Auto merge of #58575 - mati865:musl_toolchain, r=alexcrichton

Musl host toolchain

Based on https://github.com/rust-lang/rust/pull/55163 and https://github.com/rust-lang/rust/pull/57359
Depends on https://github.com/rust-lang/rust/pull/55566

CC https://github.com/rust-lang/rust/issues/57439

### How it works

Tested compiler made by `dist` on glibc and musl based distributions and verified binaries it produces:
* Ubuntu (glibc) - installed it as a target for host toolchain and observed no regressions for static (default) linking, dynamic linking apparently requires musl build libgcc so I didn't test it.
* Alpine (musl) - installed as the host toolchain, by default it links statically (executables are portable and work on glibc distributions) but with `-C target-feature=-crt-static` Rust flag it links dynamically (executables require musl built libraries).

### What's debatable

It should be decided whether this toolchain should link dynamically or statically when using it on musl distribution. I believe the distributions would prefer dynamic linking but it'd be misleading because `$ARCH-unknown-linux-musl` target links statically on the other hosts.
Another problem is using `RUSTFLAGS='-C target-feature=-crt-static'` for dynamic builds which is really uncomfortable.

To address both issues I suggest leaving `$ARCH-unknown-linux-musl` static for both host and cross target and introducing "alias triple" `$ARCH-unknown-linux-dynmusl`. It'd be the same as `$ARCH-unknown-linux-musl` (and use the same libraries to avoid duplication) but it'd link dynamically.

<del>
### Why it's still WIP (help wanted)

I'm having a hard time getting all tests to pass and I'd appreciate help.

Non-verbose error:
<details>

```
Testing proc_macro stage1 (x86_64-unknown-linux-musl -> x86_64-unknown-linux-musl)
   Compiling proc_macro v0.0.0 (/checkout/src/libproc_macro)
error[E0463]: can't find crate for `std`

error[E0463]: can't find crate for `std`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0463`.
error: aborting due to previous error

For more information about this error, try `rustc --explain E0463`.
[RUSTC-TIMING] proc_macro test:true 0.529
[RUSTC-TIMING] proc_macro test:false 0.530
error: Could not compile `proc_macro`.
warning: build failed, waiting for other jobs to finish...
error: Could not compile `proc_macro`.

To learn more, run the command again with --verbose.

command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "test" "--target" "x86_64-unknown-linux-musl" "-j" "16" "--release" "--locked" "--manifest-path" "/checkout/src/libtest/Cargo.toml" "-p" "proc_macro" "--"
expected success, got: exit code: 101

failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test --host x86_64-unknown-linux-musl --target x86_64-unknown-linux-musl
```
</details>

Verbose error:
<details>

```
Testing proc_macro stage1 (x86_64-unknown-linux-musl -> x86_64-unknown-linux-musl)
   Compiling proc_macro v0.0.0 (/checkout/src/libproc_macro)
     Running `/checkout/obj/build/bootstrap/debug/rustc --edition=2018 --crate-name proc_macro src/libproc_macro/lib.rs --color never --crate-type lib --emit=dep-info,link -C opt-level=2 -C metadata=09ddd3ecc930ab63 -C extra-filename=-09ddd3ecc930ab63 --out-dir /checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/x86_64-unknown-linux-musl/release/deps --target x86_64-unknown-linux-musl -L dependency=/checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/x86_64-unknown-linux-musl/release/deps -L dependency=/checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/release/deps -C target-feature=-crt-static`
     Running `/checkout/obj/build/bootstrap/debug/rustc --edition=2018 --crate-name proc_macro src/libproc_macro/lib.rs --color never --emit=dep-info,link -C opt-level=2 --test -C metadata=a564d363930469c8 -C extra-filename=-a564d363930469c8 --out-dir /checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/x86_64-unknown-linux-musl/release/deps --target x86_64-unknown-linux-musl -L dependency=/checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/x86_64-unknown-linux-musl/release/deps -L dependency=/checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/release/deps -C target-feature=-crt-static`
error[E0463]: can't find crate for `std`
error[E0463]: can't find crate for `std`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0463`.
error: aborting due to previous error

For more information about this error, try `rustc --explain E0463`.
[RUSTC-TIMING] proc_macro test:false 0.248
error: Could not compile `proc_macro`.

Caused by:
  process didn't exit successfully: `/checkout/obj/build/bootstrap/debug/rustc --edition=2018 --crate-name proc_macro src/libproc_macro/lib.rs --color never --crate-type lib --emit=dep-info,link -C opt-level=2 -C metadata=09ddd3ecc930ab63 -C extra-filename=-09ddd3ecc930ab63 --out-dir /checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/x86_64-unknown-linux-musl/release/deps --target x86_64-unknown-linux-musl -L dependency=/checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/x86_64-unknown-linux-musl/release/deps -L dependency=/checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/release/deps -C target-feature=-crt-static` (exit code: 1)
warning: build failed, waiting for other jobs to finish...
[RUSTC-TIMING] proc_macro test:true 0.248
error: Could not compile `proc_macro`.

Caused by:
  process didn't exit successfully: `/checkout/obj/build/bootstrap/debug/rustc --edition=2018 --crate-name proc_macro src/libproc_macro/lib.rs --color never --emit=dep-info,link -C opt-level=2 --test -C metadata=a564d363930469c8 -C extra-filename=-a564d363930469c8 --out-dir /checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/x86_64-unknown-linux-musl/release/deps --target x86_64-unknown-linux-musl -L dependency=/checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/x86_64-unknown-linux-musl/release/deps -L dependency=/checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/release/deps -C target-feature=-crt-static` (exit code: 1)

command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "test" "--target" "x86_64-unknown-linux-musl" "-j" "16" "--release" "--locked" "--manifest-path" "/checkout/src/libtest/Cargo.toml" "--verbose" "-p" "proc_macro" "--"
expected success, got: exit code: 101

failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test --host x86_64-unknown-linux-musl --target x86_64-unknown-linux-musl
```

</details>

Whole tests non-verbose output: [rust-tests.log](https://github.com/rust-lang/rust/files/2879945/rust-tests.log)

I think the error is because build system (correctly?) tries to use `obj/build/x86_64-unknown-linux-musl/stage1-test/x86_64-unknown-linux-musl/release/deps` which is empty but `obj/build/x86_64-unknown-linux-gnu/stage1-test/x86_64-unknown-linux-musl/release/deps` contains required libs.
</del>

1 files changed, 70 insertions, 0 deletions

diff --git a/src/ci/docker/scripts/musl-toolchain.sh b/src/ci/docker/scripts/musl-toolchain.sh
new file mode 100644
index 00000000000..95b7c2869c9
--- /dev/null
+++ b/src/ci/docker/scripts/musl-toolchain.sh
@@ -0,0 +1,70 @@
+# This script runs `musl-cross-make` to prepare C toolchain (Binutils, GCC, musl itself)
+# and builds static libunwind that we distribute for static target.
+#
+# Versions of the toolchain components are configurable in `musl-cross-make/Makefile` and
+# musl unlike GLIBC is forward compatible so upgrading it shouldn't break old distributions.
+# Right now we have: Binutils 2.27, GCC 6.3.0, musl 1.1.18
+set -ex
+
+hide_output() {
+  set +x
+  on_err="
+echo ERROR: An error was encountered with the build.
+cat /tmp/build.log
+exit 1
+"
+  trap "$on_err" ERR
+  bash -c "while true; do sleep 30; echo \$(date) - building ...; done" &
+  PING_LOOP_PID=$!
+  $@ &> /tmp/build.log
+  trap - ERR
+  kill $PING_LOOP_PID
+  rm /tmp/build.log
+  set -x
+}
+
+ARCH=$1
+TARGET=$ARCH-linux-musl
+
+OUTPUT=/usr/local
+shift
+
+git clone https://github.com/richfelker/musl-cross-make -b v0.9.7
+cd musl-cross-make
+
+hide_output make -j$(nproc) TARGET=$TARGET
+hide_output make install TARGET=$TARGET OUTPUT=$OUTPUT
+
+cd -
+
+# Install musl library to make binaries executable
+ln -s $OUTPUT/$TARGET/lib/libc.so /lib/ld-musl-$ARCH.so.1
+echo $OUTPUT/$TARGET/lib >> /etc/ld-musl-$ARCH.path
+
+
+export CC=$TARGET-gcc
+export CXX=$TARGET-g++
+
+LLVM=70
+
+# may have been downloaded in a previous run
+if [ ! -d libunwind-release_$LLVM ]; then
+  curl -L https://github.com/llvm-mirror/llvm/archive/release_$LLVM.tar.gz | tar xzf -
+  curl -L https://github.com/llvm-mirror/libunwind/archive/release_$LLVM.tar.gz | tar xzf -
+fi
+
+# fixme(mati865): Replace it with https://github.com/rust-lang/rust/pull/59089
+mkdir libunwind-build
+cd libunwind-build
+cmake ../libunwind-release_$LLVM \
+          -DLLVM_PATH=/build/llvm-release_$LLVM \
+          -DLIBUNWIND_ENABLE_SHARED=0 \
+          -DCMAKE_C_COMPILER=$CC \
+          -DCMAKE_CXX_COMPILER=$CXX \
+          -DCMAKE_C_FLAGS="$CFLAGS" \
+          -DCMAKE_CXX_FLAGS="$CXXFLAGS"
+
+hide_output make -j$(nproc)
+cp lib/libunwind.a $OUTPUT/$TARGET/lib
+cd - && rm -rf libunwind-build
+