The ability to print a specific item as identified by NodeId or path
seems not particularly useful, and certainly carries quite a bit of
complexity with it.

Fixes rust-lang#66476 by turning the violating example into an explicit
counterexample.

…rochenkov

*Syntactically* permit visibilities on trait items & enum variants

Fixes rust-lang#65041

Suppose we have `$vis trait_item` or `$vis enum_variant` and `$vis` is a `:vis` macro fragment. Before this PR, this would fail to parse. This is now instead allowed as per language team consensus in rust-lang#65041 (comment). (See added tests for elaboration.)

Moreover, we now also permit visibility modifiers on trait items & enum variants *syntactically* but reject them with semantic checks (in `ast_validation`):

```rust
#[cfg(FALSE)]
trait Foo { pub fn bar(); } // OK

#[cfg(FALSE)]
enum E { pub U } // OK
```

Document pitfall with `impl PartialEq<B> for A`

Fixes rust-lang#66476 by turning the violating example into an explicit
counterexample.

Remove pretty printing of specific nodes in AST

The ability to print a specific item as identified by NodeId or path
seems not particularly useful, and certainly carries quite a bit of
complexity with it.

This is intended to simplify our CLI parsing a bit and remove a
non-uncomplicated piece of it; I largely did this to remove the
dependency on NodeId from librustc/session but it's not really
necessary to do so in this invasive a way. The alternative is
moving it to librustc_interface or driver, probably.

…t, r=oli-obk

Handle statics in MIR as const pointers

This is the first PR towards the goal of removing `PlaceBase::Static`. In this PR:

* Statics are lowered to dereferencing a const pointer.
* The temporaries holding such pointers are tracked in MIR, for the most part this is only used for diagnostics. There are two exceptions:
    * The borrow checker has some checks for thread-locals that directly use this data.
    * Const checking will suppress "cannot dereference raw pointer" diagnostics for pointers to `static mut`/`extern static`. This is to maintain the current behaviour (12 tests fail otherwise).

The following are left to future PRs (I think that @spastorino will be working on the first 3):

* Applying the same treatments to promoted statics.
* Removing `PlaceBase::Static`.
* Replacing `PlaceBase` with `Local`.
* Moving the ever growing collection of metadata that we have for diagnostics in MIR passes somewhere more appropriate.

r? @oli-obk

…erb, r=Centril

follow the convention in this file to use third-person singular verbs

…=Dylan-DPC

Error code's long explanation cleanup

Continuing to clean up the error code's long explanation.

r? @Dylan-DPC

fix reoccuring typo: dereferencable -> dereferenceable

…etrochenkov

resolve: more declarative `fresh_binding`

Following up on rust-lang#64111, this PR redefines `fresh_binding` wrt. `already_bound_and` and `already_bound_or` in a more declarative and simplified fashion.

cc rust-lang#54883

r? @petrochenkov