Cannot infer type for [], but only when serde_json is used (?)

[m-ou-se]

extern crate serde_json; // Code works fine if you remove this line.

fn main() {
    Vec::<u8>::new() == [];
}

That produces:

error[E0282]: type annotations needed
 --> src/main.rs:4:25
  |
4 |     Vec::<u8>::new() == [];
  |                         ^^ cannot infer type

Somehow, referencing serde_json makes rustc unable to infer some type here. This happens on both the current stable and nightly versions of Rust.

[m-ou-se]

Looks like the problem is impl PartialEq<Value> for u8 in serde_json. Without that, impl PartialEq<u8> for u8 is the only PartialEq impl for u8.

So I suppose this is expected, although very confusing.

Here's a reproduction without a dependency:

struct Shark;

impl PartialEq<Shark> for u8 {
    fn eq(&self, other: &Shark) -> bool {
        false
    }
}

fn main() {
    Vec::<u8>::new() == [];
}

Labeling this as a diagnostics issue.

[jhpratt]

Of course having better diagnostics here is best for the immediate future, but has it ever been considered to have a "preferred" implementation where Self is the generic type? I can't recall the exact situation off-hand, but I know I've run into inference issues where that would have solved it in the past.

[estebank]

For the general diagnostic improvement, we should be pointing at all the impls that apply (and also that info when suggesting the turbofish, while we are at it). Having a way to nudge inference towards a certain type is something I'd like, but it feels like it'd have similar problems as specialization.

[estebank]

Current output:

error[E0282]: type annotations needed
  --> f71.rs:10:25
   |
10 |     Vec::<u8>::new() == [];
   |                         ^^ cannot infer type

error[E0283]: type annotations needed
  --> f71.rs:10:22
   |
10 |     Vec::<u8>::new() == [];
   |                      ^^ cannot infer type for type parameter `U`
   |
note: multiple `impl`s satisfying `u8: PartialEq<_>` found
  --> f71.rs:3:1
   |
3  | impl PartialEq<Shark> for u8 {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = note: and another `impl` found in the `core` crate: `impl PartialEq for u8;`
   = note: required for `Vec<u8>` to implement `PartialEq<[_; 0]>`

I think the diagnostics part of this is handled relatively ok.

Tagging t-lang for the "priority impl" feature suggestion.