Fix help docs for -Zallow-features
The arguments for -Zallow-features are comma-separated (`parse_opt_comma_list`), not space separated (`parse_list`).
[RFC 2397] Initial implementation
cc #51992
Because of previous experiences where ppl didn't have the time to review large PRs (or any at all), the implementation of this feature will be delivered in small chunks to hopefully make things faster.
In this initial PR, only the attribute is being declared and gated with ordinary tests.
remove E0280
After looking at #61137 I tried my hand at E0280. I'm unable to find a reasonable example that emits the error. There are a couple of old examples that compile with the current compiler ([#26217](https://github.com/rust-lang/rust/issues/26217), [#42114](https://github.com/rust-lang/rust/issues/42114), [#27113](https://github.com/rust-lang/rust/issues/27113)) and there is a [bug with chalk](b7cdb635c4/src/test/ui/chalkify/bugs/async.rs) that makes it emit the error, with a couple more chalk bugs on zulip.
It seems like the error is supposed to be emitted from unfulfilled where bounds, of which two are related to borrow checking (error in where T: 'a or where 'a: 'b) and thus tend to emit errors like "lifetime may not live long enough" from borrow checking instead. The final case is with type equality constraints (where <T as Iterator>::Item == u32), which is unimplemented ([#20041](https://github.com/rust-lang/rust/issues/20041)). That such different problems are supposed to have the same error code also seems strange to me.
Since the error seems to only be emitted when using chalk I propose to remove it and replace it with an ICE instead. A crater run might be warranted.
Pinging `@jackh726` due to removal of chalk test that now ICEs.
Consider return type when giving various method suggestions
1. Fix a bug in method probe where we weren't normalizing `xform_ret_ty` for non-`impl` method candidates. This shouldn't affect happy-path code, since we only use `xform_ret_ty` when probing methods for diagnostics (I think).
2. Pass the return type expectation down to `lookup_probe`/`probe_for_name` usages in diagnostics. Added a few UI tests to gate against bad suggestions.
3. Make a `FnCtxt::lookup_probe_for_diagnostic` which properly passes down `IsSuggestion(true)`. Should help suppress other weird notes in some corner cases.
Accept old spelling of Fuchsia target triples
The old spelling of Fuchsia target triples was changed in #106429 to add a proper vendor. Because the old spelling is widely used, some projects may need time to migrate their uses to the new triple spelling. The old spelling may eventually be removed altogether.
r? ``@tmandry``
Fix bad import suggestion with nested `use` tree
Fixes#105566Fixes#105373
Ideally, we'd find some way to turn these into structured suggestions -- perhaps on a separate line as a different `use` statement, but I have no idea how to access the span for the whole `use` from this point in the import resolution code.
