[pkg/vm] Enable const functions during platform const field evaluation.

This allows more complicated field initializers that are written using
immediately invoked closures, like

@pragma("vm:platform-const")
final bool foo = () {
  ... do some stuff ...
}();

to be properly evaluated by the VM constant evaluator.

Also throws errors at compile time if the annotated member cannot be
evaluated to a constant or if an invalid member (not an
initialized static field or a static getter) is annotated.

TEST=pkg/vm/test/transformations/vm_constant_evaluator_test

Issue: #31969
Issue: #50473
Issue: flutter/flutter#14233
Change-Id: I14be498bb5f7771f0f339baf7d3b1bec7df5903f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/348380
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Tess Strickland <[email protected]>
Reviewed-by: Alexander Markov <[email protected]>

Author: sstrickl

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

@@ -2451,7 +2451,7 @@ class ConstantEvaluator implements ExpressionVisitor<Constant> {
 
   final bool enableTripleShift;
   final bool enableAsserts;
-  bool enableConstFunctions;
+  final bool enableConstFunctions;
   bool inExtensionTypeConstConstructor = false;
 
   final Map<Constant, Constant> canonicalizationCache;

pkg/vm/lib/transformations/unreachable_code_elimination.dart

@@ -32,10 +32,56 @@ class SimpleUnreachableCodeElimination extends RemovingTransformer {
   SimpleUnreachableCodeElimination(this.constantEvaluator,
       {required this.enableAsserts, required this.soundNullSafety});
 
+  bool _shouldEvaluateMember(Member node) =>
+      constantEvaluator.hasTargetOS && constantEvaluator.isPlatformConst(node);
+
+  Never _throwPlatformConstError(Member node, String message) {
+    final uri = constantEvaluator.getFileUri(node);
+    final offset = constantEvaluator.getFileOffset(uri, node);
+    throw PlatformConstError(message, node, uri, offset);
+  }
+
+  void _checkPlatformConstMember(Member node) {
+    if (node is Field) {
+      if (!node.isStatic) {
+        _throwPlatformConstError(node, 'not a static field');
+      }
+      // Static fields currently always have an initializer set, even if it's
+      // the implicit null initializer for a nullable field.
+      assert(node.initializer != null);
+    } else if (node is Procedure) {
+      if (!node.isStatic) {
+        _throwPlatformConstError(node, 'not a static method');
+      }
+      if (!node.isGetter) {
+        _throwPlatformConstError(node, 'not a getter');
+      }
+    } else {
+      _throwPlatformConstError(node, 'not a field or method');
+    }
+  }
+
   @override
   TreeNode defaultMember(Member node, TreeNode? removalSentinel) {
     _staticTypeContext =
         StaticTypeContext(node, constantEvaluator.typeEnvironment);
+    if (_shouldEvaluateMember(node)) {
+      _checkPlatformConstMember(node);
+      // Create a StaticGet to ensure the member is evaluated at least once,
+      // and then replace the field initializer or getter body with the result.
+      final staticGet = StaticGet(node)..fileOffset = node.fileOffset;
+      final result =
+          staticGet.accept1(this, cannotRemoveSentinel) as ConstantExpression;
+      if (node is Field) {
+        node.initializer = result
+          ..fileOffset = node.initializer!.fileOffset
+          ..parent = node;
+      } else if (node is Procedure) {
+        node.function.body = ReturnStatement(result)
+          ..fileOffset = node.function.body!.fileOffset
+          ..parent = node.function;
+      }
+    }
     final result = super.defaultMember(node, removalSentinel);
     _staticTypeContext = null;
     return result;
@@ -205,20 +251,17 @@ class SimpleUnreachableCodeElimination extends RemovingTransformer {
       throw 'StaticGet from const field $target should be evaluated by front-end: $node';
     }
 
-    if (!constantEvaluator.hasTargetOS ||
-        !constantEvaluator.isPlatformConst(target)) {
+    if (!_shouldEvaluateMember(target)) {
       return super.visitStaticGet(node, removalSentinel);
     }
 
-    final result = constantEvaluator.evaluate(_staticTypeContext!, node);
-
-    if (result is UnevaluatedConstant &&
-        result.expression is InvalidExpression) {
-      return result.expression;
+    _checkPlatformConstMember(target);
+    final constant = constantEvaluator.evaluate(_staticTypeContext!, node);
+    if (constant is UnevaluatedConstant) {
+      _throwPlatformConstError(target, 'cannot evaluate to a constant');
     }
-
     final type = node.getStaticType(_staticTypeContext!);
-    return ConstantExpression(result, type)..fileOffset = node.fileOffset;
+    return ConstantExpression(constant, type)..fileOffset = node.fileOffset;
   }
 
   @override
@@ -335,3 +378,16 @@ class ContinueSwitchStatementTargetCollector extends RecursiveVisitor {
     }
   }
 }
+
+class PlatformConstError extends Error {
+  final Object? message;
+  final Member member;
+  final Uri? uri;
+  final int offset;
+
+  PlatformConstError(this.message, this.member, this.uri, this.offset);
+
+  @override
+  String toString() => '${uri ?? ''}:$offset '
+      'Error for annotated member ${member.name}: $message';
+}

pkg/vm/lib/transformations/vm_constant_evaluator.dart

@@ -23,9 +23,9 @@ import 'pragma.dart';
 /// fields and getters annotated with "vm:platform-const".
 ///
 /// To avoid restricting getters annotated with "vm:platform-const" to be just
-/// a single return statement whose body is evaluated, we treat annotated
-/// getters as const functions. If [enableConstFunctions] is false, then
-/// only annotated getters are treated this way.
+/// a single return statement whose body is evaluated, as well as handling
+/// immediately-invoked closures wrapping complex initializing code in field
+/// initializers, we enable constant evaluation of functions.
 class VMConstantEvaluator extends ConstantEvaluator {
   final TargetOS? _targetOS;
   final Map<String, Constant> _constantFields = {};
@@ -43,15 +43,16 @@ class VMConstantEvaluator extends ConstantEvaluator {
       this._targetOS,
       this._pragmaParser,
       {bool enableTripleShift = false,
-      bool enableConstFunctions = false,
       bool enableAsserts = true,
       bool errorOnUnevaluatedConstant = false,
       EvaluationMode evaluationMode = EvaluationMode.weak})
       : _platformClass = typeEnvironment.coreTypes.platformClass,
         super(dartLibrarySupport, backend, component, environmentDefines,
             typeEnvironment, errorReporter,
             enableTripleShift: enableTripleShift,
-            enableConstFunctions: enableConstFunctions,
+            // We use evaluation of const functions for getters and immediately
+            // invoked closures in field initializers.
+            enableConstFunctions: true,
             enableAsserts: enableAsserts,
             errorOnUnevaluatedConstant: errorOnUnevaluatedConstant,
             evaluationMode: evaluationMode) {
@@ -68,10 +69,10 @@ class VMConstantEvaluator extends ConstantEvaluator {
       Target target, Component component, TargetOS? targetOS, NnbdMode nnbdMode,
       {bool evaluateAnnotations = true,
       bool enableTripleShift = false,
-      bool enableConstFunctions = false,
       bool enableConstructorTearOff = false,
       bool enableAsserts = true,
       bool errorOnUnevaluatedConstant = false,
+      ErrorReporter? errorReporter,
       Map<String, String>? environmentDefines,
       CoreTypes? coreTypes,
       ClassHierarchy? hierarchy}) {
@@ -90,11 +91,10 @@ class VMConstantEvaluator extends ConstantEvaluator {
         component,
         environmentDefines,
         typeEnvironment,
-        const SimpleErrorReporter(),
+        errorReporter ?? const SimpleErrorReporter(),
         targetOS,
         ConstantPragmaAnnotationParser(coreTypes, target),
         enableTripleShift: enableTripleShift,
-        enableConstFunctions: enableConstFunctions,
         enableAsserts: enableAsserts,
         errorOnUnevaluatedConstant: errorOnUnevaluatedConstant,
         evaluationMode: EvaluationMode.fromNnbdMode(nnbdMode));
@@ -128,24 +128,14 @@ class VMConstantEvaluator extends ConstantEvaluator {
         }
       }
 
-      final initializer = target.initializer;
-      if (initializer == null) {
-        throw 'Cannot const evaluate annotated field with no initializer';
-      }
+      // The base class only evaluates const fields, so the VM constant
+      // evaluator must manually request the initializer be evaluated.
+      // instead of just calling super.visitStaticGet(target).
       return withNewEnvironment(
-          () => evaluateExpressionInContext(target, initializer));
-    }
-
-    if (target is Procedure && target.kind == ProcedureKind.Getter) {
-      // Temporarily enable const functions and use the base class to evaluate
-      // the getter with appropriate caching/recursive evaluation checks.
-      final oldEnableConstFunctions = enableConstFunctions;
-      enableConstFunctions = true;
-      final result = super.visitStaticGet(node);
-      enableConstFunctions = oldEnableConstFunctions;
-      return result;
+          () => evaluateExpressionInContext(target, target.initializer!));
     }
 
-    throw 'Expected annotated field with initializer or getter, got $target';
+    // The base class already handles constant evaluation of a getter.
+    return super.visitStaticGet(node);
   }
 }