If a symbol name can only be imported from one place for a type, and
as long as it was not glob-imported anywhere in the current crate, we
can trim its printed path and print only the name.
This has wide implications on error messages with types, for example,
shortening `std::vec::Vec` to just `Vec`, as long as there is no other
`Vec` importable anywhere.
This adds a new '-Z trim-diagnostic-paths=false' option to control this
feature.
On the good path, with no diagnosis printed, we should try to avoid
issuing this query, so we need to prevent trimmed_def_paths query on
several cases.
This change also relies on a previous commit that differentiates
between `Debug` and `Display` on various rustc types, where the latter
is trimmed and presented to the user and the former is not.
Closure typing obligations flow in both direcitons to properly infer
types. Because of this, we will get 2 type errors whenever there's
an unfulfilled obligation. To avoid this, we deduplicate them in the
`InferCtxt`.
To avoid confusion in cases where the code is
```rust
fn foo() {}
/ foo(
| bar()
| ^^^ current diagnostics point here for arg count mismatch
| );
|_^ new diagnostic span points here
```
as this leads to confusion making people think that the diagnostic is
talking about `bar`'s arg count, not `foo`'s.
Point at `fn`s definition on arg mismatch, just like we do for closures.