Error uses parameter name out of context: cannot infer type for `U`

[kornelski]

The message is technically correct, but it refers to a generic name U that is not present in user's source code, and has no meaning in the current context.

fn main() {
    assert_eq!("bar".into(), "foo".into());
}

error[E0282]: unable to infer enough type information about U
cannot infer type for U

It's even weirder that the type U is not in Into<T>'s definition, but comes from somewhere else inside stdlib.

pub trait Into<T> {
    fn into(self) -> T;
}

It would be clearer if rustc has shown Into<U> instead, or perhaps the stdlib implementation could rename the cryptic U to something self-explanatory like TypeToBeConvertedInto.

[tatref]

I think this can be troublesome for beginners, since this can pop up anywhere. Other example:

fn main() {
   use std::collections::HashMap;
   
   let h = HashMap::new();
}

Error message using nightly:

error[E0282]: type annotations needed
 --> <anon>:4:12
  |
4 |    let h = HashMap::new();
  |        -   ^^^^^^^^^^^^ cannot infer type for `K`
  |        |
  |        consider giving `h` a type

[Mark-Simulacrum]

I'm going to close this in favor of #25633.