better error message when there is a missing semicolon with an expression

[nikomatsakis]

Something like:

fn foo() {
    task::spawn {|| } //! ERROR type mismatch: expected '()' but got 'tast'
    bar();
}

fn bar() {
}

will yield an error something like the one above. It would be better if it said, "statement with non-unit return type requires a semicolon" or something like that. This would be easy-ish to fix: the ast has a type "stmt_expr" and "stmt_semi", and "stmt_expr" must have a unit type.

[lht]

My first change to the typechecking code. I'm not very confident this is the "easy-ish" fix @nikomatsakis mentioned. Please help review. It passed tests on all bots.

[nikomatsakis]

I don't think that's quite I had in mind. If I read the patch correctly, won't the new msg trigger anytime you unify unit type with non-unit type? I had in mind replacing the line bot = check_expr_with(fcx, expr, ty::mk_nil(fcx.ccx.tcx)); in check_stmt() with something that performed a similar check but issued a different error msg.

[nikomatsakis]

One other thing... this may not be a tiny patch, because it seems like using the unify routines is the right thing to be doing, but we want to change the error msg. Maybe passing in some context to the unify routine to influence the error it emits is the right thing to do...?

[nikomatsakis]

OK, last comment: what I am specifically concerned about is something like a function with type signature " fn(A)->A", where it might not be fully resolvable but could still be unified to unit.

[lht]

Thanks a lot for the information, Niko! I will try again when I understood how it works.

[catamorphism]

Removing the comment, the error message now is:

error: expected ; or } after expression but found bar

which is a parser error rather than a type error. So I think that's close enough to what we wanted.