Rust: Use defaults for type parameters

Author: paldepind

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

@@ -42,6 +42,9 @@ abstract class Type extends TType {
   /** Gets the `i`th type parameter of this type, if any. */
   abstract TypeParameter getTypeParameter(int i);
 
+  /** Gets the the default type for the `i`th type parameter, if any. */
+  TypeMention getTypeParameterDefault(int i) { none() }
+
   /** Gets a type parameter of this type. */
   final TypeParameter getATypeParameter() { result = this.getTypeParameter(_) }
 
@@ -87,6 +90,10 @@ class StructType extends StructOrEnumType, TStruct {
     result = TTypeParamTypeParameter(struct.getGenericParamList().getTypeParam(i))
   }
 
+  override TypeMention getTypeParameterDefault(int i) {
+    result = struct.getGenericParamList().getTypeParam(i).getDefaultType()
+  }
+
   override string toString() { result = struct.getName().getText() }
 
   override Location getLocation() { result = struct.getLocation() }
@@ -108,6 +115,10 @@ class EnumType extends StructOrEnumType, TEnum {
     result = TTypeParamTypeParameter(enum.getGenericParamList().getTypeParam(i))
   }
 
+  override TypeMention getTypeParameterDefault(int i) {
+    result = enum.getGenericParamList().getTypeParam(i).getDefaultType()
+  }
+
   override string toString() { result = enum.getName().getText() }
 
   override Location getLocation() { result = enum.getLocation() }
@@ -133,6 +144,10 @@ class TraitType extends Type, TTrait {
       any(AssociatedTypeTypeParameter param | param.getTrait() = trait and param.getIndex() = i)
   }
 
+  override TypeMention getTypeParameterDefault(int i) {
+    result = trait.getGenericParamList().getTypeParam(i).getDefaultType()
+  }
+
   override string toString() { result = trait.toString() }
 
   override Location getLocation() { result = trait.getLocation() }

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

@@ -88,6 +88,11 @@ class PathTypeReprMention extends TypeMention instanceof PathTypeRepr {
   override TypeMention getTypeArgument(int i) {
     result = path.getSegment().getGenericArgList().getTypeArg(i)
     or
+    // If a type argument is not given in the path, then we use the default for
+    // the type parameter if one exists for the type.
+    not exists(path.getSegment().getGenericArgList().getTypeArg(i)) and
+    result = this.resolveType().getTypeParameterDefault(i)
+    or
     // `Self` paths inside `impl` blocks have implicit type arguments that are
     // the type parameters of the `impl` block. For example, in
     //