[PATCH v2 2/2] ci(windows): build with Rust

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

The Windows runners used by Git's GitHub workflow's `windows-build` job
ship `rustup` plus a `*-pc-windows-msvc` default toolchain (see
https://github.com/actions/runner-images/blob/main/images/windows/Windows2022-Readme.md
and
https://github.com/actions/partner-runner-images/blob/main/images/arm-windows-11-image.md),
but no precompiled `std` for `*-pc-windows-gnu` or
`*-pc-windows-gnullvm`. With the Makefile now picking a GCC-compatible
target triple based on `$(MSYSTEM)`, the build step needs that
precompiled `std` to be installed before invoking `make`, otherwise
`cargo build --target <triple>` fails to find a usable `std` for the
chosen target.

Add a step between the SDK setup and the `make` invocation that selects
the matching triple from `$MSYSTEM` (which
`git-for-windows/setup-git-for-windows-sdk` exports for every subsequent
step) and runs `rustup target add` for it. The mapping mirrors what
`config.mak.uname` derives from `$(MSYSTEM)` and `$(HOST_CPU)`, just
enumerated explicitly here since CI has direct knowledge of which MSYS2
subsystems the matrix actually exercises (`CLANGARM64` for the ARM64
runner, `MINGW64` for the x86_64 runner). Technically, we only need to
handle MINGW64 at present, but the switch to UCRT64 is imminent, and the
other case arms serve as a very fine documentation of what people should
do for other MSYSTEM values.

For a `staticlib` crate-type `cargo build` does not invoke an external
linker, so no further toolchain components (e.g. the `gnullvm` LLVM
linker) need to be installed; `rustup target add` alone is sufficient.

Assisted-by: Claude Opus 4.7
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 .github/workflows/main.yml | 24 ++++++++++++++++++++++++
 ci/lib.sh                  |  3 ---
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 85cfedf5b0..0972547395 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -114,6 +114,30 @@ jobs:
     steps:
     - uses: actions/checkout@v6
     - uses: git-for-windows/setup-git-for-windows-sdk@v2
+    - name: Install GCC-compatible Rust target
+      shell: bash
+      run: |
+        # The hosted Windows runners ship a rustup-managed Rust whose
+        # default toolchain targets the MSVC ABI. That produces a
+        # `gitcore.lib` which the MinGW GCC used by the rest of the
+        # build cannot link. Install the precompiled `std` for a
+        # GCC-compatible target triple matching the MSYS2 subsystem;
+        # the Makefile selects the same triple via $(MSYSTEM) and
+        # passes it to `cargo build --target`.
+        case "$MSYSTEM" in
+        CLANGARM64) target=aarch64-pc-windows-gnullvm ;;
+        CLANG64)    target=x86_64-pc-windows-gnullvm  ;;
+        CLANG32)    target=i686-pc-windows-gnullvm    ;;
+        UCRT64)     target=x86_64-pc-windows-gnu      ;;
+        MINGW64)    target=x86_64-pc-windows-gnu      ;;
+        MINGW32)    target=i686-pc-windows-gnu        ;;
+        *) echo "::error::Unsupported MSYSTEM: $MSYSTEM"; exit 1 ;;
+        esac &&
+        rustup target add "$target" &&
+
+        # Ensure that cargo.exe is found even with the minimal SDK's restricted PATH
+        CARGO="$(type -p cargo.exe)" &&
+        echo "export PATH=\$PATH:${CARGO%/cargo.exe}" >>/etc/profile
     - name: build
       shell: bash
       env:
diff --git a/ci/lib.sh b/ci/lib.sh
index 6c52154eac..c6ccbf8c17 100755
--- a/ci/lib.sh
+++ b/ci/lib.sh
@@ -391,9 +391,6 @@ linux-asan-ubsan)
 osx-meson)
 	MESONFLAGS="$MESONFLAGS -Dcredential_helpers=osxkeychain"
 	;;
-windows-*)
-	export NO_RUST=UnfortunatelyYes
-	;;
 esac
 
 MAKEFLAGS="$MAKEFLAGS CC=${CC:-cc}"
-- 
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