |               ^^ no implementation for `Vec<{integer}> == VecDeque<{integer}>`
   |
   = help: the trait `PartialEq<VecDeque<{integer}>>` is not implemented for `Vec<{integer}>`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             `Vec<T, A1>` implements `PartialEq<Vec<U, A2>>`
             `Vec<T, A>` implements `PartialEq<&[U; N]>`
             `Vec<T, A>` implements `PartialEq<&[U]>`
             `Vec<T, A>` implements `PartialEq<&mut [U]>`
             `Vec<T, A>` implements `PartialEq<[U; N]>`
             `Vec<T, A>` implements `PartialEq<[U]>`
             `Vec<u8>` implements `PartialEq<ByteStr>`
             `Vec<u8>` implements `PartialEq<ByteString>`

error[E0277]: can't compare `Vec<{integer}>` with `Cow<'_, [{integer}]>`
  --> src/main.rs:23:15
   |
23 |     let _ = b == a;
   |               ^^ no implementation for `Vec<{integer}> == Cow<'_, [{integer}]>`
   |
   = help: the trait `PartialEq<Cow<'_, [{integer}]>>` is not implemented for `Vec<{integer}>`
help: consider dereferencing both sides of the expression
   |
23 |     let _ = *b == *a;
   |             +     +

error[E0277]: can't compare `&[{integer}]` with `VecDeque<{integer}>`
  --> src/main.rs:33:15
   |
33 |     let _ = b == a;
   |               ^^ no implementation for `&[{integer}] == VecDeque<{integer}>`
   |
   = help: the trait `PartialEq<VecDeque<{integer}>>` is not implemented for `&[{integer}]`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             `&[T]` implements `PartialEq<Vec<U, A>>`
             `&[T]` implements `PartialEq<[U; N]>`
             `&[u8; N]` implements `PartialEq<ByteStr>`
             `&[u8; N]` implements `PartialEq<ByteString>`
             `&[u8]` implements `PartialEq<ByteStr>`
             `&[u8]` implements `PartialEq<ByteString>`
             `&mut [T]` implements `PartialEq<Vec<U, A>>`
             `&mut [T]` implements `PartialEq<[U; N]>`
           and 11 others

error[E0277]: can't compare `[{integer}; 1]` with `VecDeque<{integer}>`
  --> src/main.rs:43:15
   |
43 |     let _ = b == a;
   |               ^^ no implementation for `[{integer}; 1] == VecDeque<{integer}>`
   |
   = help: the trait `PartialEq<VecDeque<{integer}>>` is not implemented for `[{integer}; 1]`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             `&[T]` implements `PartialEq<Vec<U, A>>`
             `&[T]` implements `PartialEq<[U; N]>`
             `&[u8; N]` implements `PartialEq<ByteStr>`
             `&[u8; N]` implements `PartialEq<ByteString>`
             `&[u8]` implements `PartialEq<ByteStr>`
             `&[u8]` implements `PartialEq<ByteString>`
             `&mut [T]` implements `PartialEq<Vec<U, A>>`
             `&mut [T]` implements `PartialEq<[U; N]>`
           and 11 others

error[E0277]: can't compare `&mut [{integer}]` with `VecDeque<{integer}>`
  --> src/main.rs:53:15
   |
53 |     let _ = b == a;
   |               ^^ no implementation for `&mut [{integer}] == VecDeque<{integer}>`
   |
   = help: the trait `PartialEq<VecDeque<{integer}>>` is not implemented for `&mut [{integer}]`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             `&[T]` implements `PartialEq<Vec<U, A>>`
             `&[T]` implements `PartialEq<[U; N]>`
             `&[u8; N]` implements `PartialEq<ByteStr>`
             `&[u8; N]` implements `PartialEq<ByteString>`
             `&[u8]` implements `PartialEq<ByteStr>`
             `&[u8]` implements `PartialEq<ByteString>`
             `&mut [T]` implements `PartialEq<Vec<U, A>>`
             `&mut [T]` implements `PartialEq<[U; N]>`
           and 11 others

error[E0277]: can't compare `&[{integer}; 1]` with `VecDeque<{integer}>`
  --> src/main.rs:64:15
   |
64 |     let _ = b == a;
   |               ^^ no implementation for `&[{integer}; 1] == VecDeque<{integer}>`
   |
   = help: the trait `PartialEq<VecDeque<{integer}>>` is not implemented for `&[{integer}; 1]`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             `&[T]` implements `PartialEq<Vec<U, A>>`
             `&[T]` implements `PartialEq<[U; N]>`
             `&[u8; N]` implements `PartialEq<ByteStr>`
             `&[u8; N]` implements `PartialEq<ByteString>`
             `&[u8]` implements `PartialEq<ByteStr>`
             `&[u8]` implements `PartialEq<ByteString>`
             `&mut [T]` implements `PartialEq<Vec<U, A>>`
             `&mut [T]` implements `PartialEq<[U; N]>`
           and 11 others

error[E0277]: can't compare `&mut [{integer}; 1]` with `VecDeque<{integer}>`
  --> src/main.rs:75:15
   |
75 |     let _ = b == a;
   |               ^^ no implementation for `&mut [{integer}; 1] == VecDeque<{integer}>`
   |
   = help: the trait `PartialEq<VecDeque<{integer}>>` is not implemented for `&mut [{integer}; 1]`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             `&[T]` implements `PartialEq<Vec<U, A>>`
             `&[T]` implements `PartialEq<[U; N]>`
             `&[u8; N]` implements `PartialEq<ByteStr>`
             `&[u8; N]` implements `PartialEq<ByteString>`
             `&[u8]` implements `PartialEq<ByteStr>`
             `&[u8]` implements `PartialEq<ByteString>`
             `&mut [T]` implements `PartialEq<Vec<U, A>>`
             `&mut [T]` implements `PartialEq<[U; N]>`
           and 11 others

error[E0277]: can't compare `&mut [{integer}]` with `Cow<'_, [{integer}]>`
  --> src/main.rs:85:15
   |
85 |     let _ = b == a;
   |               ^^ no implementation for `&mut [{integer}] == Cow<'_, [{integer}]>`
   |
   = help: the trait `PartialEq<Cow<'_, [{integer}]>>` is not implemented for `&mut [{integer}]`
help: consider dereferencing both sides of the expression
   |
85 |     let _ = *b == *a;
   |             +     +

For more information about this error, try `rustc --explain E0277`.
error: could not compile `rustdoc_parse` (lib) due to 8 previous errors
dai@desktopwin11:~/vscode/rustdoc_parse$ RUST_BACKTRACE=1 cargo build
   Compiling rustdoc_parse v0.1.0 (/home/dai/vscode/rustdoc_parse)
error[E0277]: can't compare `Vec<{integer}>` with `VecDeque<{integer}>`
  --> src/lib.rs:13:15
   |
13 |     let _ = b == a;
   |               ^^ no implementation for `Vec<{integer}> == VecDeque<{integer}>`
   |
   = help: the trait `PartialEq<VecDeque<{integer}>>` is not implemented for `Vec<{integer}>`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             `Vec<T, A1>` implements `PartialEq<Vec<U, A2>>`
             `Vec<T, A>` implements `PartialEq<&[U; N]>`
             `Vec<T, A>` implements `PartialEq<&[U]>`
             `Vec<T, A>` implements `PartialEq<&mut [U]>`
             `Vec<T, A>` implements `PartialEq<[U; N]>`
             `Vec<T, A>` implements `PartialEq<[U]>`
             `Vec<u8>` implements `PartialEq<ByteStr>`
             `Vec<u8>` implements `PartialEq<ByteString>`

error[E0277]: can't compare `Vec<{integer}>` with `Cow<'_, [{integer}]>`
  --> src/lib.rs:22:15
   |
22 |     let _ = b == a;
   |               ^^ no implementation for `Vec<{integer}> == Cow<'_, [{integer}]>`
   |
   = help: the trait `PartialEq<Cow<'_, [{integer}]>>` is not implemented for `Vec<{integer}>`
help: consider dereferencing both sides of the expression
   |
22 |     let _ = *b == *a;
   |             +     +

error[E0277]: can't compare `&[{integer}]` with `VecDeque<{integer}>`
  --> src/lib.rs:32:15
   |
32 |     let _ = b == a;
   |               ^^ no implementation for `&[{integer}] == VecDeque<{integer}>`
   |
   = help: the trait `PartialEq<VecDeque<{integer}>>` is not implemented for `&[{integer}]`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             `&[T]` implements `PartialEq<Vec<U, A>>`
             `&[T]` implements `PartialEq<[U; N]>`
             `&[u8; N]` implements `PartialEq<ByteStr>`
             `&[u8; N]` implements `PartialEq<ByteString>`
             `&[u8]` implements `PartialEq<ByteStr>`
             `&[u8]` implements `PartialEq<ByteString>`
             `&mut [T]` implements `PartialEq<Vec<U, A>>`
             `&mut [T]` implements `PartialEq<[U; N]>`
           and 11 others

error[E0277]: can't compare `[{integer}; 1]` with `VecDeque<{integer}>`
  --> src/lib.rs:42:15
   |
42 |     let _ = b == a;
   |               ^^ no implementation for `[{integer}; 1] == VecDeque<{integer}>`
   |
   = help: the trait `PartialEq<VecDeque<{integer}>>` is not implemented for `[{integer}; 1]`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             `&[T]` implements `PartialEq<Vec<U, A>>`
             `&[T]` implements `PartialEq<[U; N]>`
             `&[u8; N]` implements `PartialEq<ByteStr>`
             `&[u8; N]` implements `PartialEq<ByteString>`
             `&[u8]` implements `PartialEq<ByteStr>`
             `&[u8]` implements `PartialEq<ByteString>`
             `&mut [T]` implements `PartialEq<Vec<U, A>>`
             `&mut [T]` implements `PartialEq<[U; N]>`
           and 11 others

error[E0277]: can't compare `&mut [{integer}]` with `VecDeque<{integer}>`
  --> src/lib.rs:52:15
   |
52 |     let _ = b == a;
   |               ^^ no implementation for `&mut [{integer}] == VecDeque<{integer}>`
   |
   = help: the trait `PartialEq<VecDeque<{integer}>>` is not implemented for `&mut [{integer}]`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             `&[T]` implements `PartialEq<Vec<U, A>>`
             `&[T]` implements `PartialEq<[U; N]>`
             `&[u8; N]` implements `PartialEq<ByteStr>`
             `&[u8; N]` implements `PartialEq<ByteString>`
             `&[u8]` implements `PartialEq<ByteStr>`
             `&[u8]` implements `PartialEq<ByteString>`
             `&mut [T]` implements `PartialEq<Vec<U, A>>`
             `&mut [T]` implements `PartialEq<[U; N]>`
           and 11 others

error[E0277]: can't compare `&[{integer}; 1]` with `VecDeque<{integer}>`
  --> src/lib.rs:63:15
   |
63 |     let _ = b == a;
   |               ^^ no implementation for `&[{integer}; 1] == VecDeque<{integer}>`
   |
   = help: the trait `PartialEq<VecDeque<{integer}>>` is not implemented for `&[{integer}; 1]`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             `&[T]` implements `PartialEq<Vec<U, A>>`
             `&[T]` implements `PartialEq<[U; N]>`
             `&[u8; N]` implements `PartialEq<ByteStr>`
             `&[u8; N]` implements `PartialEq<ByteString>`
             `&[u8]` implements `PartialEq<ByteStr>`
             `&[u8]` implements `PartialEq<ByteString>`
             `&mut [T]` implements `PartialEq<Vec<U, A>>`
             `&mut [T]` implements `PartialEq<[U; N]>`
           and 11 others

error[E0277]: can't compare `&mut [{integer}; 1]` with `VecDeque<{integer}>`
  --> src/lib.rs:74:15
   |
74 |     let _ = b == a;
   |               ^^ no implementation for `&mut [{integer}; 1] == VecDeque<{integer}>`
   |
   = help: the trait `PartialEq<VecDeque<{integer}>>` is not implemented for `&mut [{integer}; 1]`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             `&[T]` implements `PartialEq<Vec<U, A>>`
             `&[T]` implements `PartialEq<[U; N]>`
             `&[u8; N]` implements `PartialEq<ByteStr>`
             `&[u8; N]` implements `PartialEq<ByteString>`
             `&[u8]` implements `PartialEq<ByteStr>`
             `&[u8]` implements `PartialEq<ByteString>`
             `&mut [T]` implements `PartialEq<Vec<U, A>>`
             `&mut [T]` implements `PartialEq<[U; N]>`
           and 11 others

error[E0277]: can't compare `&mut [{integer}]` with `Cow<'_, [{integer}]>`
  --> src/lib.rs:84:15
   |
84 |     let _ = b == a;
   |               ^^ no implementation for `&mut [{integer}] == Cow<'_, [{integer}]>`
   |
   = help: the trait `PartialEq<Cow<'_, [{integer}]>>` is not implemented for `&mut [{integer}]`
help: consider dereferencing both sides of the expression
   |
84 |     let _ = *b == *a;
   |             +     +

For more information about this error, try `rustc --explain E0277`.
error: could not compile `rustdoc_parse` (lib) due to 8 previous errors

[eggyal]

@rustbot label +A-collections -C-bug +C-enhancement +E-easy +T-libs-api -needs-triage

[Vrajs16]

@rustbot claim

[lms0806]

How is this issue progressing?

If possible, I'd like to resolve it.

[eggyal]

#152972 is awaiting review.

[is57primenumber]

I think the PR doesn't cover Cow type cases.