Improve error recovery in binding patterns (#4309)

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

Co-authored-by: Josh L <josh11b@users.noreply.github.com>

toolchain/parse/handle_binding_pattern.cpp

@@ -26,14 +26,12 @@ auto HandleBindingPattern(Context& context) -> void {
 
   // Handle an invalid pattern introducer for parameters and variables.
   auto on_error = [&]() {
-    CARBON_DIAGNOSTIC(ExpectedBindingPattern, Error,
-                      "Expected binding pattern.");
-    context.emitter().Emit(*context.position(), ExpectedBindingPattern);
-    // Add a placeholder for the type.
-    context.AddLeafNode(NodeKind::InvalidParse, *context.position(),
-                        /*has_error=*/true);
-    state.has_error = true;
-    context.PushState(state, State::BindingPatternFinishAsRegular);
+    if (!state.has_error) {
+      CARBON_DIAGNOSTIC(ExpectedBindingPattern, Error,
+                        "Expected binding pattern.");
+      context.emitter().Emit(*context.position(), ExpectedBindingPattern);
+      state.has_error = true;
+    }
   };
 
   // The first item should be an identifier or `self`.
@@ -53,7 +51,6 @@ auto HandleBindingPattern(Context& context) -> void {
     context.AddLeafNode(NodeKind::IdentifierName, *context.position(),
                         /*has_error=*/true);
     on_error();
-    return;
   }
 
   if (auto kind = context.PositionKind();
@@ -67,7 +64,10 @@ auto HandleBindingPattern(Context& context) -> void {
     context.PushStateForExpr(PrecedenceGroup::ForType());
   } else {
     on_error();
-    return;
+    // Add a placeholder for the type.
+    context.AddLeafNode(NodeKind::InvalidParse, *context.position(),
+                        /*has_error=*/true);
+    context.PushState(state, State::BindingPatternFinishAsRegular);
   }
 }
 

toolchain/parse/testdata/let/fail_missing_name.carbon

@@ -0,0 +1,49 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/parse/testdata/let/fail_missing_name.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/let/fail_missing_name.carbon
+
+// --- fail_runtime_binding.carbon
+
+// CHECK:STDERR: fail_runtime_binding.carbon:[[@LINE+4]]:5: ERROR: Expected binding pattern.
+// CHECK:STDERR: let : i32 = 4;
+// CHECK:STDERR:     ^
+// CHECK:STDERR:
+let : i32 = 4;
+
+// --- fail_complietime_binding.carbon
+
+// CHECK:STDERR: fail_complietime_binding.carbon:[[@LINE+3]]:5: ERROR: Expected binding pattern.
+// CHECK:STDERR: let :! bool = true;
+// CHECK:STDERR:     ^~
+let :! bool = true;
+
+// CHECK:STDOUT: - filename: fail_runtime_binding.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:       {kind: 'LetIntroducer', text: 'let'},
+// CHECK:STDOUT:         {kind: 'IdentifierName', text: ':', has_error: yes},
+// CHECK:STDOUT:         {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:       {kind: 'BindingPattern', text: ':', has_error: yes, subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'LetInitializer', text: '='},
+// CHECK:STDOUT:       {kind: 'IntLiteral', text: '4'},
+// CHECK:STDOUT:     {kind: 'LetDecl', text: ';', subtree_size: 7},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]
+// CHECK:STDOUT: - filename: fail_complietime_binding.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:       {kind: 'LetIntroducer', text: 'let'},
+// CHECK:STDOUT:         {kind: 'IdentifierName', text: ':!', has_error: yes},
+// CHECK:STDOUT:         {kind: 'BoolTypeLiteral', text: 'bool'},
+// CHECK:STDOUT:       {kind: 'CompileTimeBindingPattern', text: ':!', has_error: yes, subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'LetInitializer', text: '='},
+// CHECK:STDOUT:       {kind: 'BoolLiteralTrue', text: 'true'},
+// CHECK:STDOUT:     {kind: 'LetDecl', text: ';', subtree_size: 7},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]