Description
When compiling with MIR-borrowck via -Z borrowck-mir, the diagnostics printed when errors involves borrows/moves into the bodies of closures (where the origin path is from the outside of that closure) are pretty bad.
For example, for the test src/test/compile-fail/borrowck/borrowck-closures-two-mut.rs, the AST-borrowck error messages tend to highlight the particular uses within the closure body, while the MIR-borrowck error messages just treat the whole closure as a black box.
Specifically, look at this AST-borrowck error:
error[E0499]: cannot borrow `x` as mutable more than once at a time (Ast)
--> ../src/test/compile-fail/borrowck/borrowck-closures-two-mut.rs:55:24
|
54 | let c1 = to_fn_mut(|| set(&mut *x.f));
| -- - previous borrow occurs due to use of `x` in closure
| |
| first mutable borrow occurs here
55 | let c2 = to_fn_mut(|| set(&mut *x.f));
| ^^ - borrow occurs due to use of `x` in closure
| |
| second mutable borrow occurs here
56 | //~^ ERROR cannot borrow `x` as mutable more than once
57 | }
| - first borrow ends here
and compare that to the corresponding MIR-borrowck error:
error[E0499]: cannot borrow `x` as mutable more than once at a time (Mir)
--> ../src/test/compile-fail/borrowck/borrowck-closures-two-mut.rs:55:24
|
54 | let c1 = to_fn_mut(|| set(&mut *x.f));
| ----------------- first mutable borrow occurs here
55 | let c2 = to_fn_mut(|| set(&mut *x.f));
| ^^^^^^^^^^^^^^^^^ second mutable borrow occurs here
56 | //~^ ERROR cannot borrow `x` as mutable more than once
57 | }
| - first borrow ends here
What MIR-borrowck needs, if we can somehow get it, is the equivalent to the "previous borrow occurs due to use of x in closure" and "borrow occurs due to use of x in closure" messages, with the corresponding (very) precise spans.