ci: link the Linux Rust builds with mold

.github/actions/mold/action.yml

@@ -0,0 +1,17 @@
+name: Link with mold
+description: >
+  Install mold and link this job's x86_64 Linux Rust builds with it in place
+  of the bundled rust-lld: fewer threads and less random writing. C builds keep
+  the system linker. MOLD_RUSTFLAGS carries the flags for a step that sets the
+  target's rustflags itself or builds through RUSTFLAGS, which overrides them.
+runs:
+  using: composite
+  steps:
+    - uses: rui314/setup-mold@v1
+      with:
+        make-default: false
+    - shell: bash
+      run: |
+        flags="-C linker-features=-lld -C link-arg=-fuse-ld=mold"
+        echo "MOLD_RUSTFLAGS=$flags" >> "$GITHUB_ENV"
+        echo "CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_RUSTFLAGS=$flags" >> "$GITHUB_ENV"

.github/workflows/ci.yml

@@ -121,6 +121,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v7.0.1
+      - uses: ./.github/actions/mold
       - uses: dtolnay/rust-toolchain@stable
         with:
           components: rustfmt, clippy
@@ -251,6 +252,7 @@ jobs:
           # Build + test + header-diff job (no push); don't leave the token
           # in the checkout's git config.
           persist-credentials: false
+      - uses: ./.github/actions/mold
       - uses: dtolnay/rust-toolchain@stable
         with:
           components: clippy
@@ -383,6 +385,8 @@ jobs:
     steps:
       - uses: actions/checkout@v7.0.1
         if: needs.changes.outputs.code == 'true'
+      - uses: ./.github/actions/mold
+        if: needs.changes.outputs.code == 'true' && runner.os == 'Linux'
       - uses: dtolnay/rust-toolchain@stable
         if: needs.changes.outputs.code == 'true'
       - name: Prepend toolchain bin to PATH (macos shim workaround)
@@ -460,6 +464,8 @@ jobs:
         if: needs.changes.outputs.code == 'true'
         with:
           persist-credentials: false
+      - uses: ./.github/actions/mold
+        if: needs.changes.outputs.code == 'true' && runner.os == 'Linux'
       - uses: dtolnay/rust-toolchain@stable
         if: needs.changes.outputs.code == 'true'
       - name: Prepend toolchain bin to PATH (macos shim workaround)
@@ -494,6 +500,8 @@ jobs:
         if: needs.changes.outputs.code == 'true'
         with:
           persist-credentials: false
+      - uses: ./.github/actions/mold
+        if: needs.changes.outputs.code == 'true'
       - uses: dtolnay/rust-toolchain@stable
         if: needs.changes.outputs.code == 'true'
       - uses: Swatinem/rust-cache@v2
@@ -539,6 +547,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v7.0.1
+      - uses: ./.github/actions/mold
       - uses: dtolnay/rust-toolchain@stable
         with:
           toolchain: "1.92.0"
@@ -565,6 +574,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v7.0.1
+      - uses: ./.github/actions/mold
       - uses: dtolnay/rust-toolchain@stable
         with:
           components: clippy
@@ -854,6 +864,7 @@ jobs:
       - uses: actions/checkout@v7.0.1
         with:
           persist-credentials: false
+      - uses: ./.github/actions/mold
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
         with:
@@ -932,10 +943,13 @@ jobs:
       - uses: Swatinem/rust-cache@v2
       - uses: taiki-e/install-action@cargo-llvm-cov
       - uses: taiki-e/install-action@nextest
+      - uses: ./.github/actions/mold
       # Through nextest, as the test job runs: a process per test. Some tests
       # set process-wide state (the CLI's SIGINT disposition), and the plain
-      # harness would run them as threads of one process.
-      - run: cargo llvm-cov nextest --profile ci -p structured-zstd --features hash,std,dict-builder --lcov --output-path lcov.info
+      # harness would run them as threads of one process. cargo-llvm-cov sets
+      # RUSTFLAGS, which overrides the target's rustflags, so the mold flags
+      # go in through RUSTFLAGS and it appends its instrumentation to them.
+      - run: RUSTFLAGS="$MOLD_RUSTFLAGS" cargo llvm-cov nextest --profile ci -p structured-zstd --features hash,std,dict-builder --lcov --output-path lcov.info
         working-directory: zstd
       - uses: codecov/codecov-action@v7
         with:
@@ -980,7 +994,9 @@ jobs:
     # `-max_total_time` seconds of fresh fuzzing on top.
     name: Fuzz smoke
     needs: lint
-    timeout-minutes: 20
+    # Six targets at 90 s each plus the sanitizer builds run close to
+    # twenty minutes on a cold cache.
+    timeout-minutes: 30
     runs-on: ubuntu-latest
     env:
       # Override `rust-toolchain.toml` (which pins stable) so `cargo fuzz`
@@ -1001,6 +1017,11 @@ jobs:
     steps:
       - uses: actions/checkout@v7.0.1
       - uses: dtolnay/rust-toolchain@nightly
+      - uses: ./.github/actions/mold
+      # cargo-fuzz builds with RUSTFLAGS of its own, appending any it is
+      # given, so the mold flags reach it through RUSTFLAGS.
+      - name: Link the fuzz builds with mold
+        run: echo "RUSTFLAGS=$MOLD_RUSTFLAGS" >> "$GITHUB_ENV"
       - uses: Swatinem/rust-cache@v2
         with:
           prefix-key: fuzz
@@ -1255,6 +1276,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v7.0.1
+      - uses: ./.github/actions/mold
       - uses: dtolnay/rust-toolchain@stable
       - name: Install benchmark target
         run: rustup target add ${{ matrix.bench.target_triple }}
@@ -1290,9 +1312,12 @@ jobs:
           # decompress/level_-1_fast/decodecorpus-z000033/c_stream on
           # i9-9900K; the win comes from _bzhi_u64-backed
           # mask_lower_bits in the FSE state-update hot path.
-          CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_RUSTFLAGS: "-C target-cpu=x86-64-v3"
+          #
+          # The gnu target's baseline is added in the script below, beside
+          # the mold link flags the job's environment already holds.
           CARGO_TARGET_X86_64_UNKNOWN_LINUX_MUSL_RUSTFLAGS: "-C target-cpu=x86-64-v3"
         run: |
+          export CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_RUSTFLAGS="-C target-cpu=x86-64-v3 $MOLD_RUSTFLAGS"
           # `--no-run` builds without executing; `--message-format=json`
           # exposes the resolved binary path in the build log so we
           # can ship just the executable to shards. Building BOTH