MIR borrowck: error message confuses locals and temporaries

[arielb1]

e.g.:

fn foo() -> &u32 {
    let x = 0;
    &x
}

Gives the following error:

error[E0597]: `x` does not live long enough (Ast)
 --> foo.rs:4:6
  |
4 |     &x
  |      ^ does not live long enough
5 | }
  | - borrowed value only lives until here
  |
  = note: borrowed value must be valid for the static lifetime...

error[E0597]: borrowed value does not live long enough (Mir)
 --> foo.rs:5:2
  |
3 |     let x = 0;
  |         - temporary value created here
4 |     &x
5 | }
  | -^ temporary value dropped here while still borrowed
  | |
  | temporary value needs to live until here
  |
  = note: consider using a `let` binding to increase its lifetime

error: aborting due to 2 previous errors

The MIR error message should be more like the AST one

[arielb1]

Note that the MIR message has 2 problems

1. It calls locals "temporaries"
2. It throws away the span of the borrow, which can be confusing if a local is only borrowed in a few places in a big function.

[davidtwco]

I'll take a stab at this one.

[nikomatsakis]

The example as given does not compile. You can try these two:

fn foo() -> &'static u32 {
    let x = 0;
    &x
}

fn main() { }

and

fn foo(_: &u32) -> &u32 {
    let x = 0;
    &x
}

fn main() { }