Add experimental Iterator::contains

[bGVia3VjaGVu]

tracking issue
This is the continuation of the abandoned PR.

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[clubby789]

Why is this reimplemented when it uses the same code as the default impl?

[bGVia3VjaGVu]

It is to use SliceContains as a optimization later, but the trait has different trait bounds, so this requires further changes to the trait.

[Noratrieb]

You shouldn't add an empty specialization like this, either actually specialize it or leave it out for now.

[bGVia3VjaGVu]

There is the itertools breakage with rust analyzer and the UI tests have still been failing.

[joboet]

r? libs-api

[Amanieu]

This is blocked on #125782 which will resolve the ambiguity errors.

[bors]

☔ The latest upstream changes (presumably #135937) made this pull request unmergeable. Please resolve the merge conflicts.

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

    Checking ide-diagnostics v0.0.0 (/checkout/src/tools/rust-analyzer/crates/ide-diagnostics)
error: a method with this name may be added to the standard library in the future
   --> src/tools/rust-analyzer/crates/ide-assists/src/handlers/convert_bool_to_enum.rs:385:48
    |
385 |     if !bin_expr.lhs()?.syntax().descendants().contains(name.syntax()) {
    |                                                ^^^^^^^^
    |
    = warning: once this associated item is added to the standard library, the ambiguity may cause an error or change in behavior!
    = note: for more information, see issue #48919 <https://github.com/rust-lang/rust/issues/48919>
    = help: call with fully qualified syntax `itertools::Itertools::contains(...)` to keep using the current method
---

error: a method with this name may be added to the standard library in the future
   --> src/tools/rust-analyzer/crates/ide-assists/src/handlers/convert_bool_to_enum.rs:451:41
    |
451 |     if !receiver.syntax().descendants().contains(name.syntax()) {
    |                                         ^^^^^^^^
    |
    = warning: once this associated item is added to the standard library, the ambiguity may cause an error or change in behavior!
    = note: for more information, see issue #48919 <https://github.com/rust-lang/rust/issues/48919>
    = help: call with fully qualified syntax `itertools::Itertools::contains(...)` to keep using the current method

[Dylan-DPC]

@leb-kuchen this is no longer blocked so if you can resolve the conflicts we can push this forward

[bGVia3VjaGVu]

I have already resolved the merge conflicts. But the format compatibility lint in rust analyzer still fails. Though I haven't checked the other tests.

[alex-semenyuk]

@leb-kuchen
Thanks for your contribution
From wg-triage. Any updates on this PR?