CI: Set up explicit cache keys to share cache with exhaustive build.

Also upgrade rust-cache action to 2.7.0.

.github/workflows/ci.yml

@@ -116,7 +116,14 @@ jobs:
     - run: df -h .
 
     # Load cache before doing any Rust builds
-    - uses: Swatinem/[email protected]
+    - uses: Swatinem/[email protected]
+      with:
+        # This is not necessary for keying, but makes the GHA cache viewing page more helpful.
+        prefix-key: "v1-rust-build-${{ matrix.os }}-${{ matrix.toolchain }}-${{ matrix.depversions }}"
+        # Override the default job-id-based key explicitly so that our dependent jobs can
+        # reuse this cache.
+        shared-key: "build"
+
 
     - run: df -h .
 
@@ -245,8 +252,13 @@ jobs:
       # libasound2-dev: needed for audio via `kira`
 
     # Load cache before doing any Rust builds
-    # TODO: Do we actually want to cache this, or let it be slow but low storage?
-    - uses: Swatinem/[email protected]
+    - uses: Swatinem/[email protected]
+      with:
+        # Reuse the cache from the normal `build` job instead of creating an independent one,
+        # to reduce redundant work and save disk space — but don't *write* to that cache.
+        prefix-key: "v1-rust-build-ubuntu-stable-locked"
+        shared-key: "build"
+        save-if: false
 
     # break this out as a separate non-silenced build step
     - name: Compile xtask
@@ -288,8 +300,11 @@ jobs:
     steps:
     - uses: actions/[email protected]
 
-    - uses: Swatinem/[email protected]
-
+    - uses: Swatinem/[email protected]
+      with:
+        # This is not necessary for keying, but makes the GHA cache viewing page more helpful.
+        prefix-key: "v1-rust-miri"
+
     - name: Install Rust toolchain
       run: |
         rustup toolchain install nightly --component miri
@@ -311,7 +326,7 @@ jobs:
     - uses: actions/[email protected]
 
     # The build is faster *without* this cache.
-    # - uses: Swatinem/rust-cache@v2.0.2
+    # - uses: Swatinem/rust-cache@v2.7.0
 
     - name: Install Rust toolchain
       run: |
@@ -335,8 +350,10 @@ jobs:
     - uses: actions/[email protected]
 
     # Cache both the main workspace (for xtask builds) and the fuzzing workspace
-    - uses: Swatinem/rust-cache@v2.0.2
+    - uses: Swatinem/rust-cache@v2.7.0
       with:
+        # This is not necessary for keying, but makes the GHA cache viewing page more helpful.
+        prefix-key: "v1-rust-fuzz"
         workspaces: |
           .
           fuzz

.github/workflows/unstable-ci.yml

@@ -80,7 +80,10 @@ jobs:
       # libasound2-dev: needed for audio via `kira`
 
     # Load cache before doing any Rust builds
-    - uses: Swatinem/[email protected]
+    - uses: Swatinem/[email protected]
+      with:
+        # This is not necessary for keying, but makes the GHA cache viewing page more helpful.
+        prefix-key: "v1-rust-unstablefeatures-${{ matrix.os }}-${{ matrix.depversions }}"
 
     # break this out as a separate non-silenced build step
     - name: Compile xtask
@@ -137,8 +140,11 @@ jobs:
     steps:
     - uses: actions/[email protected]
 
-    - uses: Swatinem/[email protected]
-
+    - uses: Swatinem/[email protected]
+      with:
+        # This is not necessary for keying, but makes the GHA cache viewing page more helpful.
+        prefix-key: "v1-rust-unstablefeatures-miri"
+
     - name: Install Rust toolchain
       run: |
         rustup toolchain install nightly --component miri