The `hi` and `lo` offsets in a span are relative to a `CodeMap`, but this
doesn't seem to be terribly useful for tool consumers who don't have the
codemap, but might want the byte offset within an actual file?
Resolves#35164.
We now fetch source lines from the `external_src` member as a secondary
fallback if no regular source is present, that is, if the file map
belongs to an external crate and the source has been fetched from disk.
Minimize single span suggestions into a label
changes
```
14 | println!("☃{}", tup[0]);
| ^^^^^^
|
help: to access tuple elements, use tuple indexing syntax as shown
| println!("☃{}", tup.0);
```
into
```
14 | println!("☃{}", tup[0]);
| ^^^^^^ to access tuple elements, use `tup.0`
```
Also makes suggestions explicit in the backend in preparation of adding multiple suggestions to a single diagnostic. Currently that's already possible, but results in a full help message + modified code snippet per suggestion, and has no rate limit (might show 100+ suggestions).
Teach Diagnostics to highlight text
Support styled `Diagnostic` output:
<img width="469" alt="mismatched types error with colorized types in the note" src="93a84198-d815-11e6-88b1-0ede3c7e28ef.png">
Closes#37532 and #38901.
r? @nikomatsakis CC @jonathandturner @nagisa @nrc
