Parse invalid lambdas without crashing (#6826)

Fixes a compiler crash that occurs when a malformed lambda is provided
as an operand to an operator that strictly expects an expression

Changes:
- Emits an `InvalidParse` dummy node at the current position to act as a
placeholder for the missing body
- Changed state transitions so that `LambdaIntroducer` gets properly
wrapped into a `Lambda` node


Closes #6823

---------

Co-authored-by: Dana Jansens <danakj@orodu.net>
Co-authored-by: Carbon Infra Bot <carbon-external-infra@google.com>

toolchain/parse/handle_lambda.cpp

@@ -87,8 +87,15 @@ auto HandleLambdaBody(Context& context) -> void {
                       "expected `=>` or `{{` after return type");
     context.emitter().Emit(*context.position(),
                            ExpectedLambdaBodyAfterReturnType);
+
+    // Add a dummy node for the missing body without consuming the current
+    // token.
+    context.AddLeafNode(NodeKind::InvalidParse, *context.position(),
+                        /*has_error=*/true);
+
     state.has_error = true;
-    context.ReturnErrorOnState();
+    // Bundle all nodes into a complete lambda node.
+    context.PushState(state, StateKind::LambdaBodyFinish);
   }
 }
 

toolchain/parse/testdata/lambda/lambda.carbon

@@ -14,6 +14,37 @@ var a: auto = fn [T:! type] (x: T) -> T { return x; };
 var b: auto = fn (x: i32) => x;
 var c: auto = fn { };
 
+// --- fail_unconsumed_lambda_introducer.carbon
+
+// Tests that the compiler does not crash when a LambdaIntroducer (`fn`)
+// is left unconsumed inside an invalid binary operator sequence.
+
+// CHECK:STDERR: fail_unconsumed_lambda_introducer.carbon:[[@LINE+12]]:10: error: whitespace missing around binary operator [BinaryOperatorRequiresWhitespace]
+// CHECK:STDERR: require j%-fn->>
+// CHECK:STDERR:          ^
+// CHECK:STDERR:
+// CHECK:STDERR: fail_unconsumed_lambda_introducer.carbon:[[@LINE+8]]:16: error: expected expression [ExpectedExpr]
+// CHECK:STDERR: require j%-fn->>
+// CHECK:STDERR:                ^
+// CHECK:STDERR:
+// CHECK:STDERR: fail_unconsumed_lambda_introducer.carbon:[[@LINE+4]]:16: error: whitespace missing before binary operator [BinaryOperatorRequiresWhitespace]
+// CHECK:STDERR: require j%-fn->>
+// CHECK:STDERR:                ^
+// CHECK:STDERR:
+require j%-fn->>
+// CHECK:STDERR: fail_unconsumed_lambda_introducer.carbon:[[@LINE+12]]:1: error: expected expression [ExpectedExpr]
+// CHECK:STDERR:
+// CHECK:STDERR: ^
+// CHECK:STDERR:
+// CHECK:STDERR: fail_unconsumed_lambda_introducer.carbon:[[@LINE+8]]:1: error: expected `=>` or `{` after return type [ExpectedLambdaBodyAfterReturnType]
+// CHECK:STDERR:
+// CHECK:STDERR: ^
+// CHECK:STDERR:
+// CHECK:STDERR: fail_unconsumed_lambda_introducer.carbon:[[@LINE+4]]:1: error: expected `impls` in `require` declaration [RequireExpectedImpls]
+// CHECK:STDERR:
+// CHECK:STDERR: ^
+// CHECK:STDERR:
+
 // --- fail_expected_body.carbon
 
 fn F() {
@@ -93,6 +124,23 @@ fn G() {
 // CHECK:STDOUT:     {kind: 'VariableDecl', text: ';', subtree_size: 11},
 // CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT:   ]
+// CHECK:STDOUT: - filename: fail_unconsumed_lambda_introducer.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:       {kind: 'RequireIntroducer', text: 'require'},
+// CHECK:STDOUT:         {kind: 'IdentifierNameExpr', text: 'j'},
+// CHECK:STDOUT:             {kind: 'LambdaIntroducer', text: 'fn'},
+// CHECK:STDOUT:                 {kind: 'InvalidParse', text: '>', has_error: yes},
+// CHECK:STDOUT:                 {kind: 'InvalidParse', text: '', has_error: yes},
+// CHECK:STDOUT:               {kind: 'InfixOperatorGreater', text: '>', has_error: yes, subtree_size: 3},
+// CHECK:STDOUT:             {kind: 'ReturnType', text: '->', subtree_size: 4},
+// CHECK:STDOUT:             {kind: 'InvalidParse', text: '', has_error: yes},
+// CHECK:STDOUT:           {kind: 'Lambda', text: 'fn', has_error: yes, subtree_size: 7},
+// CHECK:STDOUT:         {kind: 'PrefixOperatorMinus', text: '-', subtree_size: 8},
+// CHECK:STDOUT:       {kind: 'InfixOperatorPercent', text: '%', subtree_size: 10},
+// CHECK:STDOUT:     {kind: 'RequireDecl', text: '>', has_error: yes, subtree_size: 12},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]
 // CHECK:STDOUT: - filename: fail_expected_body.carbon
 // CHECK:STDOUT:   parse_tree: [
 // CHECK:STDOUT:     {kind: 'FileStart', text: ''},
@@ -126,10 +174,12 @@ fn G() {
 // CHECK:STDOUT:           {kind: 'VarBindingPattern', text: ':', subtree_size: 3},
 // CHECK:STDOUT:         {kind: 'VariablePattern', text: 'var', subtree_size: 4},
 // CHECK:STDOUT:         {kind: 'VariableInitializer', text: '='},
-// CHECK:STDOUT:         {kind: 'LambdaIntroducer', text: 'fn'},
-// CHECK:STDOUT:           {kind: 'IntTypeLiteral', text: 'i32'},
-// CHECK:STDOUT:         {kind: 'ReturnType', text: '->', subtree_size: 2},
-// CHECK:STDOUT:       {kind: 'VariableDecl', text: ';', has_error: yes, subtree_size: 10},
-// CHECK:STDOUT:     {kind: 'FunctionDefinition', text: '}', subtree_size: 16},
+// CHECK:STDOUT:           {kind: 'LambdaIntroducer', text: 'fn'},
+// CHECK:STDOUT:             {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:           {kind: 'ReturnType', text: '->', subtree_size: 2},
+// CHECK:STDOUT:           {kind: 'InvalidParse', text: ';', has_error: yes},
+// CHECK:STDOUT:         {kind: 'Lambda', text: 'fn', has_error: yes, subtree_size: 5},
+// CHECK:STDOUT:       {kind: 'VariableDecl', text: ';', subtree_size: 12},
+// CHECK:STDOUT:     {kind: 'FunctionDefinition', text: '}', subtree_size: 18},
 // CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT:   ]