when temporary value is a fn call argument, highlighting can be misleading

[nikomatsakis]

In an example where the temporary value is dropped at the end of the statement, such as when the & is an argument to a fn call, the resulting error can be somewhat misleading:

error: borrowed value does not live long enough
  --> <anon>:21:22
   |
21 |     let r = map.get(&'a');
   |                      ^^^ - borrowed value only valid until here
   |                      |
   |                      does not live long enough
22 |     assert_eq!(r, Some(&string));
23 | }
   | - borrowed value must be valid until here
   |

In particular, the - borrowed value only valid until here is intended to be "attached" to the ;, but it kind of looks like it is attached to the ^^^.

One simple way to improve this would be to change the text -- temporaries are freed at one of two kinds of places:

• innermost enclosing statement (usually);
  • note though that the tail expr of a block is contained in the stmt that contains the block (transitively)
• end of block (if syntactically evident that they are assigned into a let binding)

I imagine we could detect which of these two categories we fall into and say "only valid until end of this statement" in the first case or "end of this block" in the second. That might help, though I guess people might not know what a "statement" is.

[nikomatsakis]

cc @jonathandturner

[mikhail-m1]

@jonathandturner I'am ready to fix it after merge #37405

[sophiajt]

@nikomatsakis - can you confirm this is fixed now? I think there were some improvements that landed recently

[estebank]

Current output:

rustc 1.17.0-nightly (cab4bff3d 2017-03-21)
error: borrowed value does not live long enough
  --> <anon>:21:26
   |
21 |     let r = map.get(&'a');
   |                      --- ^ temporary value dropped here while still borrowed
   |                      |
   |                      temporary value created here
22 |     assert_eq!(r, Some(&string));
23 | }
   | - temporary value needs to live until here
   |
   = note: consider using a `let` binding to increase its lifetime

error: aborting due to previous error

[nikomatsakis]

This does seem somewhat better.

[estebank]

This code compiles successfully now.