[cfe] Make IfNullIndexSet and CompoundIndexSet null-aware

This makes IfNullIndexSet and CompoundIndexSet null-aware to avoid lowerings during body building.

Inference visit methods visitIfNullPropertySet, visitIfNullSuperIndexSet, visitNullAwareCompoundSet, and visitNullAwareIfNullSet are updated to align with null-aware approach to lowering.

Change-Id: I9678191dae7ab014bfdacb2b39d271fdc852dbfc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/446041
Reviewed-by: Chloe Stefantsova <[email protected]>
Commit-Queue: Johnni Winther <[email protected]>

Author: johnniwinther

pkg/front_end/lib/src/kernel/expression_generator.dart

@@ -524,7 +524,10 @@ class PropertyAccessGenerator extends Generator {
   Expression buildIfNullAssignment(Expression value, DartType type, int offset,
       {bool voidContext = false}) {
     return new IfNullPropertySet(receiver, name, value,
-        forEffect: voidContext, readOffset: fileOffset, writeOffset: fileOffset)
+        forEffect: voidContext,
+        readOffset: fileOffset,
+        writeOffset: fileOffset,
+        isNullAware: false)
       ..fileOffset = offset;
   }
 
@@ -765,11 +768,11 @@ class NullAwarePropertyAccessGenerator extends Generator {
   @override
   Expression buildIfNullAssignment(Expression value, DartType type, int offset,
       {bool voidContext = false}) {
-    return new NullAwareIfNullSet(receiver, name, value,
+    return new IfNullPropertySet(receiver, name, value,
         forEffect: voidContext,
         readOffset: fileOffset,
-        testOffset: offset,
-        writeOffset: fileOffset)
+        writeOffset: fileOffset,
+        isNullAware: true)
       ..fileOffset = offset;
   }
 
@@ -982,27 +985,13 @@ class IndexedAccessGenerator extends Generator {
   @override
   Expression buildIfNullAssignment(Expression value, DartType type, int offset,
       {bool voidContext = false}) {
-    VariableDeclarationImpl? variable;
-    Expression receiverValue;
-    if (isNullAware) {
-      variable = _helper.createVariableDeclarationForValue(receiver);
-      receiverValue = _helper.createVariableGet(variable, fileOffset,
-          forNullGuardedAccess: true);
-    } else {
-      receiverValue = receiver;
-    }
-
-    Expression result = new IfNullIndexSet(receiverValue, index, value,
+    return new IfNullIndexSet(receiver, index, value,
         readOffset: fileOffset,
         testOffset: offset,
         writeOffset: fileOffset,
-        forEffect: voidContext)
+        forEffect: voidContext,
+        isNullAware: isNullAware)
       ..fileOffset = offset;
-    if (isNullAware) {
-      result = new NullAwareMethodInvocation(variable!, result)
-        ..fileOffset = fileOffset;
-    }
-    return result;
   }
 
   @override
@@ -1011,28 +1000,14 @@ class IndexedAccessGenerator extends Generator {
       bool voidContext = false,
       bool isPreIncDec = false,
       bool isPostIncDec = false}) {
-    VariableDeclarationImpl? variable;
-    Expression receiverValue;
-    if (isNullAware) {
-      variable = _helper.createVariableDeclarationForValue(receiver);
-      receiverValue = _helper.createVariableGet(variable, fileOffset,
-          forNullGuardedAccess: true);
-    } else {
-      receiverValue = receiver;
-    }
-
-    Expression result = new CompoundIndexSet(
-        receiverValue, index, binaryOperator, value,
+    return new CompoundIndexSet(receiver, index, binaryOperator, value,
         readOffset: fileOffset,
         binaryOffset: offset,
         writeOffset: fileOffset,
         forEffect: voidContext,
-        forPostIncDec: isPostIncDec);
-    if (isNullAware) {
-      result = new NullAwareMethodInvocation(variable!, result)
-        ..fileOffset = fileOffset;
-    }
-    return result;
+        forPostIncDec: isPostIncDec,
+        isNullAware: isNullAware)
+      ..fileOffset = offset;
   }
 
   @override
@@ -1127,7 +1102,8 @@ class ThisIndexedAccessGenerator extends Generator {
         readOffset: fileOffset,
         testOffset: offset,
         writeOffset: fileOffset,
-        forEffect: voidContext)
+        forEffect: voidContext,
+        isNullAware: isNullAware)
       ..fileOffset = offset;
   }
 
@@ -1143,7 +1119,9 @@ class ThisIndexedAccessGenerator extends Generator {
         binaryOffset: offset,
         writeOffset: fileOffset,
         forEffect: voidContext,
-        forPostIncDec: isPostIncDec);
+        forPostIncDec: isPostIncDec,
+        isNullAware: isNullAware)
+      ..fileOffset = offset;
   }
 
   @override
@@ -2077,7 +2055,8 @@ class ExplicitExtensionInstanceAccessGenerator extends Generator {
       return new IfNullPropertySet(receiver, targetName, value,
           forEffect: voidContext,
           readOffset: fileOffset,
-          writeOffset: fileOffset)
+          writeOffset: fileOffset,
+          isNullAware: false)
         ..fileOffset = offset;
     }
   }

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

@@ -1253,10 +1253,14 @@ class IfNullPropertySet extends InternalExpression {
   /// The file offset for the write operation.
   final int writeOffset;
 
+  /// `true` if the access is null-aware, i.e. of the form `o?.a ??= b`.
+  final bool isNullAware;
+
   IfNullPropertySet(this.receiver, this.propertyName, this.rhs,
       {required this.forEffect,
       required this.readOffset,
-      required this.writeOffset}) {
+      required this.writeOffset,
+      required this.isNullAware}) {
     receiver.parent = this;
     rhs.parent = this;
   }
@@ -1276,6 +1280,9 @@ class IfNullPropertySet extends InternalExpression {
   // Coverage-ignore(suite): Not run.
   void toTextInternal(AstPrinter printer) {
     printer.writeExpression(receiver);
+    if (isNullAware) {
+      printer.write('?');
+    }
     printer.write('.');
     printer.writeName(propertyName);
     printer.write(' ??= ');
@@ -1857,11 +1864,15 @@ class IfNullIndexSet extends InternalExpression {
   /// If `true`, the expression is only need for effect and not for its value.
   final bool forEffect;
 
+  /// `true` if the access is null-aware, i.e. of the form `o?[a] ??= b`.
+  final bool isNullAware;
+
   IfNullIndexSet(this.receiver, this.index, this.value,
       {required this.readOffset,
       required this.testOffset,
       required this.writeOffset,
-      required this.forEffect}) {
+      required this.forEffect,
+      required this.isNullAware}) {
     receiver.parent = this;
     index.parent = this;
     value.parent = this;
@@ -2062,12 +2073,16 @@ class CompoundIndexSet extends InternalExpression {
   /// If `true`, the expression is a post-fix inc/dec expression.
   final bool forPostIncDec;
 
+  /// `true` if the access is null-aware, i.e. of the form `o?[a] += b`.
+  final bool isNullAware;
+
   CompoundIndexSet(this.receiver, this.index, this.binaryName, this.rhs,
       {required this.readOffset,
       required this.binaryOffset,
       required this.writeOffset,
       required this.forEffect,
-      required this.forPostIncDec}) {
+      required this.forPostIncDec,
+      required this.isNullAware}) {
     receiver.parent = this;
     index.parent = this;
     rhs.parent = this;
@@ -2089,6 +2104,9 @@ class CompoundIndexSet extends InternalExpression {
   // Coverage-ignore(suite): Not run.
   void toTextInternal(AstPrinter printer) {
     printer.writeExpression(receiver);
+    if (isNullAware) {
+      printer.write('?');
+    }
     printer.write('[');
     printer.writeExpression(index);
     printer.write(']');
@@ -2218,81 +2236,6 @@ class NullAwareCompoundSet extends InternalExpression {
   }
 }
 
-/// Internal expression representing an null-aware if-null property set.
-///
-/// A null-aware if-null property set of the form
-///
-///    receiver?.name ??= value
-///
-/// is, if used for value, encoded as the expression:
-///
-///     let receiverVariable = receiver in
-///       receiverVariable == null ? null :
-///         (let readVariable = receiverVariable.name in
-///           readVariable == null ?
-///             receiverVariable.name = value : readVariable)
-///
-/// and, if used for effect, encoded as the expression:
-///
-///     let receiverVariable = receiver in
-///       receiverVariable == null ? null :
-///         (receiverVariable.name == null ?
-///           receiverVariable.name = value : null)
-///
-///
-class NullAwareIfNullSet extends InternalExpression {
-  /// The synthetic variable whose initializer hold the receiver.
-  Expression receiver;
-
-  /// The expression that reads the property from [variable].
-  Name name;
-
-  /// The expression that writes the value to the property on [variable].
-  Expression value;
-
-  /// The file offset for the read operation.
-  final int readOffset;
-
-  /// The file offset for the write operation.
-  final int writeOffset;
-
-  /// The file offset for the == operation.
-  final int testOffset;
-
-  /// If `true`, the expression is only need for effect and not for its value.
-  final bool forEffect;
-
-  NullAwareIfNullSet(this.receiver, this.name, this.value,
-      {required this.readOffset,
-      required this.writeOffset,
-      required this.testOffset,
-      required this.forEffect}) {
-    receiver.parent = this;
-    value.parent = this;
-  }
-
-  @override
-  ExpressionInferenceResult acceptInference(
-      InferenceVisitorImpl visitor, DartType typeContext) {
-    return visitor.visitNullAwareIfNullSet(this, typeContext);
-  }
-
-  @override
-  String toString() {
-    return "NullAwareIfNullSet(${toStringInternal()})";
-  }
-
-  @override
-  // Coverage-ignore(suite): Not run.
-  void toTextInternal(AstPrinter printer) {
-    printer.writeExpression(receiver);
-    printer.write('?.');
-    printer.writeName(name);
-    printer.write(' ??= ');
-    printer.writeExpression(value);
-  }
-}
-
 /// Internal expression representing a compound super index assignment.
 ///
 /// An if-null index assignment of the form `super[a] += b` is, if used for