Fix a crash on invalid found by fuzzing. (#3404)

When a `namespace` keyword has no `;` following it, we recover by
building a parse tree `Namespace` node from the `Namespace` token (as
there isn't a `;` token). Allow this correspondence on errors.

Also teach the diagnostics in this case to avoid the end-of-file token
as that's almost always going to be a less meaningful location. Instead,
we can point at the introducer which should at least be in the code that
led to the error.

toolchain/parse/handle_decl_name_and_params.cpp

@@ -27,7 +27,13 @@ static auto HandleDeclNameAndParams(Context& context, State after_name)
     CARBON_DIAGNOSTIC(ExpectedDeclName, Error,
                       "`{0}` introducer should be followed by a name.",
                       Lex::TokenKind);
-    context.emitter().Emit(*context.position(), ExpectedDeclName,
+    Lex::Token location = *context.position();
+    if (context.tokens().GetKind(location) == Lex::TokenKind::EndOfFile) {
+      // The end of file is often an especially unhelpful location. If that's
+      // the best we can do here, back up the location to the introducer itself.
+      location = state.token;
+    }
+    context.emitter().Emit(location, ExpectedDeclName,
                            context.tokens().GetKind(state.token));
     context.ReturnErrorOnState();
     context.AddLeafNode(NodeKind::InvalidParse, *context.position(),

toolchain/parse/node_kind.def

@@ -141,7 +141,9 @@ CARBON_PARSE_NODE_KIND_CHILD_COUNT(LibrarySpecifier, 1, CARBON_TOKEN(Library))
 //   _external_: Name or QualifiedDecl
 // Namespace
 CARBON_PARSE_NODE_KIND_CHILD_COUNT(NamespaceStart, 0, CARBON_TOKEN(Namespace))
-CARBON_PARSE_NODE_KIND_CHILD_COUNT(Namespace, 2, CARBON_TOKEN(Semi))
+CARBON_PARSE_NODE_KIND_CHILD_COUNT(Namespace, 2,
+                                   CARBON_TOKEN(Semi)
+                                       CARBON_IF_ERROR(CARBON_TOKEN(Namespace)))
 
 // A code block:
 //   CodeBlockStart

toolchain/parse/testdata/namespace/fail_incomplete.carbon

@@ -0,0 +1,59 @@
+// 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
+
+// CHECK:STDERR: fail_incomplete.carbon:[[@LINE+3]]:11: ERROR: `namespace` introducer should be followed by a name.
+// CHECK:STDERR: namespace 123
+// CHECK:STDERR:           ^
+namespace 123
+
+// CHECK:STDERR: fail_incomplete.carbon:[[@LINE+3]]:11: ERROR: `namespace` introducer should be followed by a name.
+// CHECK:STDERR: namespace ""
+// CHECK:STDERR:           ^
+namespace ""
+
+// CHECK:STDERR: fail_incomplete.carbon:[[@LINE+3]]:11: ERROR: `namespace` introducer should be followed by a name.
+// CHECK:STDERR: namespace +
+// CHECK:STDERR:           ^
+namespace +
+
+// CHECK:STDERR: fail_incomplete.carbon:[[@LINE+3]]:11: ERROR: `namespace` introducer should be followed by a name.
+// CHECK:STDERR: namespace bool
+// CHECK:STDERR:           ^
+namespace bool
+
+// CHECK:STDERR: fail_incomplete.carbon:[[@LINE+3]]:11: ERROR: `namespace` introducer should be followed by a name.
+// CHECK:STDERR: namespace namespace namespace namespace namespace
+// CHECK:STDERR:           ^
+namespace namespace namespace namespace namespace
+
+// CHECK:STDERR: fail_incomplete.carbon:[[@LINE+3]]:1: ERROR: `namespace` introducer should be followed by a name.
+// CHECK:STDERR: namespace
+// CHECK:STDERR: ^
+namespace
+
+// CHECK:STDOUT: - filename: fail_incomplete.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:       {kind: 'NamespaceStart', text: 'namespace'},
+// CHECK:STDOUT:       {kind: 'InvalidParse', text: '123', has_error: yes},
+// CHECK:STDOUT:     {kind: 'Namespace', text: 'namespace', has_error: yes, subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'NamespaceStart', text: 'namespace'},
+// CHECK:STDOUT:       {kind: 'InvalidParse', text: '""', has_error: yes},
+// CHECK:STDOUT:     {kind: 'Namespace', text: 'namespace', has_error: yes, subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'NamespaceStart', text: 'namespace'},
+// CHECK:STDOUT:       {kind: 'InvalidParse', text: '+', has_error: yes},
+// CHECK:STDOUT:     {kind: 'Namespace', text: 'namespace', has_error: yes, subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'NamespaceStart', text: 'namespace'},
+// CHECK:STDOUT:       {kind: 'InvalidParse', text: 'bool', has_error: yes},
+// CHECK:STDOUT:     {kind: 'Namespace', text: 'namespace', has_error: yes, subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'NamespaceStart', text: 'namespace'},
+// CHECK:STDOUT:       {kind: 'InvalidParse', text: 'namespace', has_error: yes},
+// CHECK:STDOUT:     {kind: 'Namespace', text: 'namespace', has_error: yes, subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'NamespaceStart', text: 'namespace'},
+// CHECK:STDOUT:       {kind: 'InvalidParse', text: '', has_error: yes},
+// CHECK:STDOUT:     {kind: 'Namespace', text: 'namespace', has_error: yes, subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]