Strange span in borrow error in nested match.

[ehuss]

I tried this code:

fn main() {
    let mut args = std::env::args_os();
    let _arg = match args.next() {
        Some(arg) => {
            match arg.to_str() {
                Some(s) => s,
                None => return,
            }
        }
        None => return,
    };
}

Previously, in 1.36 or earlier, it presented an error like this:

error[E0597]: `arg` does not live long enough
  --> src/main.rs:7:19
   |
5  |     let _arg = match args.next() {
   |         ---- borrow later stored here
6  |         Some(arg) => {
7  |             match arg.to_str() {
   |                   ^^^ borrowed value does not live long enough
...
13 |     };
   |     - `arg` dropped here while still borrowed

Starting with 1.37, the "dropped here while still borrowed" message points to an odd location:

error[E0597]: `arg` does not live long enough
  --> src/main.rs:7:19
   |
5  |     let _arg = match args.next() {
   |         ---- borrow later stored here
6  |         Some(arg) => {
7  |             match arg.to_str() {
   |                   ^^^ borrowed value does not live long enough
...
12 |         None => return,
   |            - `arg` dropped here while still borrowed

The earlier error looks more correct to me.

Bisected the change to nightly-2019-05-24, I would guess maybe it is #60174.

[lcnr]

Without using any methods:

fn main() {
    let _arg = match Some(()) {
        Some(arg) => {
            match Some(&arg) {
                Some(s) => s,
                None => return,
            }
        } // <- this is probably the desired location
        None => return,
    };
}

results in

error[E0597]: `arg` does not live long enough
  --> src/main.rs:6:24
   |
4  |     let _arg = match Some(()) {
   |         ---- borrow later stored here
5  |         Some(arg) => {
6  |             match Some(&arg) {
   |                        ^^^^ borrowed value does not live long enough
...
11 |         None => return,
   |            - `arg` dropped here while still borrowed

[LeSeulArtichaut]

Assigning P-medium as discussed as part of the Prioritization WG procedure.

Bisected the change to nightly-2019-05-24, I would guess maybe it is #60174.

cc @matthewjasper

[matthewjasper]

This should probably be prev_token:

rust/src/librustc_parse/parser/expr.rs

Line 1793 in 5d429f3

let hi = self.token.span;