Poor error message for unconstrained bounds

[aturon]

The following program:

#![allow(dead_code)]

fn foo() -> Result<(), ()> {
    Ok(());  // Note the stray semicolon
}

fn main() {}

generates the following error:

not-infer-sized.rs:4:5: 4:7 error: unable to infer enough type information to locate the impl of the trait `core::kinds::Sized` for the type `<generic #1>`; type annotations required
not-infer-sized.rs:4     Ok(());
                         ^~
not-infer-sized.rs:4:5: 4:7 note: the trait `core::kinds::Sized` must be implemented because it is required by `core::result::Ok`
not-infer-sized.rs:4     Ok(());

[aturon]

cc @nick29581 @nikomatsakis

[aturon]

This is happening because, due to the semicolon, the type arguments for Ok are completely unconstrained. However, this is an easy mistake to make (especially for newcomers) so it would be good to provide a better error message here.

[nrc]

This is the same problem as #17567, although a different example. Not sure if we can fix with the same fix, so I'll leave this open for now.

[nikomatsakis]

As I wrote in #17567, it's probably easiest to just update the error message to change its emphasis in cases like this. I think it's just a dup of #17567, hence closing.