This method is complementary for the feature refcell_leak added in an
earlier PR. It allows reverting the effects of leaking a borrow guard by
statically proving that such a guard could not longer exist. This was
not added to the existing `get_mut` out of concern of impacting the
complexity of the otherwise pure pointer cast and because the name
`get_mut` poorly communicates the intent of resetting remaining borrows.

Use `ast::MacroDef` instead.

Also remove `Session::imported_macro_spans`, external macros have spans now.

It was accidentally merged with the wrong version.

Correctly treat const generic parameters in fresh binding disambiguation

…tolnay

Add undo_leak to reset RefCell borrow state

This method is complementary for the feature cell_leak added in an
earlier PR. It allows *safely* reverting the effects of leaking a borrow guard by
statically proving that such a guard could not longer exist. This was
not added to the existing `get_mut` out of concern of impacting the
complexity of the otherwise pure pointer cast and because the name
`get_mut` poorly communicates the intent of resetting remaining borrows.

This is a follow-up to rust-lang#68712 and uses the same tracking issue, rust-lang#69099,
as these methods deal with the same mechanism and the idea came up
[in a review comment](rust-lang#68712 (comment)).

@dtolnay who reviewed the prior PR.
cc @RalfJung

ast: `Mac`/`Macro` -> `MacCall`

It's now obvious that these refer to macro calls rather than to macro definitions.

It's also a single name instead of two different names in different places.

`rustc_expand` usually calls macro calls in a wide sense (including attributes and derives) "macro invocations", but structures and variants renamed in this PR are only relevant to fn-like macros, so it's simpler and clearer to just call them calls.

cc rust-lang#63586 (comment)
r? @eddyb

…ckler

Implement From<&mut str> for String

I ran into this missing impl when trying to do `String::from` on the result returned from this API in the `uuid` crate:

https://docs.rs/uuid/0.8.1/uuid/adapter/struct.Hyphenated.html#method.encode_lower

I wasn't sure what to put in the stability annotation. I'd appreciate some help with that :)

rustc_metadata: Remove `rmeta::MacroDef`

And other related cleanups.

Follow-up to rust-lang#66364.
r? @Centril

resolve: Fix two issues in fresh binding disambiguation

Prevent fresh bindings from shadowing ambiguity items.
Fixes rust-lang#46079

Correctly treat const generic parameters in fresh binding disambiguation.
Fixes rust-lang#68853

def_collector: Fully visit async functions

We forgot to visit attributes previously, it caused ICEs.

Special treatment of async functions is also moved from `visit_item` to `visit_fn` to reuse more of the default visitor.

Fixes rust-lang#67778.

…-gate, r=oli-obk

Return feature gate as a `Symbol`

A minor refactoring that will be needed for rust-lang#68940. That PR is blocked on me changing the error comments in a whole lot of UI tests.

r? @oli-obk

…e-since, r=Centril

Fix "since" field for `Once::is_complete`'s `#[stable]` attribute

It was accidentally merged with the wrong version in rust-lang#68945.  Thanks @jplatte for noticing.

This also needs to be beta backported.