Better error when types differ only by their boxiness

[brson]

Instead of having to figure out what is meant by error: mismatched types: expected block but found @rec(rec(vec[@stmt] stmts,<tag#1:926>[@expr] expr,node_id id) node,span span) (types differ) rustc should just tell me that I don't need the box, maybe by using a span_note.

[brson]

Maybe for a more general solution we could display some kind of structural diff of the types involved.

[catamorphism]

I'm going to check if this is still an issue.

[catamorphism]

Still an issue. I'll either implement the more specific or the more general solution, not sure yet.

[nikomatsakis]

fixed in 2c56ba7

[brson]

So dreamy!