Type inference error when importing unrelated function.

[neuring]

I tried this code:

use serde_json::to_string;

pub fn some_function() {
    assert_eq!(3_u32, (0..3).sum());
}

Playground

I expected this to compile, but a type inference error was emitted.

error[E0283]: type annotations needed
 --> src/lib.rs:4:30
  |
4 |     assert_eq!(3_u32, (0..3).sum());
  |                              ^^^ cannot infer type for type parameter `S` declared on the associated function `sum`
  |
  = note: cannot satisfy `_: Sum<i32>`
help: consider specifying the type argument in the method call
  |
4 |     assert_eq!(3_u32, (0..3).sum::<S>());
  |                                 ^^^^^

If the import of serde_json::to_string is removed the code compiles just fine.

Meta

It fails on all stable, beta and nightly.

[PatchMixolydic]

@rustbot modify labels: +A-inference +T-compiler

[jonas-schievink]

I believe this is expected behavior. It also happens with use serde_json as _;, which just causes the compiler to load the crate. Since that also loads any impls in the crate, they can cause ambiguities.

[neuring]

@jonas-schievink There don't seem to be any impls for Sum in serde_json and neither in serde. The problem seems to be another PartialEq .impl
In searching for those I found serde-rs/json#651 which linked to #46257.
So this issue is a duplicate.