feat: add save-if option, closes #66 (#91)

README.md

@@ -53,6 +53,12 @@ sensible defaults.
     # Determines if the cache should be saved even when the workflow has failed.
     # default: "false"
     cache-on-failure: ""
+
+    # Determiners whether the cache should be saved.
+    # If `false`, the cache is only restored.
+    # Useful for jobs where the matrix is additive e.g. additional Cargo features.
+    # default: "true"
+    save-if: ""
 ```
 
 Further examples are available in the [.github/workflows](./.github/workflows/) directory.

action.yml

@@ -28,6 +28,10 @@ inputs:
   cache-on-failure:
     description: "Cache even if the build fails. Defaults to false."
     required: false
+  save-if:
+    description: "Determiners whether the cache should be saved. If `false`, the cache is only restored."
+    required: false
+    default: "true"
 outputs:
   cache-hit:
     description: "A boolean value that indicates an exact match was found."

src/save.ts

@@ -13,7 +13,9 @@ process.on("uncaughtException", (e) => {
 });
 
 async function run() {
-  if (!cache.isFeatureAvailable()) {
+  const save = core.getInput("save-if").toLowerCase() || "true";
+
+  if (!(cache.isFeatureAvailable() && save === "true")) {
     return;
   }