Running analyzer with empty memory store and resolving all files results in retained `PrefixScopeLookupResult`

[mkustermann]

When running the analyzer on flutter and it's transitive dependencies twice, once with an empty memory store and the second time with cleared library context but populated memory store we observe

• with empty memory store we have ~ 33k PrefixScopeLookupResult objects alive (~ 1 MB)
• with populated memory store we have no such objects at all

An example retaining path for those PrefixScopeLookupResult objects is:

AnalysisDriver.analysisContext (package:analyzer/src/dart/analysis/driver.dart)
    |-> LibraryContext (package:analyzer/src/dart/analysis/library_context.dart)
        |-> LinkedElementFactory (package:analyzer/src/summary2/linked_element_factory.dart)
            |-> AnalysisSessionImpl (package:analyzer/src/dart/analysis/session.dart)
                |-> ClassHierarchy (package:analyzer/src/dart/element/class_hierarchy.dart)
                    |-> _InternalLinkedHashMap (dart:collection)
                        |-> _List (dart:core)
                            |-> ClassElementImpl (package:analyzer/src/dart/element/element.dart)
                                |-> _GrowableList (dart:core)
                                    |-> _List (dart:core)
                                        |-> ConstFieldElementImpl (package:analyzer/src/dart/element/element.dart)
                                            |-> InstanceCreationExpressionImpl (package:analyzer/src/dart/ast/ast.dart)
                                                |-> ArgumentListImpl (package:analyzer/src/dart/ast/ast.dart)
                                                    |-> NodeListImpl (package:analyzer/src/dart/ast/ast.dart)
                                                        |-> _GrowableList (dart:core)
                                                            |-> _List (dart:core)
                                                                |-> NamedExpressionImpl (package:analyzer/src/dart/ast/ast.dart)
                                                                    |-> LabelImpl (package:analyzer/src/dart/ast/ast.dart)
                                                                        |-> SimpleIdentifierImpl (package:analyzer/src/dart/ast/ast.dart)
                                                                            |-> PrefixScopeLookupResult (package:analyzer/src/dart/element/scope.dart)

It appears the SimpleIdentifierImpl.scopeLookupResult is set during resolution (see resolver.dart):

  @override
  void visitSimpleIdentifier(covariant SimpleIdentifierImpl node) {
    ...
    var scopeLookupResult = nameScope.lookup(node.name);
    node.scopeLookupResult = scopeLookupResult;
    ...
  }

The fact that we don't have those objects when running with cached files in the memory store indicates that maybe the scopeLookupResult needs to be cleared at some point???

/cc @scheglov @bwilkerson

[scheglov]

Yes, we set these scopeLookupResult values while resolving AST, and could clear them after linking the element model (or resolving a unit, although now I wonder if it could be used in lints instead of LinterContext.resolveNameInScope()). But 1MB does not look huge to me, so may be it is not worth efforts.