[analyzer/ffi] Support packed Structss

This CL does not add packed structs to `dart:ffi` yet, only to the mock
SDK in the analyzer for testing the analyzer. This means we can review
and land it separately.

Bug: #38158

Split off https://dart-review.googlesource.com/c/sdk/+/186143.

Change-Id: I9c9ac9154e3dec0e05242f57cf7dddf8406df5fb
Cq-Include-Trybots: luci.dart.try:analyzer-analysis-server-linux-try,analyzer-linux-release-try,analyzer-nnbd-linux-release-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/191700
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>

pkg/analyzer/lib/error/error.dart

@@ -476,6 +476,9 @@ const List<ErrorCode> errorCodeValues = [
   FfiCode.NON_CONSTANT_TYPE_ARGUMENT,
   FfiCode.NON_NATIVE_FUNCTION_TYPE_ARGUMENT_TO_POINTER,
   FfiCode.NON_SIZED_TYPE_ARGUMENT,
+  FfiCode.PACKED_ANNOTATION,
+  FfiCode.PACKED_ANNOTATION_ALIGNMENT,
+  FfiCode.PACKED_NESTING_NON_PACKED,
   FfiCode.SIZE_ANNOTATION_DIMENSIONS,
   FfiCode.SUBTYPE_OF_FFI_CLASS_IN_EXTENDS,
   FfiCode.SUBTYPE_OF_FFI_CLASS_IN_IMPLEMENTS,

pkg/analyzer/lib/src/dart/error/ffi_code.dart

@@ -206,6 +206,35 @@ class FfiCode extends AnalyzerErrorCode {
       correction: "Try using a native integer, 'Float', 'Double', 'Pointer', "
           "or subtype of 'Struct'.");
 
+  /**
+   * No parameters.
+   */
+  static const FfiCode PACKED_ANNOTATION = FfiCode(
+      name: 'PACKED_ANNOTATION',
+      message: "Structs must have at most one 'Packed' annotation.",
+      correction: "Try removing extra 'Packed' annotations.");
+
+  /**
+   * No parameters.
+   */
+  static const FfiCode PACKED_ANNOTATION_ALIGNMENT = FfiCode(
+      name: 'PACKED_ANNOTATION_ALIGNMENT',
+      message: "Only packing to 1, 2, 4, 8, and 16 bytes is supported.",
+      correction:
+          "Try changing the 'Packed' annotation alignment to 1, 2, 4, 8, or 16.");
+
+  /**
+   * Parameters:
+   * 0: the name of the outer struct
+   * 1: the name of the struct being nested
+   */
+  static const FfiCode PACKED_NESTING_NON_PACKED = FfiCode(
+      name: 'PACKED_NESTING_NON_PACKED',
+      message:
+          "Nesting the non-packed or less tightly packed struct '{0}' in a packed struct '{1}' is not supported.",
+      correction:
+          "Try packing the nested struct or packing the nested struct more tightly.");
+
   /**
    * No parameters.
    */

pkg/analyzer/lib/src/generated/ffi_verifier.dart

@@ -51,12 +51,16 @@ class FfiVerifier extends RecursiveAstVisitor<void> {
   /// `Struct`.
   bool inStruct = false;
 
+  /// Subclass of `Struct` we are currently visiting, or `null`.
+  ClassDeclaration? struct;
+
   /// Initialize a newly created verifier.
   FfiVerifier(this.typeSystem, this._errorReporter);
 
   @override
   void visitClassDeclaration(ClassDeclaration node) {
     inStruct = false;
+    struct = null;
     // Only the Allocator, Opaque and Struct class may be extended.
     var extendsClause = node.extendsClause;
     if (extendsClause != null) {
@@ -66,10 +70,12 @@ class FfiVerifier extends RecursiveAstVisitor<void> {
         final className = ffiClass.name;
         if (className == _structClassName) {
           inStruct = true;
+          struct = node;
           if (node.declaredElement!.isEmptyStruct) {
             _errorReporter
                 .reportErrorForNode(FfiCode.EMPTY_STRUCT, node, [node.name]);
           }
+          _validatePackedAnnotation(node.metadata);
         } else if (className != _allocatorClassName &&
             className != _opaqueClassName) {
           _errorReporter.reportErrorForNode(
@@ -561,12 +567,20 @@ class FfiVerifier extends RecursiveAstVisitor<void> {
         }
         final arrayDimensions = declaredType.arrayDimensions;
         _validateSizeOfAnnotation(fieldType, annotations, arrayDimensions);
+        final arrayElement = declaredType.arrayElementType;
+        if (arrayElement.isStructSubtype) {
+          final elementClass = (arrayElement as InterfaceType).element;
+          _validatePackingNesting(struct!.declaredElement!, elementClass,
+              errorNode: fieldType);
+        }
       } else if (declaredType.isStructSubtype) {
         final clazz = (declaredType as InterfaceType).element;
         if (clazz.isEmptyStruct) {
           _errorReporter
               .reportErrorForNode(FfiCode.EMPTY_STRUCT, node, [clazz.name]);
         }
+        _validatePackingNesting(struct!.declaredElement!, clazz,
+            errorNode: fieldType);
       } else {
         _errorReporter.reportErrorForNode(FfiCode.INVALID_FIELD_TYPE_IN_STRUCT,
             fieldType, [fieldType.toSource()]);
@@ -664,6 +678,54 @@ class FfiVerifier extends RecursiveAstVisitor<void> {
     }
   }
 
+  /// Validate that the [annotations] include at most one packed annotation.
+  void _validatePackedAnnotation(NodeList<Annotation> annotations) {
+    final ffiPackedAnnotations =
+        annotations.where((annotation) => annotation.isPacked).toList();
+
+    if (ffiPackedAnnotations.isEmpty) {
+      return;
+    }
+
+    if (ffiPackedAnnotations.length > 1) {
+      final extraAnnotations = ffiPackedAnnotations.skip(1);
+      for (final annotation in extraAnnotations) {
+        _errorReporter.reportErrorForNode(
+            FfiCode.PACKED_ANNOTATION, annotation);
+      }
+    }
+
+    // Check number of dimensions.
+    final annotation = ffiPackedAnnotations.first;
+    final value = annotation.elementAnnotation?.packedMemberAlignment;
+    if (![1, 2, 4, 8, 16].contains(value)) {
+      _errorReporter.reportErrorForNode(
+          FfiCode.PACKED_ANNOTATION_ALIGNMENT, annotation);
+    }
+  }
+
+  void _validatePackingNesting(ClassElement outer, ClassElement nested,
+      {required TypeAnnotation errorNode}) {
+    final outerPacking = outer.structPacking;
+    if (outerPacking == null) {
+      // No packing for outer class, so we're done.
+      return;
+    }
+    bool error = false;
+    final nestedPacking = nested.structPacking;
+    if (nestedPacking == null) {
+      // The outer struct packs, but the nested struct does not.
+      error = true;
+    } else if (outerPacking < nestedPacking) {
+      // The outer struct packs tighter than the nested struct.
+      error = true;
+    }
+    if (error) {
+      _errorReporter.reportErrorForNode(FfiCode.PACKED_NESTING_NON_PACKED,
+          errorNode, [nested.name, outer.name]);
+    }
+  }
+
   void _validateRefIndexed(IndexExpression node) {
     var targetType = node.realTarget.staticType;
     if (!_isValidFfiNativeType(targetType,
@@ -774,6 +836,30 @@ enum _PrimitiveDartType {
   none,
 }
 
+extension on Annotation {
+  bool get isPacked {
+    final element = this.element;
+    return element is ConstructorElement &&
+        element.ffiClass != null &&
+        element.enclosingElement.name == 'Packed';
+  }
+}
+
+extension on ElementAnnotation {
+  bool get isPacked {
+    final element = this.element;
+    return element is ConstructorElement &&
+        element.ffiClass != null &&
+        element.enclosingElement.name == 'Packed';
+  }
+
+  int get packedMemberAlignment {
+    assert(isPacked);
+    final value = computeConstantValue();
+    return value!.getField('memberAlignment')!.toIntValue()!;
+  }
+}
+
 extension on Element? {
   /// Return `true` if this represents the extension `AllocatorAlloc`.
   bool get isAllocatorExtension {
@@ -855,6 +941,17 @@ extension on ClassElement {
   bool get isFfiClass {
     return library.name == FfiVerifier._dartFfiLibraryName;
   }
+
+  int? get structPacking {
+    final packedAnnotations =
+        metadata.where((annotation) => annotation.isPacked);
+
+    if (packedAnnotations.isEmpty) {
+      return null;
+    }
+
+    return packedAnnotations.first.packedMemberAlignment;
+  }
 }
 
 extension on ExtensionElement {
@@ -886,6 +983,16 @@ extension on DartType {
     return dimensions;
   }
 
+  DartType get arrayElementType {
+    DartType iterator = this;
+    while (iterator is InterfaceType &&
+        iterator.element.name == FfiVerifier._arrayClassName &&
+        iterator.element.isFfiClass) {
+      iterator = iterator.typeArguments.single;
+    }
+    return iterator;
+  }
+
   bool get isPointer {
     final self = this;
     return self is InterfaceType && self.element.isPointer;

pkg/analyzer/lib/src/test_utilities/mock_sdk.dart

@@ -677,6 +677,12 @@ extension NativeFunctionPointer<NF extends Function>
 
 class Struct extends NativeType {}
 
+class Packed {
+  final int memberAlignment;
+
+  const Packed(this.memberAlignment);
+}
+
 abstract class DynamicLibrary {}
 
 extension DynamicLibraryExtension on DynamicLibrary {

pkg/analyzer/test/src/diagnostics/packed_annotation_alignment_test.dart

@@ -0,0 +1,41 @@
+// Copyright (c) 2021, 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.
+
+import 'package:analyzer/src/dart/error/ffi_code.dart';
+import 'package:test_reflective_loader/test_reflective_loader.dart';
+
+import '../dart/resolution/context_collection_resolution.dart';
+
+main() {
+  defineReflectiveSuite(() {
+    defineReflectiveTests(PackedAnnotationAlignment);
+  });
+}
+
+@reflectiveTest
+class PackedAnnotationAlignment extends PubPackageResolutionTest {
+  test_error() async {
+    await assertErrorsInCode(r'''
+import 'dart:ffi';
+
+@Packed(3)
+class C extends Struct {
+  external Pointer<Uint8> notEmpty;
+}
+''', [
+      error(FfiCode.PACKED_ANNOTATION_ALIGNMENT, 20, 10),
+    ]);
+  }
+
+  test_no_error() async {
+    await assertNoErrorsInCode(r'''
+import 'dart:ffi';
+
+@Packed(1)
+class C extends Struct {
+  external Pointer<Uint8> notEmpty;
+}
+''');
+  }
+}

pkg/analyzer/test/src/diagnostics/packed_annotation_test.dart

@@ -0,0 +1,52 @@
+// Copyright (c) 2021, 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.
+
+import 'package:analyzer/src/dart/error/ffi_code.dart';
+import 'package:test_reflective_loader/test_reflective_loader.dart';
+
+import '../dart/resolution/context_collection_resolution.dart';
+
+main() {
+  defineReflectiveSuite(() {
+    defineReflectiveTests(PackedAnnotation);
+  });
+}
+
+@reflectiveTest
+class PackedAnnotation extends PubPackageResolutionTest {
+  test_error() async {
+    await assertErrorsInCode(r'''
+import 'dart:ffi';
+
+@Packed(1)
+@Packed(1)
+class C extends Struct {
+  external Pointer<Uint8> notEmpty;
+}
+''', [
+      error(FfiCode.PACKED_ANNOTATION, 31, 10),
+    ]);
+  }
+
+  test_no_error_1() async {
+    await assertNoErrorsInCode(r'''
+import 'dart:ffi';
+
+class C extends Struct {
+  external Pointer<Uint8> notEmpty;
+}
+''');
+  }
+
+  test_no_error_2() async {
+    await assertNoErrorsInCode(r'''
+import 'dart:ffi';
+
+@Packed(1)
+class C extends Struct {
+  external Pointer<Uint8> notEmpty;
+}
+''');
+  }
+}