MIR borrowck: missing suggestion to use a reference

[arielb1]

When moving out of a value into a new let binding, AST borrowck suggests using a reference, but MIR borrowck does not.

e.g. for E0508:

struct NonCopy;

fn main() {
    let array = [NonCopy; 1];
    let _value = array[0];  //[ast]~ ERROR [E0508]
                            //[mir]~^ ERROR [E0508]
}

We have the following error message

error[E0508]: cannot move out of type `[NonCopy; 1]`, a non-copy array (Ast)
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/E0508.rs:18:18
   |
18 |     let _value = array[0];  //[ast]~ ERROR [E0508]
   |                  ^^^^^^^^
   |                  |
   |                  cannot move out of here
   |                  help: consider using a reference instead: `&array[0]`

error[E0508]: cannot move out of type `[NonCopy; 1]`, a non-copy array (Mir)
  --> /home/ariel/Rust/rust-master/src/test/compile-fail/E0508.rs:18:18
   |
18 |     let _value = array[0];  //[ast]~ ERROR [E0508]
   |                  ^^^^^^^^ cannot move out of here

error: aborting due to 2 previous errors

See that the help is only in the AST. We should port that help message to MIR borrowck.

The help message is generated in this place in AST borrowck:

rust/src/librustc_borrowck/borrowck/gather_loans/move_error.rs

Lines 77 to 87 in 02b4d3d

	Some(&MovePlace { pat_source: PatternSource::LetDecl(ref e), .. }) => {
	// ignore patterns that are found at the top-level of a `let`;
	// see `get_pattern_source()` for details
	let initializer =
	e.init.as_ref().expect("should have an initializer to get an error");
	if let Ok(snippet) = bccx.tcx.sess.codemap().span_to_snippet(initializer.span) {
	err.span_suggestion(initializer.span,
	"consider using a reference instead",
	format!("&{}", snippet));
	}
	}