…nloaded

Before:
```
downloading https://ci-artifacts.rust-lang.org/rustc-builds/bf5cad8e775fb326465e5c1b98693e5d259da156/rust-std-nightly-x86_64-unknown-linux-gnu.tar.xz
curl: (22) The requested URL returned error: 404
```

After:
```
downloading https://ci-artifacts.rust-lang.org/rustc-builds/bf5cad8e775fb326465e5c1b98693e5d259da156/rust-std-nightly-x86_64-unknown-linux-gnu.tar.xz
curl: (22) The requested URL returned error: 404

error: failed to download pre-built rustc from CI

note: old builds get deleted after a certain time
help: if trying to compile an old commit of rustc, disable `download-rustc` in config.toml:

[rust]
download-rustc = false
```

An odd feature of Rust is that `Foo` is invalid, but `Bar` is okay:

    type Foo<'a, 'b> = Box<dyn PartialEq<Foo<'a, 'b>>>;
    type Bar<'a, 'b> = impl PartialEq<Bar<'a, 'b>>;

To get it right, track every time rustdoc descends into a type alias,
so if it shows up twice, it can be write the path instead of
infinitely expanding it.

…lacrum

download-rustc: Give a better error message if artifacts can't be dowloaded

It should be very rare in practice to happen; people would need to both have `download-ci-llvm` disabled and `download-rustc` enabled. I think it may be more common if we start turning this on by default, though.

Helps with rust-lang#81930.

Before:
```
downloading https://ci-artifacts.rust-lang.org/rustc-builds/bf5cad8e775fb326465e5c1b98693e5d259da156/rust-std-nightly-x86_64-unknown-linux-gnu.tar.xz
curl: (22) The requested URL returned error: 404
```

After:
```
downloading https://ci-artifacts.rust-lang.org/rustc-builds/bf5cad8e775fb326465e5c1b98693e5d259da156/rust-std-nightly-x86_64-unknown-linux-gnu.tar.xz
curl: (22) The requested URL returned error: 404

error: failed to download pre-built rustc from CI

note: old builds get deleted after a certain time
help: if trying to compile an old commit of rustc, disable `download-rustc` in config.toml:

[rust]
download-rustc = false
```

…le, r=GuillaumeGomez

rustdoc: catch and don't blow up on impl Trait cycles

Fixes rust-lang#110629

An odd feature of Rust is that `Foo` is invalid, but `Bar` is okay:

    type Foo<'a, 'b> = Box<dyn PartialEq<Foo<'a, 'b>>>;
    type Bar<'a, 'b> = impl PartialEq<Bar<'a, 'b>>;

To get it right, track every time rustdoc descends into a type alias, so if it shows up twice, it can be write the path instead of infinitely expanding it.

Make ConstProp some tests unit.

Part of the effort to tidy up mir-opt test suite.

…nonur

bootstrap: Fix compile error: unused-mut

Compile errors:

```
   Compiling bootstrap v0.0.0 (/home/hev/rust/rust/src/bootstrap)
error: variable does not need to be mutable
    --> config.rs:1312:17
     |
1312 |             let mut build_target = config
     |                 ----^^^^^^^^^^^^
     |                 |
     |                 help: remove this `mut`
     |
     = note: `-D unused-mut` implied by `-D warnings`

error: could not compile `bootstrap` (lib) due to previous error
```

…-Simulacrum

Output some bootstrap messages on stderr

Fixes rust-lang#110995

…unction, r=jyn514

Remove unneeded function call in `core::option`.

r? `@jyn514`