use of undeclared lifetime emits incorrect help message

[lcnr]

In case there is an undeclared lifetime in an ADT deriving Eq a wrong help message is emited

#[derive(Eq, PartialEq)]
struct Test {
    a: &'b str,
}

causes the following error

error[E0261]: use of undeclared lifetime name `'b`
 --> src/lib.rs:3:9
  |
2 | struct Test {
  |            - help: consider introducing lifetime `'b` here: `<'b>`
3 |     a: &'b str,
  |         ^^ undeclared lifetime

error[E0261]: use of undeclared lifetime name `'b`
 --> src/lib.rs:3:9
  |
3 |     a: &'b str,
  |         ^^ undeclared lifetime
  |
help: consider introducing lifetime `'b` here
  |
1 | #[derive(<'b>, PartialEq)]
  |          ^^^^
help: consider introducing lifetime `'b` here
  |
1 | #[derive(<'b>, PartialEq)]
  |          ^^^^

thx, bye

[lcnr]

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=e320116b13b1bc736651defba5f2fe93

[estebank]

There are two things here:

1. the suggestion should try and identify spans coming from macro expansion and not suggest changes
2. #[derive(Eq)] in this case expands to the following:

#[automatically_derived]
#[allow(unused_qualifications)]
impl ::core::cmp::Eq for Test {
    #[inline]
    #[doc(hidden)]
    fn assert_receiver_is_total_eq(&self) -> () {
        { let _: ::core::cmp::AssertParamIsEq<&'b str>; }
    }
}

Note the &'b str param, which could avoid having any lifetimes and thing would work correctly. We can probably change the desugaring to not specify the lifetime and then we would avoid the second unnecessary duplicated error.

@petrochenkov would you happen to know if there's any clear negative in doing 2 above and if not, point me in the right direction in librustc_expand to make that change?

[lcnr]

I believe that we can't remove the explicit lifetime here.
This would allow Eq for &'a T even if Eq is only implemented for &'static T.

use std::marker::PhantomData;

trait A {}

struct Foo;

impl A for &'static Foo {}

struct AssertImpl<T: A>(PhantomData<T>);

pub fn f<'a>(_: &'a ()) {
    let _: AssertImpl::<&Foo>; 
}