Add CI compiling rustc

.github/workflows/nightly_rustc.yml

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+name: Nightly compilation of rustc with rustc_codegen_gcc
+
+on:
+  pull_request:
+  # TODO: remove pull_request and add schedule to run during the night.
+
+env:
+  # Enable backtraces for easier debugging
+  RUST_BACKTRACE: 1
+
+jobs:
+  build:
+    runs-on: ubuntu-24.04
+
+    strategy:
+      fail-fast: false
+      matrix:
+        arch:
+          - host_target: "x86_64-unknown-linux-gnu"
+            cross_target: ""
+            gcc_urls: >
+              https://github.com/rust-lang/gcc/releases/latest/download/gcc-15.deb
+          - host_target: "m68k-unknown-linux-gnu"
+            cross_target: "m68k"
+            gcc_urls: >
+              https://github.com/rust-lang/gcc/releases/latest/download/gcc-15.deb
+
+    steps:
+    - uses: actions/checkout@v4
+      with:
+        path: "rustc_codegen_gcc"
+
+    - uses: actions/checkout@v4
+      with:
+        repository: "rust-lang/rust"
+        path: "rust"
+        fetch-depth: 10
+
+    - run: |
+        echo $HOME
+        echo "*****"
+        ls
+        echo "*****"
+        ls ..
+        echo "*****"
+        ls rust
+        echo "*****"
+
+    # `rustup show` installs from rust-toolchain.toml
+    - name: Setup rust toolchain
+      run: rustup show
+
+    - name: Setup rust cache
+      uses: Swatinem/rust-cache@v2
+
+    - name: Download and install artifacts
+      run: |
+        urls=(${{ matrix.arch.gcc_urls }})
+        for url in "${urls[@]}"; do
+            curl -L -o package.deb $url
+            sudo dpkg --force-overwrite -i package.deb
+        done
+
+    - name: Download cross-compiling libgccjit.so and move it in libgccjit-libs-dir
+      if: ${{ matrix.arch.cross_target != '' }}
+      run: |
+        curl -LO https://github.com/cross-cg-gcc-tools/cross-gcc/releases/latest/download/gcc-${{ matrix.arch.cross_target }}-15.deb
+
+        sudo dpkg-deb -x gcc-${{ matrix.arch.cross_target }}-15.deb cross-gcc
+        dir=$HOME/libgccjit-libs-dir/x86_64-unknown-linux-gnu/${{ matrix.arch.host_target }}
+        mkdir -p $dir
+        # TODO: create a symlink instead of a copy.
+        cp cross-gcc/usr/lib/libgccjit.so.0.0.1 $dir/libgccjit.so
+
+        echo "$(pwd)/cross-gcc/usr/bin" >> $GITHUB_PATH
+
+    - name: Download native libgccjit.so and move it in libgccjit-libs-dir
+      if: ${{ matrix.arch.cross_target != '' }}
+      run: |
+        curl -LO https://github.com/cross-cg-gcc-tools/native-gcc/releases/latest/download/libgccjit-${{ matrix.arch.cross_target }}.so
+
+        dir=$HOME/libgccjit-libs-dir/${{ matrix.arch.host_target }}/${{ matrix.arch.host_target }}
+        mkdir -p $dir
+        mv libgccjit-${{ matrix.arch.cross_target }}.so $dir/libgccjit.so
+
+    - name: Patch linux-raw-sys and rustix.
+      if: ${{ matrix.arch.cross_target != '' }}
+      run: |
+        cd rust
+
+        cat <<'EOF' > patch.toml
+        [patch.crates-io]
+        linux-raw-sys = { git = "https://github.com/antoyo/linux-raw-sys", branch = "m68k-support" }
+        rustix = { git = "https://github.com/antoyo/rustix", branch = "m68k-support" }
+        EOF
+
+        cat patch.toml >> Cargo.toml
+        cat patch.toml >> compiler/rustc_codegen_gcc/Cargo.toml
+
+        cargo update -p rustix
+        cd compiler/rustc_codegen_gcc/
+        cargo update -p rustix
+
+        cargo tree -i linux-raw-sys 2>&1 | grep "was not used in the crate graph" && (echo "Unused patches found"; cargo tree; exit 1) || exit 0
+
+    - name: Compile rustc
+      run: |
+        echo $PATH
+        cd rust
+        ./x build --host=${{ matrix.arch.host_target }} --stage 2 --config ../rustc_codegen_gcc/tests/bootstraps/bootstrap.${{ matrix.arch.host_target }}.toml
+
+    - name: Link toolchain
+      run: |
+        cd rust
+        ls
+        echo ****
+        ls build
+        echo ****
+        ls build/${{ matrix.arch.host_target }}
+        rustup toolchain link my-toolchain build/${{ matrix.arch.host_target }}/stage2
+
+    - name: Smoke test
+      run: |
+        rustc +my-toolchain -V > output
+        grep rustc output
+
+    - name: Compile test program
+      run: |
+        cd rustc_codegen_gcc/tests/hello-world
+        cargo +my-toolchain build
+
+        objcopy --dump-section .comment=comment target/debug/hello_world
+        grep "rustc version .* with libgccjit" comment
+        grep 'GCC: ' comment
+
+        cargo +my-toolchain run > hello_world_stdout
+
+        expected_output="40"
+        test $(cat hello_world_stdout) == $expected_output || (echo "Output differs. Actual output: $(cat hello_world_stdout)"; exit 1)

tests/bootstraps/bootstrap.m68k-unknown-linux-gnu.toml

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+profile = "compiler"
+change-id = 140732
+
+[build]
+target = ["m68k-unknown-linux-gnu"]
+
+[rust]
+codegen-backends = ["gcc"]
+deny-warnings = false
+debug-assertions = false
+debug-assertions-std = false
+
+[gcc]
+download-ci-gcc = true
+libgccjit-libs-dir = "/home/runner/libgccjit-libs-dir"
+
+[target.m68k-unknown-linux-gnu]
+cc = "/home/runner/work/rustc_codegen_gcc/rustc_codegen_gcc/cross-gcc/usr/bin/m68k-unknown-linux-gnu-gcc"

tests/bootstraps/bootstrap.x86_64-unknown-linux-gnu.toml

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+profile = "compiler"
+change-id = 140732
+
+[rust]
+codegen-backends = ["gcc"]
+deny-warnings = false
+debug-assertions = false
+debug-assertions-std = false
+
+[gcc]
+download-ci-gcc = true