Only allow designators when parsing `where __ = ...` (#4304)

Implements
[TODO](https://github.com/carbon-language/carbon-lang/pull/4275/files#r1751000646)
introduced in #4275 . Note that this enforces the restriction
syntactically in parse, following the
[design](https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/generics/details.md#rewrite-constraints).

---------

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

toolchain/diagnostics/diagnostic_kind.def

@@ -129,6 +129,7 @@ CARBON_DIAGNOSTIC_KIND(ExpectedAliasInitializer)
 
 // Where requirement diagnostics.
 CARBON_DIAGNOSTIC_KIND(ExpectedRequirementOperator)
+CARBON_DIAGNOSTIC_KIND(RequirementEqualAfterNonDesignator)
 
 // ============================================================================
 // Semantics diagnostics

toolchain/parse/handle_requirement.cpp

@@ -9,9 +9,23 @@
 namespace Carbon::Parse {
 
 auto HandleRequirementBegin(Context& context) -> void {
-  context.PopAndDiscardState();
-  // TODO: Peek ahead for `.designator = ...`, and give it special handling.
-  context.PushState(State::RequirementOperator);
+  auto state = context.PopState();
+
+  // Peek ahead for `.designator = ...`, and give it special handling.
+  if (context.PositionKind() == Lex::TokenKind::Period &&
+      context.PositionKind(Lookahead::NextToken) ==
+          Lex::TokenKind::Identifier &&
+      context.PositionKind(static_cast<Lookahead>(2)) ==
+          Lex::TokenKind::Equal) {
+    auto period = context.Consume();
+    context.AddNode(NodeKind::IdentifierName, context.Consume(),
+                    /*has_error=*/false);
+    context.AddNode(NodeKind::DesignatorExpr, period, /*has_error=*/false);
+    state.token = context.Consume();
+    context.PushState(state, State::RequirementOperatorFinish);
+  } else {
+    context.PushState(State::RequirementOperator);
+  }
   context.PushStateForExpr(PrecedenceGroup::ForRequirements());
 }
 
@@ -19,14 +33,22 @@ auto HandleRequirementOperator(Context& context) -> void {
   auto state = context.PopState();
 
   switch (context.PositionKind()) {
-    case Lex::TokenKind::Impls: {
+    // Accept either `impls` or `==`
+    case Lex::TokenKind::Impls:
+    case Lex::TokenKind::EqualEqual:
       break;
-    }
+
+    // Reject `=` since correct usage is consumed in `HandleRequirementBegin`.
     case Lex::TokenKind::Equal: {
-      break;
-    }
-    case Lex::TokenKind::EqualEqual: {
-      break;
+      if (!state.has_error) {
+        CARBON_DIAGNOSTIC(
+            RequirementEqualAfterNonDesignator, Error,
+            "Requirement can only use `=` after `.member` designator.");
+        context.emitter().Emit(*context.position(),
+                               RequirementEqualAfterNonDesignator);
+      }
+      context.ReturnErrorOnState();
+      return;
     }
     default: {
       if (!state.has_error) {

toolchain/parse/state.def

@@ -618,6 +618,10 @@ CARBON_PARSE_STATE(IfExprFinish)
 // an expression.
 // TODO: Also a `require` declaration?
 //
+// expr where .designator = ...
+//            ^~~~~~~~~~~~~
+//   1. Expr
+//   2. RequirementOperatorFinish
 // expr where ...
 //           ^
 //   1. Expr

toolchain/parse/testdata/where_expr/fail_rewrite.carbon

@@ -0,0 +1,197 @@
+// 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/where_expr/fail_rewrite.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/where_expr/fail_rewrite.carbon
+
+// Tests for reporting errors when a rewrite requirement doesn't follow
+// `where .designator = rewrite`.
+
+// --- fail_not_designator.carbon
+
+// CHECK:STDERR: fail_not_designator.carbon:[[@LINE+4]]:37: ERROR: Requirement can only use `=` after `.member` designator.
+// CHECK:STDERR: fn NotDesignator(T:! type where i32 = .U);
+// CHECK:STDERR:                                     ^
+// CHECK:STDERR:
+fn NotDesignator(T:! type where i32 = .U);
+
+// --- fail_designator_in_parens.carbon
+
+// CHECK:STDERR: fail_designator_in_parens.carbon:[[@LINE+4]]:40: ERROR: Requirement can only use `=` after `.member` designator.
+// CHECK:STDERR: fn DesignatorInParens(V:! I where (.J) = bool);
+// CHECK:STDERR:                                        ^
+// CHECK:STDERR:
+fn DesignatorInParens(V:! I where (.J) = bool);
+
+// --- fail_dot_self.carbon
+
+// CHECK:STDERR: fail_dot_self.carbon:[[@LINE+4]]:30: ERROR: Requirement can only use `=` after `.member` designator.
+// CHECK:STDERR: fn DotSelf(W:! K where .Self = f32);
+// CHECK:STDERR:                              ^
+// CHECK:STDERR:
+fn DotSelf(W:! K where .Self = f32);
+
+// --- fail_dot_keyword.carbon
+
+// CHECK:STDERR: fail_dot_keyword.carbon:[[@LINE+4]]:28: ERROR: Expected identifier or `Self` after `.`.
+// CHECK:STDERR: fn DotKeyword(W:! K where .and = u8);
+// CHECK:STDERR:                            ^~~
+// CHECK:STDERR:
+fn DotKeyword(W:! K where .and = u8);
+
+// --- fail_postfix_after_designator.carbon
+
+// CHECK:STDERR: fail_postfix_after_designator.carbon:[[@LINE+4]]:43: ERROR: Requirement can only use `=` after `.member` designator.
+// CHECK:STDERR: fn PostfixAfterDesignator(X:! L where .M* = u64*);
+// CHECK:STDERR:                                           ^
+// CHECK:STDERR:
+fn PostfixAfterDesignator(X:! L where .M* = u64*);
+
+// --- fail_binary_op_after_designator.carbon
+
+// CHECK:STDERR: fail_binary_op_after_designator.carbon:[[@LINE+4]]:48: ERROR: Requirement can only use `=` after `.member` designator.
+// CHECK:STDERR: fn BinaryOpAfterDesignator(Y:! N where .O + .P = {});
+// CHECK:STDERR:                                                ^
+// CHECK:STDERR:
+fn BinaryOpAfterDesignator(Y:! N where .O + .P = {});
+
+// --- fail_after_and.carbon
+
+// CHECK:STDERR: fail_after_and.carbon:[[@LINE+3]]:43: ERROR: Requirement can only use `=` after `.member` designator.
+// CHECK:STDERR: fn AfterAnd(Z:! Q where .R impls S and () = .A);
+// CHECK:STDERR:                                           ^
+fn AfterAnd(Z:! Q where .R impls S and () = .A);
+
+// CHECK:STDOUT: - filename: fail_not_designator.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:       {kind: 'FunctionIntroducer', text: 'fn'},
+// CHECK:STDOUT:       {kind: 'IdentifierName', text: 'NotDesignator'},
+// CHECK:STDOUT:         {kind: 'TuplePatternStart', text: '('},
+// CHECK:STDOUT:           {kind: 'IdentifierName', text: 'T'},
+// CHECK:STDOUT:               {kind: 'TypeTypeLiteral', text: 'type'},
+// CHECK:STDOUT:             {kind: 'WhereOperand', text: 'where', subtree_size: 2},
+// CHECK:STDOUT:             {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:           {kind: 'WhereExpr', text: 'where', has_error: yes, subtree_size: 4},
+// CHECK:STDOUT:         {kind: 'CompileTimeBindingPattern', text: ':!', has_error: yes, subtree_size: 6},
+// CHECK:STDOUT:       {kind: 'TuplePattern', text: ')', has_error: yes, subtree_size: 8},
+// CHECK:STDOUT:     {kind: 'FunctionDecl', text: ';', subtree_size: 11},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]
+// CHECK:STDOUT: - filename: fail_designator_in_parens.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:       {kind: 'FunctionIntroducer', text: 'fn'},
+// CHECK:STDOUT:       {kind: 'IdentifierName', text: 'DesignatorInParens'},
+// CHECK:STDOUT:         {kind: 'TuplePatternStart', text: '('},
+// CHECK:STDOUT:           {kind: 'IdentifierName', text: 'V'},
+// CHECK:STDOUT:               {kind: 'IdentifierNameExpr', text: 'I'},
+// CHECK:STDOUT:             {kind: 'WhereOperand', text: 'where', subtree_size: 2},
+// CHECK:STDOUT:               {kind: 'ParenExprStart', text: '('},
+// CHECK:STDOUT:                 {kind: 'IdentifierName', text: 'J'},
+// CHECK:STDOUT:               {kind: 'DesignatorExpr', text: '.', subtree_size: 2},
+// CHECK:STDOUT:             {kind: 'ParenExpr', text: ')', subtree_size: 4},
+// CHECK:STDOUT:           {kind: 'WhereExpr', text: 'where', has_error: yes, subtree_size: 7},
+// CHECK:STDOUT:         {kind: 'CompileTimeBindingPattern', text: ':!', has_error: yes, subtree_size: 9},
+// CHECK:STDOUT:       {kind: 'TuplePattern', text: ')', has_error: yes, subtree_size: 11},
+// CHECK:STDOUT:     {kind: 'FunctionDecl', text: ';', subtree_size: 14},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]
+// CHECK:STDOUT: - filename: fail_dot_self.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:       {kind: 'FunctionIntroducer', text: 'fn'},
+// CHECK:STDOUT:       {kind: 'IdentifierName', text: 'DotSelf'},
+// CHECK:STDOUT:         {kind: 'TuplePatternStart', text: '('},
+// CHECK:STDOUT:           {kind: 'IdentifierName', text: 'W'},
+// CHECK:STDOUT:               {kind: 'IdentifierNameExpr', text: 'K'},
+// CHECK:STDOUT:             {kind: 'WhereOperand', text: 'where', subtree_size: 2},
+// CHECK:STDOUT:               {kind: 'SelfTypeName', text: 'Self'},
+// CHECK:STDOUT:             {kind: 'DesignatorExpr', text: '.', subtree_size: 2},
+// CHECK:STDOUT:           {kind: 'WhereExpr', text: 'where', has_error: yes, subtree_size: 5},
+// CHECK:STDOUT:         {kind: 'CompileTimeBindingPattern', text: ':!', has_error: yes, subtree_size: 7},
+// CHECK:STDOUT:       {kind: 'TuplePattern', text: ')', has_error: yes, subtree_size: 9},
+// CHECK:STDOUT:     {kind: 'FunctionDecl', text: ';', subtree_size: 12},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]
+// CHECK:STDOUT: - filename: fail_dot_keyword.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:       {kind: 'FunctionIntroducer', text: 'fn'},
+// CHECK:STDOUT:       {kind: 'IdentifierName', text: 'DotKeyword'},
+// CHECK:STDOUT:         {kind: 'TuplePatternStart', text: '('},
+// CHECK:STDOUT:           {kind: 'IdentifierName', text: 'W'},
+// CHECK:STDOUT:               {kind: 'IdentifierNameExpr', text: 'K'},
+// CHECK:STDOUT:             {kind: 'WhereOperand', text: 'where', subtree_size: 2},
+// CHECK:STDOUT:               {kind: 'IdentifierName', text: 'and', has_error: yes},
+// CHECK:STDOUT:             {kind: 'DesignatorExpr', text: '.', has_error: yes, subtree_size: 2},
+// CHECK:STDOUT:           {kind: 'WhereExpr', text: 'where', has_error: yes, subtree_size: 5},
+// CHECK:STDOUT:         {kind: 'CompileTimeBindingPattern', text: ':!', has_error: yes, subtree_size: 7},
+// CHECK:STDOUT:       {kind: 'TuplePattern', text: ')', has_error: yes, subtree_size: 9},
+// CHECK:STDOUT:     {kind: 'FunctionDecl', text: ';', subtree_size: 12},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]
+// CHECK:STDOUT: - filename: fail_postfix_after_designator.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:       {kind: 'FunctionIntroducer', text: 'fn'},
+// CHECK:STDOUT:       {kind: 'IdentifierName', text: 'PostfixAfterDesignator'},
+// CHECK:STDOUT:         {kind: 'TuplePatternStart', text: '('},
+// CHECK:STDOUT:           {kind: 'IdentifierName', text: 'X'},
+// CHECK:STDOUT:               {kind: 'IdentifierNameExpr', text: 'L'},
+// CHECK:STDOUT:             {kind: 'WhereOperand', text: 'where', subtree_size: 2},
+// CHECK:STDOUT:                 {kind: 'IdentifierName', text: 'M'},
+// CHECK:STDOUT:               {kind: 'DesignatorExpr', text: '.', subtree_size: 2},
+// CHECK:STDOUT:             {kind: 'PostfixOperatorStar', text: '*', subtree_size: 3},
+// CHECK:STDOUT:           {kind: 'WhereExpr', text: 'where', has_error: yes, subtree_size: 6},
+// CHECK:STDOUT:         {kind: 'CompileTimeBindingPattern', text: ':!', has_error: yes, subtree_size: 8},
+// CHECK:STDOUT:       {kind: 'TuplePattern', text: ')', has_error: yes, subtree_size: 10},
+// CHECK:STDOUT:     {kind: 'FunctionDecl', text: ';', subtree_size: 13},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]
+// CHECK:STDOUT: - filename: fail_binary_op_after_designator.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:       {kind: 'FunctionIntroducer', text: 'fn'},
+// CHECK:STDOUT:       {kind: 'IdentifierName', text: 'BinaryOpAfterDesignator'},
+// CHECK:STDOUT:         {kind: 'TuplePatternStart', text: '('},
+// CHECK:STDOUT:           {kind: 'IdentifierName', text: 'Y'},
+// CHECK:STDOUT:               {kind: 'IdentifierNameExpr', text: 'N'},
+// CHECK:STDOUT:             {kind: 'WhereOperand', text: 'where', subtree_size: 2},
+// CHECK:STDOUT:                 {kind: 'IdentifierName', text: 'O'},
+// CHECK:STDOUT:               {kind: 'DesignatorExpr', text: '.', subtree_size: 2},
+// CHECK:STDOUT:                 {kind: 'IdentifierName', text: 'P'},
+// CHECK:STDOUT:               {kind: 'DesignatorExpr', text: '.', subtree_size: 2},
+// CHECK:STDOUT:             {kind: 'InfixOperatorPlus', text: '+', subtree_size: 5},
+// CHECK:STDOUT:           {kind: 'WhereExpr', text: 'where', has_error: yes, subtree_size: 8},
+// CHECK:STDOUT:         {kind: 'CompileTimeBindingPattern', text: ':!', has_error: yes, subtree_size: 10},
+// CHECK:STDOUT:       {kind: 'TuplePattern', text: ')', has_error: yes, subtree_size: 12},
+// CHECK:STDOUT:     {kind: 'FunctionDecl', text: ';', subtree_size: 15},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]
+// CHECK:STDOUT: - filename: fail_after_and.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:       {kind: 'FunctionIntroducer', text: 'fn'},
+// CHECK:STDOUT:       {kind: 'IdentifierName', text: 'AfterAnd'},
+// CHECK:STDOUT:         {kind: 'TuplePatternStart', text: '('},
+// CHECK:STDOUT:           {kind: 'IdentifierName', text: 'Z'},
+// CHECK:STDOUT:               {kind: 'IdentifierNameExpr', text: 'Q'},
+// CHECK:STDOUT:             {kind: 'WhereOperand', text: 'where', subtree_size: 2},
+// CHECK:STDOUT:                 {kind: 'IdentifierName', text: 'R'},
+// CHECK:STDOUT:               {kind: 'DesignatorExpr', text: '.', subtree_size: 2},
+// CHECK:STDOUT:               {kind: 'IdentifierNameExpr', text: 'S'},
+// CHECK:STDOUT:             {kind: 'RequirementImpls', text: 'impls', subtree_size: 4},
+// CHECK:STDOUT:             {kind: 'RequirementAnd', text: 'and'},
+// CHECK:STDOUT:               {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:             {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:           {kind: 'WhereExpr', text: 'where', has_error: yes, subtree_size: 10},
+// CHECK:STDOUT:         {kind: 'CompileTimeBindingPattern', text: ':!', has_error: yes, subtree_size: 12},
+// CHECK:STDOUT:       {kind: 'TuplePattern', text: ')', has_error: yes, subtree_size: 14},
+// CHECK:STDOUT:     {kind: 'FunctionDecl', text: ';', subtree_size: 17},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]

toolchain/parse/typed_nodes.h

@@ -1037,8 +1037,7 @@ struct DesignatorExpr {
 struct RequirementEqual {
   static constexpr auto Kind = NodeKind::RequirementEqual.Define(
       {.category = NodeCategory::Requirement, .child_count = 2});
-  // TODO: Enforce that the lhs is always a DesignatorExpr.
-  AnyExprId lhs;
+  DesignatorExprId lhs;
   Lex::EqualTokenIndex token;
   AnyExprId rhs;
 };