Link to edition guide instead of issues for 2021 lints.
This changes the 2021 lints to not link to github issues, but to the edition guide instead.
Fixes #86996
* On suggestions that include deletions, use a diff inspired output format
* When suggesting addition, use `+` as underline
* Color highlight modified span
These are all testing corner-cases in the compiler.
Adding a new warning broke these test cases, but --cap-lints stops
it from actually breaking things in production.
Review feedback asked for the test to be generalized to include macros
2.0; that generalization is dyn-2015-idents-in-decl-macros-unlinted.rs
As a drive-by, I also decided to revise the test to make it clear
*why* we cannot generally lint these cases. (I already had similar
demonstrations in dyn-2015-edition-keyword-ident-lint.rs, but it does
not hurt to try to emphasize matters.)
I also added some commentary on the cases where we could choose to
make the lint smarter, namely the situations where a macro is
*definitely* using `dyn` as an identifier (because it is using it as a
path component).
Back-story: After reflection this morning, I realized that the
previous form of this test would allow the macro invocation to treat
the `dyn` input as a raw-identifier rather than a keyword, and since
the input was discarded by that version of the macro, the test would
pass despite the detail that the input `dyn` should not have been
parsed as a raw-identifier.
This revision fixes that oversight, by actually *using* the macro
input to construct a `Box<dyn Trait>` type.
