"type annotations needed" on associated type in trait of unknown type should point to that unknown type

[theemathas]

Code

struct Foo;
impl Foo {
    fn method(&self) {}
}

trait Trait {
    type Assoc;
}
impl Trait for i32 {
    type Assoc = Foo;
}

fn get_assoc<T: Trait>(_: T) -> T::Assoc {
    unimplemented!()
}

fn conjure<T>() -> T {
    unimplemented!()
}

fn main() {
    let arg = conjure();  // This should be an i32
    get_assoc(arg).method();
}

Current output

error[E0282]: type annotations needed
  --> src/main.rs:23:5
   |
23 |     get_assoc(arg).method();
   |     ^^^^^^^^^^^^^^ cannot infer type

For more information about this error, try `rustc --explain E0282`.
error: could not compile `playground` (bin "playground") due to 1 previous error

Desired output

Something that points to `arg` or `conjure`

Rationale and extra context

Someone ran into this diagnostic in https://users.rust-lang.org/t/cannot-infer-type-of-vec-of-mpsc-sender/130579, and assumed that they had to annotate the type returned from get_assoc.

The above code is minimized to be self-contained. The original code which produced this issue was less obvious, since it involved the SliceIndex trait internally used to implement Vec indexing, as follows:

fn conjure<T>() -> T {
    unimplemented!()
}

struct Foo;
impl Foo {
    fn method(&self) {}
}

fn main() {
    let data: Vec<Foo> = vec![];
    let index = conjure();  // This should be a usize
    data[index].method();
}

error[E0282]: type annotations needed
  --> src/main.rs:13:5
   |
13 |     data[index].method();
   |     ^^^^^^^^^^^ cannot infer type

For more information about this error, try `rustc --explain E0282`.
error: could not compile `playground` (bin "playground") due to 1 previous error

Other cases

Rust Version

Reproducible on the playground with 1.89.0-nightly (2025-06-10 1677d46cb128cc8f285d)

Anything else?

No response

[xizheyin]

@rustbot claim

[sww1235]

Thank you not only for the clear explanation, but identifying it as a point of improvement for the compiler and submitting an issue!