Rust: Address review comments

Author: paldepind

rust/ql/lib/codeql/rust/internal/Type.qll

@@ -133,26 +133,32 @@ class DataType extends Type, TDataType {
 
 /** A struct type. */
 class StructType extends DataType {
-  StructType() { super.getTypeItem() instanceof Struct }
+  private Struct struct;
+
+  StructType() { struct = super.getTypeItem() }
 
   /** Gets the struct that this struct type represents. */
-  override Struct getTypeItem() { result = super.getTypeItem() }
+  override Struct getTypeItem() { result = struct }
 }
 
 /** An enum type. */
 class EnumType extends DataType {
-  EnumType() { super.getTypeItem() instanceof Enum }
+  private Enum enum;
+
+  EnumType() { enum = super.getTypeItem() }
 
   /** Gets the enum that this enum type represents. */
-  override Enum getTypeItem() { result = super.getTypeItem() }
+  override Enum getTypeItem() { result = enum }
 }
 
 /** A union type. */
 class UnionType extends DataType {
-  UnionType() { super.getTypeItem() instanceof Union }
+  private Union union;
+
+  UnionType() { union = super.getTypeItem() }
 
   /** Gets the union that this union type represents. */
-  override Union getTypeItem() { result = super.getTypeItem() }
+  override Union getTypeItem() { result = union }
 }
 
 /** A trait type. */

rust/ql/lib/codeql/rust/internal/TypeInference.qll

@@ -907,7 +907,7 @@ private module StructExprMatchingInput implements MatchingInputSig {
       result = this.getTypeParameter(_) and
       path = TypePath::singleton(result)
       or
-      // type of the struct itself
+      // type of the struct or enum itself
       dpos.isStructPos() and
       path.isEmpty() and
       result = TDataType(this.getTypeItem())
@@ -2860,15 +2860,15 @@ private class TupleLikeStruct extends TupleLikeConstructor instanceof Struct {
 
   override TypeItem getTypeItem() { result = this }
 
-  override TupleField getTupleField(int i) { result = this.(Struct).getTupleField(i) }
+  override TupleField getTupleField(int i) { result = Struct.super.getTupleField(i) }
 }
 
 private class TupleLikeVariant extends TupleLikeConstructor instanceof Variant {
   TupleLikeVariant() { this.isTuple() }
 
   override TypeItem getTypeItem() { result = super.getEnum() }
 
-  override TupleField getTupleField(int i) { result = this.(Variant).getTupleField(i) }
+  override TupleField getTupleField(int i) { result = Variant.super.getTupleField(i) }
 }
 
 /**