ci: reuse compiler state across validation runs (#3956)

* ci: retain zig build caches across validation runs

* ci: reduce debug information in validation builds

* ci: build only the ghostty test target for scalar checks

* ci: measure debug profiles and remove duplicate macos caching

* ci: compare scalar build scopes on one runner

* ci: keep the zig compiler path stable across runs

* ci: run macos and windows lint alongside tests

* ci: inspect remaining zig cache misses

* ci: remove completed measurement probes

* ci: compare windows linkers without changing release builds

* ci: test linux incremental reuse and correct the linker probe

* ci: measure windows incremental state reuse

* test: verify incremental reuse after a source change

* ci: finalize measured compiler cache optimizations

1 files changed, 65 insertions, 13 deletions

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 535cd871..a481594e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -11,6 +11,9 @@ permissions:
 
 env:
   RUST_TOOLCHAIN_VERSION: 1.96.1
+  # Keep file/line backtraces without generating type and variable debug information.
+  CARGO_PROFILE_DEV_DEBUG: "1"
+  CARGO_PROFILE_TEST_DEBUG: "1"
 
 concurrency:
   group: ci-${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
@@ -89,13 +92,28 @@ jobs:
         uses: mlugg/setup-zig@d1434d08867e3ee9daa34448df10607b98908d29 # v2.2.1
         with:
           version: 0.16.0
-          use-cache: ${{ matrix.kind != 'windows' }}
+          use-cache: ${{ runner.os == 'Linux' }}
+          # Linux builds both SIMD/ReleaseFast and scalar/ReleaseSafe variants.
+          cache-size-limit: 4096
 
-      - name: Configure Zig cache on macOS
-        if: runner.os == 'macOS'
+      # Zig's C-header cache keys include the compiler installation path.
+      - name: Stabilize Zig installation path on Unix
+        if: matrix.kind == 'unix'
+        run: |
+          stable_zig="$RUNNER_TEMP/herdr-zig-0.16.0"
+          mv "$(dirname "$(command -v zig)")" "$stable_zig"
+          echo "$stable_zig" >> "$GITHUB_PATH"
+          echo "ZIG=$stable_zig/zig" >> "$GITHUB_ENV"
+
+      - name: Stabilize Zig installation path on Windows
+        if: matrix.kind == 'windows'
+        shell: pwsh
         run: |
-          echo "ZIG_GLOBAL_CACHE_DIR=$GITHUB_WORKSPACE/.zig-cache" >> "$GITHUB_ENV"
-          echo "ZIG_LOCAL_CACHE_DIR=$GITHUB_WORKSPACE/.zig-cache" >> "$GITHUB_ENV"
+          $source = Split-Path (Get-Command zig).Source
+          $stable = Join-Path $env:RUNNER_TEMP "herdr-zig-0.16.0"
+          Move-Item -LiteralPath $source -Destination $stable
+          $stable | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
+          "ZIG=$(Join-Path $stable 'zig.exe')" | Out-File -FilePath $env:GITHUB_ENV -Encoding utf8 -Append
 
       - name: Restore Zig build cache
         if: runner.os == 'macOS' || matrix.kind == 'windows'
@@ -106,18 +124,33 @@ jobs:
           restore-keys: |
             zig-build-v1-${{ runner.os }}-${{ runner.arch }}-0.16.0-
 
-      - name: Install macOS build tools
-        if: runner.os == 'macOS'
-        run: HOMEBREW_NO_AUTO_UPDATE=1 brew install cmake ninja
-
       - name: Restore cargo cache
         uses: Swatinem/rust-cache@e18b497796c12c097a38f9edb9d0641fb99eee32 # v2
         with:
           cache-bin: false
           key: ${{ matrix.os }}
 
-      - name: Run checks
-        if: matrix.kind == 'unix'
+      # Save before rust-cache's post-step removes incremental state. Cargo still
+      # rebuilds workspace targets; only rustc's validated query state is retained.
+      - name: Restore Windows incremental compiler state
+        if: matrix.kind == 'windows'
+        uses: actions/cache@0057852bfaa89a56745cba8c7296529d2fc39830 # v4
+        with:
+          path: target/debug/incremental
+          key: ci-incremental-v1-${{ runner.os }}-${{ runner.arch }}-${{ env.RUST_TOOLCHAIN_VERSION }}-${{ env.CARGO_PROFILE_DEV_DEBUG }}-${{ env.CARGO_PROFILE_TEST_DEBUG }}-${{ hashFiles('Cargo.lock', 'Cargo.toml', '.cargo/config.toml', 'build.rs') }}-${{ github.sha }}
+          restore-keys: |
+            ci-incremental-v1-${{ runner.os }}-${{ runner.arch }}-${{ env.RUST_TOOLCHAIN_VERSION }}-${{ env.CARGO_PROFILE_DEV_DEBUG }}-${{ env.CARGO_PROFILE_TEST_DEBUG }}-${{ hashFiles('Cargo.lock', 'Cargo.toml', '.cargo/config.toml', 'build.rs') }}-
+
+      - name: Run Linux lint
+        if: runner.os == 'Linux'
+        run: just lint
+
+      - name: Run Linux tests
+        if: runner.os == 'Linux'
+        run: CARGO_INCREMENTAL=1 just ci-tests '${{ matrix.nextest_filter }}'
+
+      - name: Run macOS checks
+        if: runner.os == 'macOS'
         run: just ci '${{ matrix.nextest_filter }}'
 
       - name: Check scalar libghostty with runtime safety
@@ -125,12 +158,14 @@ jobs:
         env:
           LIBGHOSTTY_VT_SIMD: "false"
           LIBGHOSTTY_VT_OPTIMIZE: ReleaseSafe
-        run: just test-one ghostty
+        run: CARGO_INCREMENTAL=1 just test-one ghostty
 
       - name: Run Windows checks
         if: matrix.kind == 'windows'
         shell: pwsh
-        run: just check
+        run: |
+          $env:CARGO_INCREMENTAL = "1"
+          just check
 
       - name: Smoke ConPTY pane
         if: matrix.kind == 'windows'
@@ -162,6 +197,23 @@ jobs:
           version: 0.16.0
           use-cache: false
 
+      - name: Stabilize Zig installation path
+        shell: pwsh
+        run: |
+          $source = Split-Path (Get-Command zig).Source
+          $stable = Join-Path $env:RUNNER_TEMP "herdr-zig-0.16.0"
+          Move-Item -LiteralPath $source -Destination $stable
+          $stable | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
+          "ZIG=$(Join-Path $stable 'zig.exe')" | Out-File -FilePath $env:GITHUB_ENV -Encoding utf8 -Append
+
+      - name: Restore Zig build cache
+        uses: actions/cache@0057852bfaa89a56745cba8c7296529d2fc39830 # v4
+        with:
+          path: .zig-cache
+          key: zig-build-v1-conpty-package-windows-2022-${{ runner.arch }}-0.16.0-${{ github.run_id }}-${{ github.run_attempt }}
+          restore-keys: |
+            zig-build-v1-conpty-package-windows-2022-${{ runner.arch }}-0.16.0-
+
       - name: Restore Cargo cache
         uses: Swatinem/rust-cache@e18b497796c12c097a38f9edb9d0641fb99eee32 # v2
         with: