…d a long lost assertion

This reverts part of commit 9712fa4.

Co-Authored-By: Ralf Jung <post@ralfj.de>

Added tvOS as targets

This is a first attempt of adding support tvOS as described in rust-lang#48862. It's got a lot of overlap with [src/librustc_target/spec/apple_ios_base.rs](https://github.com/rust-lang/rust/blob/31dd4f4acbcbdb02b0745d2136399ed664a28050/src/librustc_target/spec/apple_ios_base.rs).

I thought about refactoring `apple_ios_base.rs` to include this as well but that would require each of the ios and tvos targets to be of the something like the form `let base = opts(AppleOS::TV, Arch::Arm64)?;` I also did the same thing for watchOS because from what I can tell, all three targets (iOS, tvOS, and watchOS) have the same logic but have different parameters being sent to `xcrun`. Thoughts?

As far as the `data_layout` and other parameters to `Target`, I did as much research as I could but it really seems that processor in the [iPhone 11 is the same as the apple TV](https://en.wikipedia.org/wiki/Apple-designed_processors) so I didn't change any of those parameters.

I did get this to build and tested that it's actually running the the below logic (because the parameter to `xcrun` is `appletvos` not `tvos`).

I didn't manage to get it to actually compile a file with `fn main(){}` because I don't have the stdlib for `aarch64-apple-tvos` compiled it seems. Is there documentation for this?

Similar to the ending of rust-lang#63467, I'm not sure what to do next.

…rk-Simulacrum

Document unsafe blocks in core::fmt

r? @RalfJung
CC: @rust-lang/wg-unsafe-code-guidelines
rust-lang#66219

Sorry for the hiatus, but here's a few more files with the unsafe blocks documented! I think working on it smaller chunks like this will be easier for everyone.

…ta, r=Mark-Simulacrum

Toolstate: remove redundant beta-week check.

I made a bit of a mistake in rust-lang#69624.  The "beta regression" doesn't need to be checked twice.

I also rolled up rust-lang#69693 to avoid merge conflicts.

…etroalbini

Rename rustc guide

This is in preparation for rust-lang/rustc-dev-guide#470
Needs to be merged after we actually rename the guide.

Have used this to rename:

`git grep -l 'rustc_guide' | xargs sed -i 's/rustc_guide/rustc_dev_guide/g'`
`git grep -l 'rustc-guide' | xargs sed -i 's/rustc-guide/rustc-dev-guide/g'`
`git grep -l 'rustc guide' | xargs sed -i 's/rustc guide/rustc dev guide/g'`

…lfJung

Compute the correct layout for variants of uninhabited enums

r? @eddyb
cc @RalfJung

fixes rust-lang#69191
cc rust-lang#69763

…=oli-obk

miri: ICE on invalid terminators

We've run a lot of MIR in Miri (including some generators) and never seen these.

@tmandry is it correct that `Yield` and `GeneratorDrop` get lowered away?

@eddyb @oli-obk what's with this `Abort` that does not seem to ever actually exist? Codegen *does* seem to handle it, so I wonder why Miri can get away without that. In fact, codegen handles it twice:

https://github.com/rust-lang/rust/blob/1d5241c96208ca7d925442b1a5fa45ad18717a6f/src/librustc_codegen_ssa/mir/block.rs#L796

https://github.com/rust-lang/rust/blob/1d5241c96208ca7d925442b1a5fa45ad18717a6f/src/librustc_codegen_ssa/mir/mod.rs#L296

…ov,eddyb

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

rustdoc: remove unused import

```
    Checking rustdoc v0.0.0 (/home/matthias/vcs/github/rust/src/librustdoc)
warning: unused import: `rustc_span::symbol::sym`
  --> src/librustdoc/clean/inline.rs:15:5
   |
15 | use rustc_span::symbol::sym;
   |     ^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default
```