error message for inherent impl on type that is external could be more explanatory

[cyrgani]

Code

use std::rc::Rc;
pub struct Foo;

pub type Function = Rc<Foo>;

impl Function {
    
}

Current output

error[E0116]: cannot define inherent `impl` for a type outside of the crate where the type is defined
 --> src/lib.rs:6:1
  |
6 | impl Function {
  | ^^^^^^^^^^^^^ impl for type defined outside of crate
  |
  = note: define and implement a trait or new type instead

Desired output

error[E0116]: cannot define inherent `impl` for a type outside of the crate where the type is defined
 --> src/lib.rs:6:1
  |
6 | impl Function {
  | ^^^^^^^^^^^^^ impl for type defined outside of crate
  |
  = note: define and implement a trait or new type instead (`pub struct Function(Rc<Foo>);`)
  = note: `type` does not define a new type, only a type alias
  = note: `Function` is actually `Rc<Foo>`, a type not defined in the current crate

Rationale and extra context

To a beginner, it looks as if Function was a new type, so the error message should be a bit more explanatory why it is not.

Other cases

Rust Version

1.89.0-nightly

(2025-05-27 45f256d9d7cffb66185c)

Anything else?

@rustbot label D-terse

[xizheyin]

@rustbot claim