E0308 because `Self` is not equivalent to concrete type in constructor (lifetime mismatch)

[PieterPenninckx]

I tried this code:

struct S<'a> {
    x: &'a str
}

impl S<'static> {
    fn f(self) {}
}

impl<'a> S<'a> {
    fn f1() -> S<'static> {
        Self{x: ""} // error[E0308]: mismatched types
    }
    fn f2() -> S<'static> {
        S{x: ""}
    }
    
    fn g1() {
        S::f(Self{x: ""}) // error[E0308]: mismatched types
    }
    fn g2() {
        S::f(S{x: ""})
    }
}

fn main() {}

I expect this code to compile without error because I would assume that Self is equivalent to S in this case. I expect this equivalence because Self is used as an alias of S in the body of a function and lifetimes cannot be annotated in function bodies (edit: this turns out to be incorrect, see below).

Instead, this happened:

error[E0308]: mismatched types
  --> test.rs:11:9
   |
11 |         Self{x: ""} // error[E0308]: mismatched types
   |         ^^^^^^^^^^^ lifetime mismatch
   |
   = note: expected struct `S<'static>`
              found struct `S<'a>`
note: the lifetime `'a` as defined on the impl at 9:6...
  --> test.rs:9:6
   |
9  | impl<'a> S<'a> {
   |      ^^
   = note: ...does not necessarily outlive the static lifetime

error[E0308]: mismatched types
  --> test.rs:18:14
   |
18 |         S::f(Self{x: ""}) // error[E0308]: mismatched types
   |              ^^^^^^^^^^^ lifetime mismatch
   |
   = note: expected struct `S<'static>`
              found struct `S<'a>`
note: the lifetime `'a` as defined on the impl at 9:6...
  --> test.rs:9:6
   |
9  | impl<'a> S<'a> {
   |      ^^
   = note: ...does not necessarily outlive the static lifetime

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0308`.

Meta

rustc --version --verbose:

rustc 1.42.0 (b8cedc004 2020-03-09)
binary: rustc
commit-hash: b8cedc00407a4c56a3bda1ed605c6fc166655447
commit-date: 2020-03-09
host: x86_64-unknown-linux-gnu
release: 1.42.0
LLVM version: 9.0

Same behaviour with the nightly version of Rust:

rustc 1.44.0-nightly (1057dc97a 2020-03-20)
binary: rustc
commit-hash: 1057dc97afce39ff6a224966ece3ed438af4c1f5
commit-date: 2020-03-20
host: x86_64-unknown-linux-gnu
release: 1.44.0-nightly
LLVM version: 9.0

Probably related to: #69224

[tesuji]

The error is legit and right.

[tesuji]

Self is not the same as type name.

[tesuji]

In the impl block, Self is alias for S<'a>, not S alone.
Anyway, this maybe a duplicate of #66176.

[jonas-schievink]

Yeah, this is working as intended, so closing.

[PieterPenninckx]

For people reading this in the future: in contrast to what I initially believed, lifetimes can be annotated in function bodies (at least in certain cases), as illustrated in the following code which compiles:

struct S<'a> { x: &'a str }

impl S<'static> { fn f(self) {} }

impl<'a> S<'a> {
    fn f2() -> S<'static> {
        S::<'static>{x: ""} // Lifetime annotation in function body
    }
  
    fn g2() {
        S::f(S::<'static>{x: ""})  // Lifetime annotation in function body
    }
}

fn main() {}

With this knowledge, the error indeed makes sense to me.