…g, r=alexcrichton

matching.

This breaks code like:

    struct Foo<'a,'b> {
        x: &'a int,
        y: &'b int,
    }

    trait Tr {
        fn foo(x: Self) {}
    }

    impl<'a,'b> Tr for Foo<'a,'b> {
        fn foo(x: Foo<'b,'a>) {} // <-- bad
    }

Change this code to not contain a lifetime mismatch error. For example:

    struct Foo<'a,'b> {
        x: &'a int,
        y: &'b int,
    }

    trait Tr {
        fn foo(x: Self) {}
    }

    impl<'a,'b> Tr for Foo<'a,'b> {
        fn foo(x: Foo<'a,'b>) {} // OK
    }

Closes #15517.

[breaking-change]

r? @alexcrichton