Include env vars in the generated cache key

README.md

@@ -26,6 +26,10 @@ An optional key that is added to the automatic cache key.
 : `sharedKey`
 An additional key that is stable over multiple jobs.
 
+: `envVars`
+A space-separated list of regular expressions that define additional environment variable filters.
+These are added to an additional cache key that's generated from environment variable contents.
+
 : `working-directory`
 The working directory the action operates in, is case the cargo project is not
 located in the repo root.

action.yml

@@ -8,6 +8,9 @@ inputs:
   sharedKey:
     description: "An additional cache key that is stable over multiple jobs"
     required: false
+  envVars:
+    description: "Additional environment variables to include in the cache key, separeted by spaces"
+    required: false
   working-directory:
     description: "The working directory this action should operate in"
     required: false

src/common.ts

@@ -71,6 +71,9 @@ export async function getCacheConfig(): Promise<CacheConfig> {
     }
   }
 
+  const extraEnvKeys = core.getInput("envVars").split(/\s+/);
+
+  key += `${getEnvKey(extraEnvKeys)}-`;
   key += await getRustKey();
 
   return {
@@ -105,6 +108,45 @@ export async function getCargoBins(): Promise<Set<string>> {
   }
 }
 
+/**
+ * Create a key hash, generated from environment variables.
+ *
+ * The available environment variables are filtered by a set of defaults that are common for Rust
+ * projects and should apply to almost all runs, as they modify the Rustc compiler's, Clippy's and
+ * other tools' behavior.
+ *
+ * @param extraKeys additional user-provided keys that are added to the default list. These are
+ * treated as regular expressions ({@link RegExp}), and will each be surrounded by a `^` and `$`,
+ * to make sure they are matched against the whole env var name.
+ * @returns An SHA-1 hash over all the environment variable values, whose names were not filtered
+ * out. The hash is returned as hex-string, **reduced to half its length**.
+ */
+function getEnvKey(extraKeys: string[]): string {
+  const hasher = crypto.createHash("sha1");
+  const defaultValidKeys = [
+    /^CARGO_.+$/,
+    /^CC_.+$/,
+    /^CXX_.+$/,
+    /^RUSTC_.+$/,
+    /^RUSTC$/,
+    /^RUSTDOC$/,
+    /^RUSTDOCFLAGS$/,
+    /^RUSTFLAGS$/,
+    /^RUSTFMT$/,
+  ];
+
+  // Combine default key filters with user-provided ones.
+  const keyFilter = defaultValidKeys.concat(extraKeys.map((key) => new RegExp(`^${key}$`)));
+
+  for (const [key, value] of Object.entries(process.env)) {
+    if (keyFilter.some((re) => re.test(key)) && value) {
+      hasher.update(`${key}=${value}`);
+    }
+  }
+
+  return hasher.digest("hex").slice(0, 20);
+}
+
 async function getRustKey(): Promise<string> {
   const rustc = await getRustVersion();
   return `${rustc.release}-${rustc.host}-${rustc["commit-hash"].slice(0, 12)}`;