Co-Authored-By: Mazdak Farrokhzad <twingoow@gmail.com>

…down to doctests

This removes the implicit dependency on the environment variables set
when running `./x.py test`

Update: review feedback
Update: placate tidy

…elf`

(Without this commit, you still get an error (a very similar one, at
that), but it complains about use of forward declaration, which is
confusing since people do not necessarily think of `Self` as being
declared at all.)

Update: incorporate review feedback.

See rust-lang#64931.

Update: incorporate review feedback.

…oats

Stabilize todo macro

The `todo!` macro is just another name for `unimplemented!`.

Tracking issue: rust-lang#59277

This PR needs a FCP to merge.

r? @withoutboats

Deprecate `#![plugin]` & `#[plugin_registrar]`

This PR deprecates `#![plugin]` and `#[plugin_registrar]`.

~A removal deadline is set: 1.44.0. This will be in 9 months from now and should give everyone who is still relying on the feature ample time to rid themselves of this dependency.~

cc rust-lang#29597

r? @Mark-Simulacrum

proc_macro API: Expose `macro_rules` hygiene

Proc macros do not have direct access to our oldest and most stable hygiene kind - `macro_rules` hygiene.

To emulate it macro authors have to go through two steps - first generate a temporary `macro_rules` item (using a derive, at least until rust-lang#64035 is merged), then generate a macro call to that item. Popular crates like [proc_macro_hack](https://crates.io/crates/proc-macro-hack) use this trick to generate hygienic identifiers from proc macros.

I'd say that these workarounds with nested macro definitions have more chances to hit some corner cases in our hygiene system, in which we don't have full confidence.
So, let's provide a direct access to `macro_rules` hygiene instead.

This PR does that by adding a new method `Span::mixed_site` (bikeshedding is welcome) in addition to existing `Span::call_site` (stable) and `Span::def_site` (unstable).
Identifiers with this span resolve at def-site in for local variables, labels and `$crate`, and resolve at call-site for everything else, i.e. exactly like identifiers produced by `macro_rules`.

This API addition opens the way to stabilizing proc macros in expression positions (rust-lang#54727), for which use of call-site hygiene or workarounds with temporary items would be quite unfortunate.
(`macro_rules` expanded in expression position, on the other hand, are stable since 1.0 and widely used.)

r? @dtolnay @alexcrichton

… r=centril

add regression test for rust-lang#60218

Fixes rust-lang#60218

…ure-doctests, r=QuietMisdreavus

Prevent rustdoc feature doctests

Part of rust-lang#61351

cc @ollie27

…pe-param-default, r=alexreg

Disallow Self in type param defaults of ADTs

Fix rust-lang#61631

(also includes a drive-by fix to a typo in some related diagnostic output.)

Replace mentions of IRC with Discord

Revival of rust-lang#61531

closes rust-lang#61524

what was the outcome of this? rust-lang#61531 (comment)

should this be changed in this PR as well? rust-lang#61531 (comment)

…ikomatsakis

Set RUST_BACKTRACE=0 in tests that include a backtrace in stderr

This removes the implicit dependency on the environment variables set
when running `./x.py test`

…-E0556, r=petrochenkov

Add long error explanation for E0556

Part of rust-lang#61137

…s, r=oli-obk

Make visit projection iterative

r? @oli-obk

/cc @nikomatsakis

typo: fix typo in E0392

See rust-lang#64931.

---
Cc: @Centril @estebank