NLL: Wrong variable name in E0502 (double borrow) error message

[ashtneoi]

On nightly, the following code...

#![feature(nll)]

struct Bar;

impl Bar {
    fn bar(&mut self, _: impl Fn()) { }
}

struct Foo { thing: Bar, number: usize }

impl Foo {
    fn foo(&mut self) {
        self.thing.bar(|| {
            &self.number;
        });
    }
}

...yields this error message:

error[E0502]: cannot borrow `self.thing` as mutable because it is also borrowed as immutable
  --> src/lib.rs:13:9
   |
13 |            self.thing.bar(|| {
   |            ^              -- immutable borrow occurs here
   |   _________|
   |  |_________|
   | ||
14 | ||             &self.number;
   | ||              ---- previous borrow occurs due to use of `self.thing` in closure
15 | ||         });
   | ||          ^
   | ||__________|
   | |___________mutable borrow occurs here
   |             borrow later used here

Since the closure actually borrows self.number, not self.thing, I feel like the error should say self instead of self.thing. It would also be nice if it weren't an error in the first place, but that's a separate issue (let me know if I should report it separately).

---

Meta

rustc --version --verbose:

rustc 1.28.0-nightly (1ffb32147 2018-05-31)
binary: rustc
commit-hash: 1ffb321477d85fb3c95a0be2a0cd89c04dff0d8d
commit-date: 2018-05-31
host: x86_64-unknown-linux-gnu
release: 1.28.0-nightly
LLVM version: 6.0

[nikomatsakis]

This is clearly a similar problem as to what we see in #51512, though not precisely the same code. We are using the Place that was accessed, but we should be using the Place that was borrowed -- in the case of a closure, we should really use just the local variable, since closures always capture an individual variable.