I keep forgetting that rust calls this `and_then` and trying to search for `flatmap`.
`and_then`'s docs even mention "Some languages call this operation flatmap",
but it doesn't show up as a result in the search at `https://doc.rust-lang.org/std/?search=flatmap`

This reverts commit 827f656.

Incremental is not sound to use across stages. Arbitrary changes to the compiler can invalidate the
incremental cache - even changes to normal queries, not incremental itself! - and we do not
currently enable `incremental-verify-ich` in bootstrap. Since 2018, we highly recommend and nudge
users towards stage 1 builds instead of stage 2, and using `keep-stage` for anything other than
libstd is very rare.

I don't think the risk of unsoundness is worth the very minor speedup when building libstd. Disable
incremental to avoid spurious panics and miscompilations when building with the stage 1 and 2
sysroot.

Previously, this would print a message for each doctest, which was quite
verbose:
```
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/exploit-mitigations.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/instrument-coverage.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/json.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/linker-plugin-lto.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/lints/groups.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/lints/index.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/lints/levels.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/aarch64-apple-ios-sim.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/aarch64-nintendo-switch-freestanding.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/apple-watchos.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/armeb-unknown-linux-gnueabi.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/armv4t-none-eabi.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/armv5te-none-eabi.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/armv6k-nintendo-3ds.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/armv7-sony-vita-newlibeabihf.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/armv7-unknown-linux-uclibceabi.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/armv7-unknown-linux-uclibceabihf.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/esp-idf.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/fuchsia.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/kmc-solid.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/loongarch-linux.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/m68k-unknown-linux-gnu.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/mipsel-sony-psx.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/nto-qnx.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/openbsd.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/openharmony.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/riscv32imac-unknown-xous-elf.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/unknown-uefi.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/wasm64-unknown-unknown.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/x86_64-fortanix-unknown-sgx.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/platform-support/x86_64-unknown-none.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/profile-guided-optimization.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/target-tier-policy.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/targets/custom.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/targets/index.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/tests/index.md
doc tests for: /home/jyn/src/rust/src/doc/rustc/src/what-is-rustc.md
```

This works by building std from source unconditionally instead of downloading it, for library tests only.

This was somewhat complicated because of the following requirements:
1. Unconditionally downloading libstd breaks `x test std`, because `coretests` requires the std loaded from the sysroot to match the std that's currently being tested.
2. Unconditionally rebuilding libstd breaks `x test ui-fulldeps librustdoc`, because anything loading `rustc_private` needs to use the same libstd that rustc was built with.

Break the knot by introducing a new `stage2-test-sysroot`, used only for testing `std` itself. This
holds a freshly compiled std, while `stage2` and `ci-rustc-sysroot` still hold the downloaded std.

This also extends the existing `cp_filtered` in Sysroot to apply to the `rust-std` component, not just the `rustc-dev` component.

… not inlined

…e-export was not inlined

add `#[doc(alias="flatmap")]` to `Option::and_then`

I keep forgetting that rust calls this `and_then` and trying to search for `flatmap`. `and_then`'s docs even mention "Some languages call this operation flatmap", but it doesn't show up as a result in the search at `https://doc.rust-lang.org/std/?search=flatmap`

Fix `x test core` when download-rustc is enabled

Fix `x test --stage 2 core` when download-rustc is enabled

This works by building std from source instead of downloading it, for library tests only.

This was somewhat complicated because of the following requirements:
1. Unconditionally downloading libstd breaks `x test core`, because `coretests` requires the std loaded from the sysroot to match the std that's currently being tested.
2. Unconditionally rebuilding libstd breaks `x test ui-fulldeps librustdoc`, because anything loading `rustc_private` needs to use the same libstd that rustc was built with.

Break the knot by introducing a new `stage2-test-sysroot`, used only for testing `std` itself. This
holds a freshly compiled std, while `stage2` and `ci-rustc-sysroot` still hold the downloaded std.

This also extends the existing `cp_filtered` in Sysroot to apply to the `rust-std` component, not just the `rustc-dev` component, to avoid having both versions of std in `stage2-test-sysroot`.

Fixes rust-lang#110352.

…-Simulacrum

Revert "Enable incremental independent of stage"

This reverts commit 827f656.

Incremental is not sound to use across stages. Arbitrary changes to the compiler can invalidate the incremental cache - even changes to normal queries, not incremental itself! - and we do not currently enable `incremental-verify-ich` in bootstrap. Since 2018, we highly recommend and nudge users towards stage 1 builds instead of stage 2, and using `keep-stage` for anything other than libstd is very rare.

I don't think the risk of unsoundness is worth the very minor speedup when building libstd. Disable incremental to avoid spurious panics and miscompilations when building with the stage 1 and 2 sysroot.

Combined with rust-lang#111329, this should fix rust-lang#76720.

r? `@Mark-Simulacrum`

Add portable-simd mention

Adds a triagebot message for portable-simd

doc: improve explanation

…intermediate, r=notriddle

Fix bug where private item with intermediate doc hidden re-export was not inlined

This fixes this bug:

```rust
mod private {
    /// Original.
    pub struct Bar3;
}

/// Hidden.
#[doc(hidden)]
pub use crate::private::Bar3;
/// Visible.
pub use self::Bar3 as Reexport;
```

In this case, `private::Bar3` should be inlined and renamed `Reexport` but instead we have:

```
pub use self::Bar3 as Reexport;
```

and no links.

There were actually two issues: the first one is that we forgot to check if the next intermediate re-export was doc hidden. The second was that we made the `#[doc(hidden)]` attribute inheritable, which shouldn't be possible.

r? `@notriddle`