[cfe] Desugar pattern variable assignments

Part of #49749

Change-Id: Ibae19d0e64f023aea047b007d73f3cee9910d259
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/277683
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Chloe Stefantsova <[email protected]>

Author: chloestefantsova

pkg/front_end/lib/src/fasta/kernel/body_builder.dart

@@ -8483,8 +8483,17 @@ class BodyBuilder extends StackListenerImpl
     Pattern pattern;
     if (variable.lexeme == "_") {
       pattern = new WildcardPattern(patternType, variable.charOffset);
+    } else if (inAssignmentPattern) {
+      Expression variableUse =
+          toValue(scopeLookup(scope, variable.lexeme, variable));
+      if (variableUse is VariableGet) {
+        pattern = new AssignedVariablePattern(variableUse.variable,
+            offset: variable.charOffset);
+      } else {
+        // Recover by using [WildcardPattern] instead.
+        pattern = new WildcardPattern(patternType, variable.charOffset);
+      }
     } else {
-      // TODO(paulberry): use inAssignmentPattern.
       pattern = new VariablePattern(
           patternType,
           variable.lexeme,
@@ -8552,7 +8561,24 @@ class BodyBuilder extends StackListenerImpl
     // TODO(johnniwinther,cstefantsova): Handle metadata.
     pop(NullValue.Metadata) as List<Expression>?;
     push(new PatternVariableDeclaration(pattern, initializer,
-        offset: keyword.charOffset, isFinal: isFinal));
+        fileOffset: keyword.charOffset, isFinal: isFinal));
+  }
+
+  @override
+  void handlePatternAssignment(Token equals) {
+    debugEvent("PatternAssignment");
+    assert(checkState(equals, [
+      unionOfKinds([
+        ValueKinds.Expression,
+        ValueKinds.Generator,
+        ValueKinds.ProblemBuilder,
+      ]),
+      ValueKinds.Pattern
+    ]));
+    Expression expression = popForValue();
+    Pattern pattern = toPattern(pop());
+    push(new PatternAssignment(pattern, expression,
+        fileOffset: equals.charOffset));
   }
 }
 

pkg/front_end/lib/src/fasta/kernel/forest.dart

@@ -386,6 +386,14 @@ class Forest {
       ..fileEndOffset = fileEndOffset;
   }
 
+  /// Return a representation of a block expression.
+  BlockExpression createBlockExpression(
+      int fileOffset, Block body, Expression value) {
+    // ignore: unnecessary_null_comparison
+    assert(fileOffset != null);
+    return new BlockExpression(body, value)..fileOffset = fileOffset;
+  }
+
   /// Return a representation of a break statement.
   Statement createBreakStatement(int fileOffset, Object? label) {
     // ignore: unnecessary_null_comparison

pkg/front_end/lib/src/fasta/kernel/internal_ast.dart

@@ -5020,8 +5020,8 @@ class PatternVariableDeclaration extends InternalStatement {
   final bool isFinal;
 
   PatternVariableDeclaration(this.pattern, this.initializer,
-      {required int offset, required this.isFinal}) {
-    fileOffset = offset;
+      {required int fileOffset, required this.isFinal}) {
+    super.fileOffset = fileOffset;
   }
 
   @override
@@ -5048,6 +5048,72 @@ class PatternVariableDeclaration extends InternalStatement {
   }
 }
 
+class PatternAssignment extends InternalExpression {
+  final Pattern pattern;
+  final Expression expression;
+
+  PatternAssignment(this.pattern, this.expression, {required int fileOffset}) {
+    super.fileOffset = fileOffset;
+  }
+
+  @override
+  ExpressionInferenceResult acceptInference(
+      InferenceVisitorImpl visitor, DartType typeContext) {
+    return visitor.visitPatternAssignment(this, typeContext);
+  }
+
+  @override
+  String toString() {
+    return "PatternAssignment(${toStringInternal()})";
+  }
+}
+
+class AssignedVariablePattern extends Pattern {
+  final VariableDeclaration variable;
+
+  AssignedVariablePattern(this.variable, {required int offset}) : super(offset);
+
+  @override
+  PatternInferenceResult acceptInference(InferenceVisitorImpl visitor,
+      {required SharedMatchContext context}) {
+    return visitor.visitAssignedVariablePattern(this, context: context);
+  }
+
+  @override
+  List<VariableDeclaration> get declaredVariables => [variable];
+
+  @override
+  void toTextInternal(AstPrinter printer) {
+    printer.write(variable.name!);
+  }
+
+  @override
+  String toString() {
+    return "AssignedVariablePattern(${toStringInternal()})";
+  }
+
+  @override
+  PatternTransformationResult transform(
+      Expression matchedExpression,
+      DartType matchedType,
+      Expression variableInitializingContext,
+      InferenceVisitorBase inferenceVisitor) {
+    // condition: let _ = `variable` = `matchedExpression` in true
+    return new PatternTransformationResult([
+      new PatternTransformationElement(
+          kind: PatternTransformationElementKind.regular,
+          condition: inferenceVisitor.engine.forest.createLet(
+              inferenceVisitor.engine.forest.createVariableDeclarationForValue(
+                  inferenceVisitor.engine.forest.createVariableSet(
+                      fileOffset, variable, matchedExpression)),
+              inferenceVisitor.engine.forest
+                  .createBoolLiteral(fileOffset, true))
+            ..fileOffset = fileOffset,
+          variableInitializers: [])
+    ]);
+  }
+}
+
 final Pattern dummyPattern = new ExpressionPattern(dummyExpression);
 
 /// Internal statement for a if-case statements:
@@ -5617,7 +5683,7 @@ class VariablePattern extends Pattern {
 
   @override
   String toString() {
-    return "VariableBinder(${toStringInternal()})";
+    return "VariablePattern(${toStringInternal()})";
   }
 }
 

pkg/front_end/lib/src/fasta/type_inference/inference_visitor.dart

@@ -9304,6 +9304,82 @@ class InferenceVisitorImpl extends InferenceVisitorBase
     return const PatternInferenceResult();
   }
 
+  ExpressionInferenceResult visitPatternAssignment(
+      PatternAssignment node, DartType typeContext) {
+    Expression expression = node.expression;
+    ExpressionTypeAnalysisResult<DartType> analysisResult =
+        analyzePatternAssignment(node, node.pattern, expression);
+    Node? rewrite = popRewrite();
+    if (!identical(expression, rewrite)) {
+      expression = rewrite as Expression;
+    }
+
+    // TODO(cstefantsova): Do we need a more precise type for the variable?
+    VariableDeclaration matchedExpressionVariable = engine.forest
+        .createVariableDeclarationForValue(expression,
+            type: const DynamicType());
+    VariableDeclaration isPatternMatchingFailed = engine.forest
+        .createVariableDeclarationForValue(
+            engine.forest.createBoolLiteral(node.fileOffset, true),
+            type: coreTypes.boolNonNullableRawType);
+
+    // patternMatchedSet: `isPatternMatchingFailed` = false;
+    //   ==> VAR = true;
+    Statement patternMatchedSet = engine.forest.createExpressionStatement(
+        node.fileOffset,
+        engine.forest.createVariableSet(
+            node.fileOffset,
+            isPatternMatchingFailed,
+            engine.forest.createBoolLiteral(node.fileOffset, false)));
+
+    PatternTransformationResult transformationResult = node.pattern.transform(
+        engine.forest
+            .createVariableGet(node.fileOffset, matchedExpressionVariable),
+        const DynamicType(),
+        engine.forest
+            .createVariableGet(node.fileOffset, matchedExpressionVariable),
+        this);
+
+    List<Statement> replacementStatements = _transformationResultToStatements(
+        node.fileOffset, transformationResult, [patternMatchedSet]);
+
+    replacementStatements = [
+      matchedExpressionVariable,
+      isPatternMatchingFailed,
+      ...replacementStatements,
+      // TODO(cstefantsova): Figure out the right exception to throw.
+      engine.forest.createIfStatement(
+          node.fileOffset,
+          engine.forest
+              .createVariableGet(node.fileOffset, isPatternMatchingFailed),
+          engine.forest.createExpressionStatement(
+              node.fileOffset,
+              new Throw(
+                  new ConstructorInvocation(
+                      coreTypes.reachabilityErrorConstructor,
+                      engine.forest.createArguments(node.fileOffset, []))
+                    ..fileOffset = node.fileOffset)
+                ..fileOffset = node.fileOffset),
+          null),
+    ];
+
+    Expression replacement = engine.forest.createBlockExpression(
+        node.fileOffset,
+        engine.forest.createBlock(
+            node.fileOffset, node.fileOffset, replacementStatements),
+        engine.forest
+            .createVariableGet(node.fileOffset, matchedExpressionVariable));
+    return new ExpressionInferenceResult(
+        analysisResult.resolveShorting(), replacement);
+  }
+
+  PatternInferenceResult visitAssignedVariablePattern(
+      AssignedVariablePattern node,
+      {required SharedMatchContext context}) {
+    analyzeAssignedVariablePattern(context, node, node.variable);
+    return const PatternInferenceResult();
+  }
+
   @override
   shared.RecordType<DartType>? asRecordType(DartType type) {
     if (type is RecordType) {

pkg/front_end/test/text_representation/internal_ast_text_representation_test.dart

@@ -1318,14 +1318,14 @@ void _testPatternVariableDeclaration() {
   testStatement(
       new PatternVariableDeclaration(
           new ExpressionPattern(new IntLiteral(0)), new IntLiteral(1),
-          isFinal: false, offset: TreeNode.noOffset),
+          isFinal: false, fileOffset: TreeNode.noOffset),
       '''
 var 0 = 1;''');
 
   testStatement(
       new PatternVariableDeclaration(
           new ExpressionPattern(new IntLiteral(0)), new IntLiteral(1),
-          isFinal: true, offset: TreeNode.noOffset),
+          isFinal: true, fileOffset: TreeNode.noOffset),
       '''
 final 0 = 1;''');
 }

pkg/front_end/testcases/patterns/simple_assignments.dart

@@ -0,0 +1,43 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+test1(dynamic x) {
+  int a;
+  [a] = x;
+  return a;
+}
+
+test2(dynamic x) {
+  int a, b, c;
+  [c, ...] = [a && b, ...] = x;
+  return a + b + c;
+}
+
+main() {
+  expectEquals(test1([1]), 1);
+  expectThrows(() => test1([]));
+  expectThrows(() => test1([1, 2, 3]));
+  expectThrows(() => test1("foo"));
+  expectThrows(() => test1(null));
+
+  expectEquals(test2([1]), 3);
+  expectThrows(() => test2(1));
+}
+
+expectEquals(x, y) {
+  if (x != y) {
+    throw "Expected ${x} to be equal to ${y}.";
+  }
+}
+
+expectThrows(void Function() f) {
+  bool hasThrown = true;
+  try {
+    f();
+    hasThrown = false;
+  } catch (e) {}
+  if (!hasThrown) {
+    throw "Expected the function to throw.";
+  }
+}

pkg/front_end/testcases/patterns/simple_assignments.dart.strong.expect

@@ -0,0 +1,80 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+import "dart:_internal" as _in;
+
+static method test1(dynamic x) → dynamic {
+  core::int a;
+  block {
+    final dynamic #t1 = x;
+    final core::bool #t2 = true;
+    final dynamic #t3 = #t1;
+    if(#t3 is core::List<dynamic> && #t3{core::List<dynamic>}.{core::List::length}{core::int}.{core::num::==}(1){(core::Object) → core::bool}) {
+      final dynamic #t4 = #t3{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic};
+      if(let final dynamic #t5 = a = #t4 in true) {
+        #t2 = false;
+      }
+    }
+    if(#t2)
+      throw new _in::ReachabilityError::•();
+  } =>#t1;
+  return a;
+}
+static method test2(dynamic x) → dynamic {
+  core::int a;
+  core::int b;
+  core::int c;
+  block {
+    final dynamic #t6 = block {
+      final dynamic #t7 = x;
+      final core::bool #t8 = true;
+      final dynamic #t9 = #t7;
+      if(#t9 is core::List<dynamic> && #t9{core::List<dynamic>}.{core::List::length}{core::int}.{core::num::>=}(1){(core::num) → core::bool}) {
+        final dynamic #t10 = #t9{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic};
+        final dynamic #t11 = #t10;
+        if((let final dynamic #t12 = a = #t11 in true) && (let final dynamic #t13 = b = #t11 in true)) {
+          #t8 = false;
+        }
+      }
+      if(#t8)
+        throw new _in::ReachabilityError::•();
+    } =>#t7;
+    final core::bool #t14 = true;
+    final dynamic #t15 = #t6;
+    if(#t15 is core::List<dynamic> && #t15{core::List<dynamic>}.{core::List::length}{core::int}.{core::num::>=}(1){(core::num) → core::bool}) {
+      final dynamic #t16 = #t15{core::List<dynamic>}.{core::List::[]}(0){(core::int) → dynamic};
+      if(let final dynamic #t17 = c = #t16 in true) {
+        #t14 = false;
+      }
+    }
+    if(#t14)
+      throw new _in::ReachabilityError::•();
+  } =>#t6;
+  return a.{core::num::+}(b){(core::num) → core::int}.{core::num::+}(c){(core::num) → core::int};
+}
+static method main() → dynamic {
+  self::expectEquals(self::test1(<core::int>[1]), 1);
+  self::expectThrows(() → void => self::test1(<dynamic>[]));
+  self::expectThrows(() → void => self::test1(<core::int>[1, 2, 3]));
+  self::expectThrows(() → void => self::test1("foo"));
+  self::expectThrows(() → void => self::test1(null));
+  self::expectEquals(self::test2(<core::int>[1]), 3);
+  self::expectThrows(() → void => self::test2(1));
+}
+static method expectEquals(dynamic x, dynamic y) → dynamic {
+  if(!(x =={core::Object::==}{(core::Object) → core::bool} y)) {
+    throw "Expected ${x} to be equal to ${y}.";
+  }
+}
+static method expectThrows(() → void f) → dynamic {
+  core::bool hasThrown = true;
+  try {
+    f(){() → void};
+    hasThrown = false;
+  }
+  on core::Object catch(final core::Object e) {
+  }
+  if(!hasThrown) {
+    throw "Expected the function to throw.";
+  }
+}