Warn when deleting files, and error on stray pubspec.yamls

lib/src/entrypoint.dart

@@ -1343,6 +1343,7 @@ See https://dart.dev/go/sdk-constraint
   /// but still has an old package config or lockfile.
   void _removeStrayLockAndConfigFiles() {
     final visited = <String>{};
+    var deletedAny = false;
     for (final package in workspaceRoot.transitiveWorkspace) {
       if (package.pubspec.resolution == Resolution.workspace) {
         for (final dir in parentDirs(package.dir)) {
@@ -1353,11 +1354,26 @@ See https://dart.dev/go/sdk-constraint
             // No reason to delete from the same directory twice.
             break;
           }
-          deleteEntry(p.join(dir, 'pubspec.lock'));
-          deleteEntry(p.join(dir, '.dart_tool', 'package_config.json'));
+          void deleteIfPresent(String path, String type) {
+            fileExists(path);
+            log.warning('Deleting old $type: `$path`.');
+            deleteEntry(path);
+            deletedAny = true;
+          }
+
+          deleteIfPresent(p.join(dir, 'pubspec.lock'), 'lock-file');
+          deleteIfPresent(
+            p.join(dir, '.dart_tool', 'package_config.json'),
+            'package config',
+          );
         }
       }
     }
+    if (deletedAny) {
+      log.warning(
+        'See https://dart.dev/go/workspaces-no-inbetween-packages for details.',
+      );
+    }
   }
 
   /// Returns a list of changes to constraints of workspace pubspecs updated to

lib/src/package.dart

@@ -435,9 +435,12 @@ $symlinkResolvedDir => ${p.canonicalize(symlinkResolvedParent)}
 /// * If a package name occurs twice.
 /// * If two packages in the workspace override the same package name.
 /// * A workspace package is overridden.
+/// * A pubspec not included in the workspace exists in a directory
+///   between the root and a workspace package.
 void validateWorkspace(Package root) {
   if (root.workspaceChildren.isEmpty) return;
 
+  /// Maps the `p.canonicalize`d dir of each workspace-child to its parent.
   final includedFrom = <String, String>{};
   final stack = [root];
 
@@ -500,6 +503,38 @@ Consider removing one of the overrides.
 Cannot override workspace packages.
 
 Package `$override` at `${overriddenWorkspacePackage.presentationDir}` is overridden in `${package.pubspecPath}`.
+''');
+      }
+    }
+  }
+
+  // Check for pubspec.yaml files between the root and any workspace package.
+  final visited = <String>{};
+  for (final package in root.transitiveWorkspace) {
+    // Run through all parent directories until we meet another workspace
+    // package.
+    for (final dir in parentDirs(package.dir).skip(1)) {
+      // Stop
+      if (includedFrom.containsKey(p.canonicalize(dir)) ||
+          p.equals(dir, root.dir)) {
+        break;
+      }
+      if (!visited.add(dir)) {
+        // We have been here before.
+        break;
+      }
+      final pubspecCandidate = p.join(dir, 'pubspec.yaml');
+      if (fileExists(pubspecCandidate)) {
+        fail('''
+The file `$pubspecCandidate` is located in a directory between the workspace root at
+`${root.dir}` and a workspace package at `${package.dir}`. But is not a member of the
+workspace.
+
+This blocks the resolution of the package at `${package.dir}`.
+
+Consider removing it.
+
+See https://dart.dev/go/workspaces-no-inbetween-packages for details.
 ''');
       }
     }

test/workspace_test.dart

@@ -809,7 +809,39 @@ foo:foomain''',
     );
   });
 
-  test('Removes lock files and package configs from workspace members',
+  test('Reports error if pubspec inside workspace is not part of the workspace',
+      () async {
+    await dir(appPath, [
+      libPubspec(
+        'myapp',
+        '1.2.3',
+        extras: {
+          'workspace': ['pkgs/a', 'pkgs/a/example'],
+        },
+        sdk: '^3.5.0',
+      ),
+      dir('pkgs', [
+        libPubspec('not_in_workspace', '1.0.0'),
+        dir(
+          'a',
+          [
+            libPubspec('a', '1.1.1', resolutionWorkspace: true),
+            dir('example', [
+              libPubspec('example', '0.0.0', resolutionWorkspace: true),
+            ]),
+          ],
+        ),
+      ]),
+    ]).create();
+    await pubGet(
+      environment: {'_PUB_TEST_SDK_VERSION': '3.5.0'},
+      error: contains(
+        'The file `./pkgs/pubspec.yaml` is located in a directory between the workspace root',
+      ),
+    );
+  });
+
+  test('Removes lock files and package configs from inside the workspace',
       () async {
     await dir(appPath, [
       libPubspec(
@@ -865,7 +897,22 @@ foo:foomain''',
     createLockFileAndPackageConfig(pkgsDir);
     createLockFileAndPackageConfig(inside);
 
-    await pubGet(environment: {'_PUB_TEST_SDK_VERSION': '3.5.0'});
+    await pubGet(
+      environment: {'_PUB_TEST_SDK_VERSION': '3.5.0'},
+      warning: allOf(
+        contains('Deleting old lock-file: `./pkgs/a/pubspec.lock'),
+        contains(
+          'Deleting old package config: `./pkgs/a/.dart_tool/package_config.json`',
+        ),
+        contains('Deleting old lock-file: `./pkgs/pubspec.lock'),
+        contains(
+          'Deleting old package config: `./pkgs/.dart_tool/package_config.json`',
+        ),
+        contains(
+          'See https://dart.dev/go/workspaces-no-inbetween-packages for details.',
+        ),
+      ),
+    );
 
     validateLockFileAndPackageConfig(
       outideWorkpace,