Elision in method can take `'static` from `self`, perhaps surprising

[cramertj]

rustc currently compiles the following with no warnings or errors:

struct Foo;

impl Foo {
    pub fn get_mut(&'static self, x: &mut u8) -> &mut u8 {
        unsafe { &mut *(x as *mut _) }
    }
}

However, attempting to make this function safe reveals that the elided lifetime of x isn't the lifetime in the return type-- it's instead copying over the 'static lifetime into the return type! Rustdoc actually documents that the signature of this method is fn get_mut(&'static self, x: &mut u8) -> &'static mut u8.

The following code produces an error:

struct Foo;

impl Foo {
    pub fn get_mut(&'static self, x: &mut u8) -> &mut u8 {
        x
    }
}

error[E0312]: lifetime of reference outlives lifetime of borrowed content...
 --> src/main.rs:7:9
  |
7 |         x
  |         ^
  |
  = note: ...the reference is valid for the static lifetime...
note: ...but the borrowed content is only valid for the anonymous lifetime #1 defined on the method body at 6:5
 --> src/main.rs:6:5
  |
6 | /     pub fn get_mut(&'static self, x: &mut u8) -> &mut u8 {
7 | |         x
8 | |     }
  | |_____^

error: aborting due to previous error

error: Could not compile `playground`.

This seems like it could be fixed by disallowing elision in this case via a lint.

cc @aturon

[leoyvens]

Sounds like a case of "disallow elided lifetimes that refer to explicit lifetimes", I recall @nikomatsakis being in favor of forbidding this.

[nikomatsakis]

I agree this is undesirable, but as @leodasvacas notes, I would say 'no more undesirable than any other named lifetime'. We should implement that lint.

[nikomatsakis]

I retitled the issue to be less dramatic =)

[Enselic]

Triage: Marking as E-help-wanted to implement a lint that warns when an elided lifetime ends up being a named lifetime. There seems to be a broad consensus that this would be good to lint for.

[GrigorenkoPV]

@rustbot claim