Auto merge of #138677 - shepmaster:consistent-elided-lifetime-syntax, r=traviscross,jieyouxu

Add a new `mismatched-lifetime-syntaxes` lint

The lang-team [discussed this](https://hackmd.io/nf4ZUYd7Rp6rq-1svJZSaQ) and I attempted to [summarize](rust-lang/rust#120808 (comment)) 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](rust-lang/rust#48686)! Some examples:

    ```rust
    // 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:

    ```rust
    // 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](rust-lang/rust#91639) 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.

Author: bors

tests/pass/fat_ptr.rs

@@ -19,11 +19,11 @@ fn fat_ptr_via_local(a: &[u8]) -> &[u8] {
     x
 }
 
-fn fat_ptr_from_struct(s: FatPtrContainer) -> &[u8] {
+fn fat_ptr_from_struct(s: FatPtrContainer<'_>) -> &[u8] {
     s.ptr
 }
 
-fn fat_ptr_to_struct(a: &[u8]) -> FatPtrContainer {
+fn fat_ptr_to_struct(a: &[u8]) -> FatPtrContainer<'_> {
     FatPtrContainer { ptr: a }
 }