Also slightly refactor pointer bounds checks to avoid creating unnecessary temporary Errors

It was unused.

Regions in TypeckTables will be erased, so are unusable for error
reporting.

Also skip duplicated region solving entirely with `-Zborrowck=mir`.

keyword docs for else and inkeyword docs for else and in.

First cut of else and in keyword docs. Comments and suggestions more than welcome.

…an-DPC

clean up E0308 explanation

r? @Dylan-DPC

…komatsakis

Erase regions in writeback

Regions in `TypeckTables` (except canonicalized user annotations) are now erased. Further, we no longer do lexical region solving on item bodies with `-Zborrowck=mir`.

cc rust-lang#68261
r? @nikomatsakis

…andry

Use smaller discriminants for generators

Closes rust-lang#69815

I'm not yet sure about the runtime performance impact of this, so I'll try running this on some benchmarks (if I can find any). (Update: No impact on the benchmarks I've measured on)

* [x] Add test with a generator that has exactly 256 total states
* [x] Add test with a generator that has more than 256 states so that it needs to use a u16 discriminant
* [x] Add tests for the size of `Option<[generator]>`
* [x] Add tests for the `discriminant_value` intrinsic in all cases

Expansion-driven outline module parsing

After this PR, the parser will not do any conditional compilation or loading of external module files when `mod foo;` is encountered. Instead, the parser only leaves `mod foo;` in place in the AST, with no items filled in. Expansion later kicks in and will load the actual files and do the parsing. This entails that the following is now valid:

```rust
#[cfg(FALSE)]
mod foo {
    mod bar {
        mod baz; // `foo/bar/baz.rs` doesn't exist, but no error!
    }
}
```

Fixes rust-lang#64197.

r? @petrochenkov

Miri error reform

Some time ago we started moving Miri errors into a few distinct categories, but we never classified all the old errors. That's what this PR does.

~~This is on top of rust-lang#69762; [relative diff](RalfJung/rust@validity-errors...RalfJung:miri-error-cleanup).~~

r? @oli-obk

Fixes rust-lang/const-eval#4

…=oli-obk

Make methods declared by `newtype_index` macro `const`

Crates that use the macro to define an `Idx` type need to enable `#![feature(const_if_match, const_panic)]`.

Remove some imports to the rustc crate

- When we have `NestedVisitorMap::None`, we use `type Map = dyn intravisit::Map<'v>;` instead of the actual map. This doesn't actually result in dynamic dispatch (in the future we may want to use an associated type default to simplify the code).

- Use `rustc_session::` imports instead of `rustc::{session, lint}`.

r? @Zoxc

…petrochenkov

Fix repr pretty display

Fixes rust-lang#70027.

r? @varkor

…mulacrum

Tidy: fix running rustfmt twice

`./x.py test tidy` runs rustfmt twice. This is because `Build::build` runs `execute_cli` twice (once dry, once not). This can be quite slow (and prints a bunch of things twice).

I'm not sure if this is really the best place to check the dry_run status.