Confusing 'implicit copy' warnings when copying into heap closures without a capture clause

[bblum]

One of my teammates during ICFP (#2928) who didn't already know Rust was boggled by a particularly insistent warning.

a.rs:5:17: 5:18 warning: implicitly copying a non-implicitly-copyable value
a.rs:5         bar(copy v, i)
                        ^

Rustc was really complaining that v was being copied into a heap closure. The code looked like this:

fn bar(+v: ~[int], i: uint) -> int { v[i] }

fn foo(v: ~[int]) -> fn@(uint) -> int {
    ret |i| {
        bar(copy v, i)
    }   
}

fn main() { #error["%d", foo(~[5])(0)]; }

The solution was to write copy v in the capture clause.

Perhaps the warning message could be more informative? Could it hint that the copy comes from building the heap closure, and suggest writing a capture clause?

[nikomatsakis]

this shouldn't be hard to do, the code in question is in kind.rs. Right now it all calls things like check_copy() with no indication of why the value must be copyable.

[catamorphism]

I'm going to remove the RFC tag since making an error message more helpful is something we should always do where possible; it doesn't require consensus :-)