Explanation of E0207 is not what actually happens

[yshui]

Unconstrained lifetime parameters are actually allowed:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=e4deca509f64a6d600ae49904e966124

Unlike what's described here:

rust/src/librustc_typeck/error_codes.rs

Lines 1941 to 1950 in d3e2cec

	E0207: r##"
	Any type parameter or lifetime parameter of an `impl` must meet at least one of
	the following criteria:
	- it appears in the _implementing type_ of the impl, e.g. `impl<T> Foo<T>`
	- for a trait impl, it appears in the _implemented trait_, e.g.
	`impl<T> SomeTrait<T> for Foo`
	- it is bound as an associated type, e.g. `impl<T, U> SomeTrait for T
	where T: AnotherTrait<AssocType=U>`

For comparison, unconstrained type parameters do generate error:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=a4d2a865a8a311e0eb46f8ed8583c671

[ayushmishra2005]

@yshui I would like to address this. What should be error message for E0207?

[yshui]

@ayushmishra2005 Maybe just don't mention lifetime parameter?

[ayushmishra2005]

@yshui I think this has been fixed already. Can you please try again?

[yshui]

@ayushmishra2005 I don't think it has been: https://github.com/rust-lang/rust/blob/master/src/librustc_error_codes/error_codes/E0207.md still mentions lifetime parameter.

[ayushmishra2005]

@yshui Sorry My bad. I missed checking this file. Let me fix and raise PR.

[yshui]

@ayushmishra2005 Thanks 👍