feat: error for behavior change with no matching types

messages/project.decompose.md

@@ -15,12 +15,16 @@ Which sourceBehaviorOption to enable.
 - Switch the project to use decomposed custom labels
   <%= config.bin %> <%= command.id %> --behavior DecomposeCustomLabels --source-dir .
 
-- Switch one packageDirectory to use decomposed custom labels
-  <%= config.bin %> <%= command.id %> --behavior DecomposeCustomLabels --source-dir force-app
+- Without changing any existing files, see what the command would have produced.
+  <%= config.bin %> <%= command.id %> --behavior DecomposeCustomLabels --dry-run
 
 # flags.dry-run.summary
 
-Explain what the command would do but don't modify the project.
+Explain what the command would do.
+
+# flags.dry-run.description
+
+Doesn't modify existing files. Lists files that would be deleted, explains modifications to sfdx-project.json, and outputs the resulting modifications to a new folder for review.
 
 # flags.preserve-temp-dir.summary
 
@@ -50,3 +54,7 @@ The command will move metadata into main/default which doesn't seem like what yo
 # success.dryRun
 
 Files were created in %s outside your package directories for inspection.
+
+# error.noTargetTypes
+
+The project contains no packageDirectories with metadata that matches the specified behavior %s.

src/commands/project/decompose.ts

@@ -56,9 +56,9 @@ export default class ProjectDecompose extends SfCommand<ProjectDecomposeResult>
     if (await flags['target-org']?.supportsSourceTracking()) {
       throw messages.createError('error.trackingNotSupported');
     }
+    const projectJson = getValidatedProjectJson(flags.behavior, this.project!);
     const packageDirsWithDecomposable = await getDecomposablePackageDirectories(this.project!, flags.behavior);
     const filesToDelete = await convertToMdapi(packageDirsWithDecomposable);
-    const projectJson = getValidatedProjectJson(flags.behavior, this.project!);
     const backupPjsonContents = flags['dry-run'] ? await readFile(projectJson.getPath()) : '';
 
     // flip the preset in the sfdx-project.json, even for dry-run, since the registry will need for conversions

src/utils/decomposition.ts

@@ -34,8 +34,8 @@ export const TMP_DIR = process.env.SF_MDAPI_TEMP_DIR ?? 'decompositionConverterT
 export const getDecomposablePackageDirectories = async (
   project: SfProject,
   preset: string
-): Promise<ComponentSetAndPackageDirPath[]> =>
-  (
+): Promise<ComponentSetAndPackageDirPath[]> => {
+  const output = (
     await Promise.all(
       project
         .getPackageDirectories()
@@ -46,6 +46,11 @@ export const getDecomposablePackageDirectories = async (
     .filter(componentSetIsNonEmpty)
     // we do this after filtering componentSets to reduce false positives (ex: dir does not have main/default but also has nothing to decompose)
     .map(validateMainDefault(project.getPath()));
+  if (output.length === 0) {
+    loadMessages().createError('error.noTargetTypes', [preset]);
+  }
+  return output;
+};
 
 /** converts the composed metadata to mdapi format in a temp dir */
 export const convertToMdapi = async (packageDirsWithDecomposable: ComponentSetAndPackageDirPath[]): Promise<string[]> =>