chore(core): warn when db cache is disabled (#28929)

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## Current Behavior
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## Expected Behavior
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## Related Issue(s)
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Fixes #

packages/nx/src/tasks-runner/cache.ts

@@ -20,6 +20,7 @@ import { verifyOrUpdateNxCloudClient } from '../nx-cloud/update-manager';
 import { getCloudOptions } from '../nx-cloud/utilities/get-cloud-options';
 import { isCI } from '../utils/is-ci';
 import { output } from '../utils/output';
+import { logger } from '../utils/logger';
 
 export type CachedResult = {
   terminalOutput: string;
@@ -29,13 +30,50 @@ export type CachedResult = {
 };
 export type TaskWithCachedResult = { task: Task; cachedResult: CachedResult };
 
+// This function is called once during tasks runner initialization. It checks if the db cache is enabled and logs a warning if it is not.
 export function dbCacheEnabled(nxJson: NxJsonConfiguration = readNxJson()) {
-  return (
-    !IS_WASM &&
-    process.env.NX_DISABLE_DB !== 'true' &&
-    nxJson.useLegacyCache !== true &&
-    process.env.NX_DB_CACHE !== 'false'
-  );
+  // If the user has explicitly disabled the db cache, we can warn...
+  if (
+    nxJson.useLegacyCache ||
+    process.env.NX_DISABLE_DB === 'true' ||
+    process.env.NX_DB_CACHE === 'false'
+  ) {
+    let readMoreLink = 'https://nx.dev/deprecated/legacy-cache';
+    if (
+      nxJson.tasksRunnerOptions?.default?.runner &&
+      !['nx-cloud', 'nx/tasks-runners/default', '@nrwl/nx-cloud'].includes(
+        nxJson.tasksRunnerOptions.default.runner
+      )
+    ) {
+      readMoreLink += '#tasksrunneroptions';
+    } else if (
+      process.env.NX_REJECT_UNKNOWN_LOCAL_CACHE === '0' ||
+      process.env.NX_REJECT_UNKNOWN_LOCAL_CACHE === 'false'
+    ) {
+      readMoreLink += '#nxrejectunknownlocalcache';
+    }
+    logger.warn(
+      `Nx is configured to use the legacy cache. This cache will be removed in Nx 21. Read more at ${readMoreLink}.`
+    );
+    return false;
+  }
+  // ...but if on wasm and the the db cache isnt supported we shouldn't warn
+  if (IS_WASM) {
+    return false;
+  }
+  // Below this point we are using the db cache.
+  if (
+    // The NX_REJECT_UNKNOWN_LOCAL_CACHE env var is not supported with the db cache.
+    // If the user has tried to use it, we can point them to powerpack as it
+    // provides a similar featureset.
+    process.env.NX_REJECT_UNKNOWN_LOCAL_CACHE === '0' ||
+    process.env.NX_REJECT_UNKNOWN_LOCAL_CACHE === 'false'
+  ) {
+    logger.warn(
+      'NX_REJECT_UNKNOWN_LOCAL_CACHE=0 is not supported with the new database cache. Read more at https://nx.dev/deprecated/legacy-cache#nxrejectunknownlocalcache.'
+    );
+  }
+  return true;
 }
 
 // Do not change the order of these arguments as this function is used by nx cloud