Unrolled build for #149713

Rollup merge of #149713 - mati865:gnullvm-doc, r=petrochenkov

Update windows-gnullvm platform support doc

Author: rust-timer

src/doc/rustc/src/platform-support/windows-gnullvm.md

@@ -2,9 +2,11 @@
 
 **Tier: 2 (with host tools)**
 
-Windows targets similar to `*-windows-gnu` but using UCRT as the runtime and various LLVM tools/libraries instead of GCC/Binutils.
+Windows targets similar to `*-windows-gnu` but using UCRT as the runtime and various LLVM tools/libraries instead of
+GCC/Binutils.
 
 Target triples available so far:
+
 - `aarch64-pc-windows-gnullvm`
 - `i686-pc-windows-gnullvm`
 - `x86_64-pc-windows-gnullvm`
@@ -16,48 +18,44 @@ Target triples available so far:
 
 ## Requirements
 
-The easiest way to obtain these targets is cross-compilation, but native build from `x86_64-pc-windows-gnu` is possible with few hacks which I don't recommend.
-Std support is expected to be on par with `*-windows-gnu`.
+Building those targets requires an LLVM-based C toolchain, for example, [llvm-mingw][1] or [MSYS2][2] with CLANG*
+environment.
 
-Binaries for this target should be at least on par with `*-windows-gnu` in terms of requirements and functionality.
+Binaries for this target should be at least on par with `*-windows-gnu` in terms of requirements and functionality,
+except for implicit self-contained mode (explained in [the section below](#building-rust-programs)).
 
 Those targets follow Windows calling convention for `extern "C"`.
 
 Like with any other Windows target, created binaries are in PE format.
 
 ## Building the target
 
-These targets can be easily cross-compiled
-using [llvm-mingw](https://github.com/mstorsjo/llvm-mingw) toolchain or [MSYS2 CLANG*](https://www.msys2.org/docs/environments/) environments.
-Just fill `[target.*]` sections for both build and resulting compiler and set installation prefix in `bootstrap.toml`.
-Then run `./x.py install`.
-In my case I had ran `./x.py install --host x86_64-pc-windows-gnullvm --target x86_64-pc-windows-gnullvm` inside MSYS2 MINGW64 shell
-so `x86_64-pc-windows-gnu` was my build toolchain.
-
-Native bootstrapping is doable in two ways:
-- cross-compile gnullvm host toolchain and use it as build toolchain for the next build,
-- copy libunwind libraries and rename them to mimic libgcc like here: https://github.com/msys2/MINGW-packages/blob/68e640756df2df6df6afa60f025e3f936e7b977c/mingw-w64-rust/PKGBUILD#L108-L109, stage0 compiler will be mostly broken but good enough to build the next stage.
-
-The second option might stop working anytime, so it's not recommended.
+Both native and cross-compilation builds are supported and function similarly to other Rust targets.
 
 ## Building Rust programs
 
-Rust does ship a pre-compiled std library for those targets.
-That means one can easily cross-compile for those targets from other hosts if C proper toolchain is installed.
+Rust ships both std and host tools for those targets. That allows using them as both the host and the target.
 
-Alternatively full toolchain can be built as described in the previous section.
+When used as the host and building pure Rust programs, no additional C toolchain is required.
+The only requirements are to install `rust-mingw` component and to set `rust-lld` as the linker.
+Otherwise, you will need to install the C toolchain mentioned previously.
+There is no automatic fallback to `rust-lld` when the C toolchain is missing yet, but it may be added in the future.
 
 ## Testing
 
-Created binaries work fine on Windows or Wine using native hardware. Testing AArch64 on x86_64 is problematic though and requires spending some time with QEMU.
-Most of x86_64 testsuite does pass when cross-compiling,
-with exception for `rustdoc` and `ui-fulldeps` that fail with and error regarding a missing library,
-they do pass in native builds though.
-The only failing test is std's `process::tests::test_proc_thread_attributes` for unknown reason.
+Created binaries work fine on Windows and Linux with Wine using native hardware.
+Testing AArch64 on x86_64 is problematic, though, and requires launching a whole AArch64 system with QEMU.
+
+Most of the x86_64 testsuite does pass, but because it isn't run on CI, different failures are expected over time.
 
 ## Cross-compilation toolchains and C code
 
-Compatible C code can be built with Clang's `aarch64-pc-windows-gnu`, `i686-pc-windows-gnullvm` and `x86_64-pc-windows-gnu` targets as long as LLVM-based C toolchains are used.
-Those include:
-- [llvm-mingw](https://github.com/mstorsjo/llvm-mingw)
-- [MSYS2 with CLANG* environment](https://www.msys2.org/docs/environments)
+Compatible C code can be built with Clang's `aarch64-pc-windows-gnu`, `i686-pc-windows-gnullvm` and
+`x86_64-pc-windows-gnu` targets as long as LLVM-based C toolchains are used. Those include:
+
+- [llvm-mingw][1]
+- [MSYS2][2] with CLANG* environment
+
+[1]: https://github.com/mstorsjo/llvm-mingw
+
+[2]: https://www.msys2.org/docs/environments