Repeated "mutable/immutable borrow" error messages

[tommyip]

Minimal example:

fn do_something<T>(collection: &mut Vec<T>) {
    let a = &collection;
    collection.swap(1, 2);
}

Error message (ran in playground):

rustc 1.17.0 (56124baa9 2017-04-24)
error[E0502]: cannot borrow `*collection` as mutable because `collection` is also borrowed as immutable
 --> <anon>:3:5
  |
2 |     let a = &collection;
  |              ---------- immutable borrow occurs here
3 |     collection.swap(1, 2);
  |     ^^^^^^^^^^ mutable borrow occurs here
4 | }
  | - immutable borrow ends here

error[E0502]: cannot borrow `*collection` as mutable because `collection` is also borrowed as immutable
 --> <anon>:3:5
  |
2 |     let a = &collection;
  |              ---------- immutable borrow occurs here
3 |     collection.swap(1, 2);
  |     ^^^^^^^^^^ mutable borrow occurs here
4 | }
  | - immutable borrow ends here

error: aborting due to 2 previous errors

Compilation failed.

Tested on nightly as well (rustc 1.19.0-nightly (0ed1ec9f9 2017-05-18))

[est31]

Related: #40067

[estebank]

report_error_if_loans_conflict calls report_error_if_loan_conflicts_with_restriction twice without checking the return value. Modify it to return Result<(), DiagnosticBuilder<'a>> and check where it is being called ether both calls have returned an (cancel one, emit the other, return false), wether only one was created (emit it, return false) or none has (return true).

[tommyip]

I'll fix this, thanks for the instruction!

[lochsh]

I would happily have a go at this, if that's ok with @tommyip 😄

[tommyip]

Sure @lochsh go for it!

[joshleeb]

@lochsh have you had time to work on this? If not could I give it a go? :)

[lochsh]

@joshleeb I totally thought I was going to have time, but I had a crazy week at work this week. Go ahead and give it a go :) I'll find another bug to work on when I definitely have time 👍

[dsheets]

It looks like the PR to fix this was merged but has a few dangling threads. There seems to be some confusion about short-circuiting functionality and the necessity of the patch and also the error count still reads 2 rather than 1. Should a new issue be opened with those issues and this issue closed?