You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: ty.rs
    path of file: ./repos/prusti-dev/vir/defs/high/operations_internal
    
    the code of the file until where you have to start completion: use super::super::ast::{
    expression::{visitors::ExpressionFolder, *},
    ty::{
        visitors::{default_walk_reference, default_walk_type, TypeFolder, TypeWalker},
        *,
    },
    type_decl::DiscriminantValue,
};
use rustc_hash::FxHashMap;
use std::collections::BTreeMap;

impl Type {
    /// Return a type that represents a variant of the given enum.
    #[must_use]
    pub fn variant(self, variant: VariantIndex) -> Self {
        match self {
            Type::Enum(Enum {
   
    
    fn substitute_types(self, substs: &FxHashMap<TypeVar, Type>) -> Self {
        struct TypeSubstitutor<'a> {
            substs: &'a FxHashMap<TypeVar, Type>,
        }
        impl<'a> TypeFolder for TypeSubstitutor<'a> {
            fn fold_type_var_enum(&mut self, var: TypeVar) -> Type {
                if let Some(new_type) = self.substs.get(&var).cloned() {
                    new_type
                } else {
                    Type::TypeVar(var)
                }
            }
        }
        let mut substitutor = TypeSubstitutor { substs };
        substitutor.fold_type(self)
    }