Before this, the diagnostic errors would only point at the return type
when changing it would be a possible solution to a type error. Add a
label to the return type without a suggestion to change in order to make
the source of the expected type obvious.
Follow up to #42850, fixes#25133, fixes#41897.
Prior to this PR, when we aborted because a "critical pass" failed, we
displayed the number of errors from that critical pass. While that's the
number of errors that caused compilation to abort in *that place*,
that's not what people really want to know. Instead, always report the
total number of errors, and don't bother to track the number of errors
from the last pass that failed.
This changes the compiler driver API to handle errors more smoothly,
and therefore is a compiler-api-[breaking-change].
Fixes#42793.
- Point out the origin of a type requirement when it is the return type
of a method
- Point out possibly missing semicolon when the return type is () and
the implicit return makes sense as a statement
- Suggest changing the return type of methods with default return type
- Don't suggest changing the return type on fn main()
- Don't suggest changing the return type on impl fn
