Filter LSP completion requests when offset is mid-symbol

Change-Id: I28bf5ddda1d7d6f1177a602d9f2dd3091f35966d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151847
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Danny Tuppeny <danny@tuppeny.com>

pkg/analysis_server/lib/src/lsp/handlers/handler_completion.dart

@@ -362,26 +362,38 @@ class CompletionHandler
   }
 
   /// Return the pattern to match suggestions against, from the identifier
-  /// to the left of the caret. Return the empty string if cannot find the
-  /// identifier.
+  /// to the left of (or spanning) the caret. Return the empty string if cannot
+  /// find the identifier.
+  ///
+  /// If the caret is within the identifier, the returned pattern will be truncated
+  /// to the position of the caret. For example at:
+  ///
+  ///     new MyClass^Foo
+  ///
+  /// will return "MyClass" as the search pattern.
   String _prefixMatchingPattern(DartCompletionRequestImpl request) {
-    final nodeAtOffsetVisitor =
-        _IdentifierEndingAtOffsetVisitor(request.offset);
-    request.target.containingNode.accept(nodeAtOffsetVisitor);
+    final nodeSpanningOffsetVisitor =
+        _IdentifierSpanningOffsetVisitor(request.offset);
+    request.target.containingNode.accept(nodeSpanningOffsetVisitor);
+    final node = nodeSpanningOffsetVisitor.matchingNode;
 
-    return nodeAtOffsetVisitor.matchingNode?.name ?? '';
+    final prefix = node != null && request.offset - node.offset < node.length
+        ? node.name.substring(0, request.offset - node.offset)
+        : node?.name;
+
+    return prefix ?? '';
   }
 }
 
-/// An AST visitor to locate a [SimpleIdentifier] that ends at the provided offset.
-class _IdentifierEndingAtOffsetVisitor extends RecursiveAstVisitor<void> {
+/// An AST visitor to locate a [SimpleIdentifier] that spans the provided offset.
+class _IdentifierSpanningOffsetVisitor extends RecursiveAstVisitor<void> {
   final int offset;
   SimpleIdentifier _matchingNode;
-  _IdentifierEndingAtOffsetVisitor(this.offset);
+  _IdentifierSpanningOffsetVisitor(this.offset);
   SimpleIdentifier get matchingNode => _matchingNode;
   @override
   void visitSimpleIdentifier(SimpleIdentifier node) {
-    if (node.end == offset) {
+    if (node.offset <= offset && node.end >= offset) {
       _matchingNode = node;
     }
   }

pkg/analysis_server/test/lsp/completion_test.dart

@@ -417,7 +417,7 @@ class CompletionTest extends AbstractLspAnalysisServerTest {
     expect(updated, contains('a.abcdefghij'));
   }
 
-  Future<void> test_prefixFilter() async {
+  Future<void> test_prefixFilter_endOfSymbol() async {
     final content = '''
     class UniqueNamedClassForLspOne {}
     class UniqueNamedClassForLspTwo {}
@@ -425,7 +425,7 @@ class CompletionTest extends AbstractLspAnalysisServerTest {
 
     main() {
       // Should match only Two and Three
-      class UniqueNamedClassForLspT^
+      UniqueNamedClassForLspT^
     }
     ''';
 
@@ -437,6 +437,46 @@ class CompletionTest extends AbstractLspAnalysisServerTest {
     expect(res.any((c) => c.label == 'UniqueNamedClassForLspThree'), isTrue);
   }
 
+  Future<void> test_prefixFilter_midSymbol() async {
+    final content = '''
+    class UniqueNamedClassForLspOne {}
+    class UniqueNamedClassForLspTwo {}
+    class UniqueNamedClassForLspThree {}
+
+    main() {
+      // Should match only Two and Three
+      UniqueNamedClassForLspT^hree
+    }
+    ''';
+
+    await initialize();
+    await openFile(mainFileUri, withoutMarkers(content));
+    final res = await getCompletion(mainFileUri, positionFromMarker(content));
+    expect(res.any((c) => c.label == 'UniqueNamedClassForLspOne'), isFalse);
+    expect(res.any((c) => c.label == 'UniqueNamedClassForLspTwo'), isTrue);
+    expect(res.any((c) => c.label == 'UniqueNamedClassForLspThree'), isTrue);
+  }
+
+  Future<void> test_prefixFilter_startOfSymbol() async {
+    final content = '''
+    class UniqueNamedClassForLspOne {}
+    class UniqueNamedClassForLspTwo {}
+    class UniqueNamedClassForLspThree {}
+
+    main() {
+      // Should match all three
+      ^UniqueNamedClassForLspT
+    }
+    ''';
+
+    await initialize();
+    await openFile(mainFileUri, withoutMarkers(content));
+    final res = await getCompletion(mainFileUri, positionFromMarker(content));
+    expect(res.any((c) => c.label == 'UniqueNamedClassForLspOne'), isTrue);
+    expect(res.any((c) => c.label == 'UniqueNamedClassForLspTwo'), isTrue);
+    expect(res.any((c) => c.label == 'UniqueNamedClassForLspThree'), isTrue);
+  }
+
   Future<void> test_suggestionSets() async {
     newFile(
       join(projectFolderPath, 'other_file.dart'),