Add a new mismatched-lifetime-syntaxes lint

[shepmaster]

The lang-team discussed this and I attempted to summarize their decision. The summary-of-the-summary is:

• Using two different kinds of syntax for elided lifetimes is confusing. In rare cases, it may even lead to unsound code! Some examples:

  // Lint will warn about these
  fn(v: ContainsLifetime) -> ContainsLifetime<'_>;
  fn(&'static u8) -> &u8;

• Matching up references with no lifetime syntax, references with anonymous lifetime syntax, and paths with anonymous lifetime syntax is an exception to the simplest possible rule:

  // Lint will not warn about these
  fn(&u8) -> &'_ u8;
  fn(&'_ u8) -> &u8;
  fn(&u8) -> ContainsLifetime<'_>;

• Having a lint for consistent syntax of elided lifetimes will make the future goal of warning-by-default for paths participating in elision much simpler.

---

This new lint attempts to accomplish the goal of enforcing consistent syntax. In the process, it supersedes and replaces the existing elided-named-lifetimes lint, which means it starts out life as warn-by-default.

[rustbot]

r? @wesleywiser

rustbot has assigned @wesleywiser.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[shepmaster]

Thanks to:

• @GrigorenkoPV for the original implementation of elided_named_lifetimes.
• @compiler-errors for the push to write the lint using HIR instead of in rustc_resolve.
• @estebank for the diagnostic help and wording feedback.

Addressed.

[bors]

☔ The latest upstream changes (presumably #138719) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

rustc_macros::diagnostics was changed

cc @davidtwco, @compiler-errors, @TaKO8Ki

Some changes occurred in exhaustiveness checking

cc @Nadrieril

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[bors]

☔ The latest upstream changes (presumably #138965) made this pull request unmergeable. Please resolve the merge conflicts.