assert_eq(x, y) is not the same as assert_eq(y, x) because of type inferrence

[jyn514]

I tried this code (playground):

fn main() {
    assert_eq!(0_i64, S::zero());
    assert_eq!(S::zero(), 0_i64);
}

struct S;

trait Zeroed<T> {
    fn zero() -> T;
}

impl Zeroed<i16> for S {
    fn zero() -> i16 {
        0
    }
}

impl Zeroed<i64> for S {
    fn zero() -> i64 {
        0
    }
}

I expected to see this happen: The program compiles and runs successfully.

Instead, this happened:

error[E0282]: type annotations needed
 --> src/main.rs:3:16
  |
3 |     assert_eq!(S::zero(), 0_i64);
  |                ^^^^^^^ cannot infer type for type parameter `T` declared on the trait `Zeroed`

Note that the assert_eq!(0_i64, S::zero()); works as expected.

Meta

rustc --version --verbose:

rustc 1.42.0 (b8cedc004 2020-03-09)
binary: rustc
commit-hash: b8cedc00407a4c56a3bda1ed605c6fc166655447
commit-date: 2020-03-09
host: x86_64-unknown-linux-gnu
release: 1.42.0
LLVM version: 9.0

The same error is present on nightly:

rustc 1.44.0-nightly (2fbb07525 2020-03-26)
binary: rustc
commit-hash: 2fbb07525e2f07a815e780a4268b11916248b5a9
commit-date: 2020-03-26
host: x86_64-unknown-linux-gnu
release: 1.44.0-nightly
LLVM version: 9.0

[jyn514]

Not sure the T-lang label is right here, I think this could be fixed in the standard library.

[jonas-schievink]

How so? There's already a PartialEq impl for i64

[contrun]

Here is a related problem

fn main() {
    let xs: Vec<u64> = vec![1, 2, 3];
    assert_eq!(6u64, xs.iter().sum()); // This works
    // assert_eq!(xs.iter().sum(), 6u64); // This does not work.
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=4f26089c7d9d2fc303bcbf38facdf8e3