Confusing 'implicit copy' warnings with outstanding loans

[bblum]

fn borrow(_x: &mut ~[()]) { } 

fn consume(+_x: ~[()]) { } 

fn main() {
    let mut x = ~[()];
    let xref = &mut x;
    borrow(xref);
    consume(x);
}

This code compiles, but gives an implicit copy warning on consume(x). Really the reason for this is because of the outstanding loan -- if { let xref ... borrow(xref); } is enclosed in its own block, the warning goes away.

Perhaps the warning should give a bit more info about why the implicit copy is necessary?

(incidentally, if consume's argument is changed from copy-mode to move-mode, it doesn't compile at all, with the "outstanding loan" error.)

[nikomatsakis]

The problem here is that the loan is issued for the entire block. We could improve the error message if borrow check would note the "last uses" that it canceled (and, presumably, the loan that caused it to do so). Then the kind checker could report "this would be a last use, but for the loan issued on line foo, trying enclosing it in a block".