Unexpected "cannot infer type"

[Spxg]

I tried this code:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=609d454db4ecdbe3026330593c586d79

// delete this import to compile success
use serde_json;

fn main() {
    let a = 0usize;
    let b = 0i32;
    assert!(a == b as _);
}

I expected to see this happen: can infer type

Instead, this happened: cannot infer type, but delete use serde_json; will compile success

In fact, if the compilation succeeds at this point, it may be broken later when used with other dependencies. Spxg/sqlite-wasm-rs#58

[moxian]

Minimized:

enum Value {}

// delete this impl to make compilation succeed
impl std::cmp::PartialEq<Value> for usize {
    fn eq(&self, _: &Value) -> bool { todo!() }
}

fn main() {
    let a = 0usize;
    let b = 0i32;
    assert!(a == b as _);
}

error[E0282]: type annotations needed
  --> src\main.rs:11:23
   |
11 |     assert!(a == b as _);
   |                       ^ cannot infer type

For more information about this error, try `rustc --explain E0282`.

Same issue as #89254, although this one does not provide good diagnostic.

@rustbot label: +A-inference +T-compiler +A-diagnostics -needs-triage

[xizheyin]

@rustbot claim