Trait resolution with index operator and mixed lifetimes

[Jarcho]

As far as I can tell the error generated by this is wrong.

fn index_op<'a>(map: &std::collections::HashMap<&'static str, ()>, key: &'a str) {
    map[&key];
}

error[E0759]: `key` has lifetime `'a` but it needs to satisfy a `'static` lifetime requirement
 --> src/main.rs:2:9
  |
1 | fn index_op<'a>(map: &std::collections::HashMap<&'static str, ()>, key: &'a str) {
  |                                                                         ------- this data with lifetime `'a`...
2 |     map[&key];
  |         ^^^^ ...is captured here, requiring it to live as long as `'static`

If the lifetime comes from an impl block, a completely different error message is given.

trait Foo<'a> {
    type Key;
    fn index_op(map: &std::collections::HashMap<&'static str, ()>, key: Self::Key);
}
impl<'a> Foo<'a> for () {
    type Key = &'a str;
    fn index_op(map: &std::collections::HashMap<&'static str, ()>, key: Self::Key) {
        map[&key];
    }
}

error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'a` due to conflicting requirements
 --> src/main.rs:7:84
  |
7 |       fn index_op(map: &std::collections::HashMap<&'static str, ()>, key: Self::Key) {
  |  ____________________________________________________________________________________^
8 | |         map[&key];
9 | |     }
  | |_____^
  |
note: first, the lifetime cannot outlive the lifetime `'a` as defined here...
 --> src/main.rs:5:6
  |
5 | impl<'a> Foo<'a> for () {
  |      ^^
note: ...so that the types are compatible
 --> src/main.rs:7:84
  |
7 |       fn index_op(map: &std::collections::HashMap<&'static str, ()>, key: Self::Key) {
  |  ____________________________________________________________________________________^
8 | |         map[&key];
9 | |     }
  | |_____^
  = note: expected `Foo<'a>`
             found `Foo<'_>`
  = note: but, the lifetime must be valid for the static lifetime...
note: ...so that the types are compatible
 --> src/main.rs:8:9
  |
8 |         map[&key];
  |         ^^^^^^^^^
  = note: expected `Borrow<&str>`
             found `Borrow<&'static str>`

Which is saying &'static str doesn't implement Borrow<&'a str>, which is technically true. The closest matching impl is impl<T> Borrow<T> for T which would mean it implements Borrow<&'static str> not Borrow<&'a str>. My understanding is trait resolution is not supposed to take lifetimes into account when determining if a type implements a trait which would make this error incorrect.

When either key is a &'static str, or the index method is used directly the errors do not occur.

use std::ops::Index;

trait Foo<'a> {
    type Key;
    fn index_op(map: &std::collections::HashMap<&'static str, ()>, key: Self::Key);
}
impl<'a> Foo<'a> for () {
    type Key = &'static str;
    fn index_op(map: &std::collections::HashMap<&'static str, ()>, key: Self::Key) {
        map[&key];
    }
}

fn index_op(map: &std::collections::HashMap<&'static str, ()>, key: &'static str) {
    map[&key];
}

and

use std::ops::Index;

trait Foo<'a> {
    type Key;
    fn index_op(map: &std::collections::HashMap<&'static str, ()>, key: Self::Key);
}
impl<'a> Foo<'a> for () {
    type Key = &'a str;
    fn index_op(map: &std::collections::HashMap<&'static str, ()>, key: Self::Key) {
        map.index(&key);
    }
}

fn index_op<'a>(map: &std::collections::HashMap<&'static str, ()>, key: &'a str) {
    map.index(&key);
}

This came up while looking at rust-lang/rust-clippy#1725

Tested on the playground (stable 1.57)

[fmease]

Both snippets now compile successfully, closing as fixed. Please notify me if I missed something and I should reopen the issue.