In some places we use `Vec<Attribute>` and some places we use
`ThinVec<Attribute>` (a.k.a. `AttrVec`). This results in various points
where we have to convert between `Vec` and `ThinVec`.
This commit changes the places that use `Vec<Attribute>` to use
`AttrVec`. A lot of this is mechanical and boring, but there are
some interesting parts:
- It adds a few new methods to `ThinVec`.
- It implements `MapInPlace` for `ThinVec`, and introduces a macro to
avoid the repetition of this trait for `Vec`, `SmallVec`, and
`ThinVec`.
Overall, it makes the code a little nicer, and has little effect on
performance. But it is a precursor to removing
`rustc_data_structures::thin_vec::ThinVec` and replacing it with
`thin_vec::ThinVec`, which is implemented more efficiently.
then we just suggest the first legal position where you could inject a use.
To do this, I added `inject_use_span` field to `ModSpans`, and populate it in
parser (it is the span of the first token found after inner attributes, if any).
Then I rewrote the use-suggestion code to utilize it, and threw out some stuff
that is now unnecessary with this in place. (I think the result is easier to
understand.)
Then I added a test of issue 87613.
