# Multiple defining uses of Type Alias Impl Traits

Disclaimer: this blog post comes from some notes I took meanwhile working on an Rust compiler issue and it’s not very polished but I’ve decided to publish it anyway.

A bug was reported to the compiler that involves type checking, inference, traits and in particular type alias impl traits (TAIT) with multiple defining uses and some other particular things about them. In this post we are going to explain how the system works as I was investigating this myself in order to fix this issue, what is the issue exactly about and then describe how we’ve solved the issue.

First, let’s get started with the code that causes the problem:

#![feature(min_type_alias_impl_trait)]

type X<A, B> = impl Into<&'static A>;

fn f<A, B: 'static>(a: &'static A, _b: B) -> (X<A, B>, X<B, A>) {
(a, a)
}

fn main() {
println!("{}", <X<_, _> as Into<&String>>::into(f(&[1isize, 2, 3], String::new()).1));
}


If you try to run this program using nightly (with one before the fix landed) you’ll get:

[santiago@galago tait_issue (master)]\$ cargo run
Compiling tait_issue v0.1.0 (/tmp/tait_issue)
Finished dev [unoptimized + debuginfo] target(s) in 0.53s
Running target/debug/tait_issue
Segmentation fault (core dumped)


This program should not compile but it does and when you run it you get a segfault. So, this is a soundness issue in our type system. We want every program that type checks to be a valid one; but that is not what happens with this example: the compiler type checks but the program is invalid.

Let’s see how the compiler type checks this and other simpler programs in order to get a better understanding of what’s going on.

## Type checking TAIT: a simple example

Let’s type check the following example:

type X<A> = impl Into<A>;
fn foo<T>(x: T) -> X<T> { x }


The idea is that impl Into<A> in that position ‘desugars’ into a kind of ‘existential type’, something whose value must be inferred. It is inferred when there is a “defining use”, which is basically a use of the type alias in a context that defines the value of the “hidden” type.

For the purposes of type checking, is important to consider that the following code which our example uses, lives on stdlib …

impl<T> From<T> for T {
fn from(t: T) -> T {
t
}
}

impl<T, U> Into<U> for T
where
U: From<T>,
{
fn into(self) -> U {
U::from(self)
}
}


So written as program clauses, from stdlib we have: <T> T: From<T> <T, U> T: Into<U> if U: From<T>

Let’s type check foo:

First of all, as we’ve said previously we need to convert the return type, which would be impl Into<T> into an existential type … So our foo function will look like this:

fn foo<T>(x: T) -> ?X { x }


and we will have the following clause: ?X: Into<T> because X<T> was really the return type.

Type checking foo’s body, which is not important for the purposes of this work, will give us that the type being returned is T because x is returned and its type is T. Then, we can deduce the following clause: T = ?X

Unifying those two clauses we end up with: T: Into<T>

Then, unifying the clause <T, U> T: Into<U> if U: From<T> with our current program clause we have <T> T: Into<T> if T: From<T>. So T: Into<T> holds if T: From<T> holds, and T: From<T> holds because it’s one of the clauses that also comes from stdlib.

So foo signature type checks and ?X is T.

What happens after that is that we use the inferred ?X value T, to figure out what’s the real type of X<A>, which in our case we conclude that it’s T.

## Type Alias Impl Traits Rules

Let’s consider the following example:

type X<A, B> = impl Sized;


In this case, what would be the “hidden” type?, it could either be A or B, those are types that the inference context can use to define the actual types.

If the defining use is:

fn foo<T, Z>(x: T, y: Z) -> X<T, Z> { x }


the “hidden” type would be A. Type alias can be treated as type X<A, B> = A.

If the defining use is:

fn foo<T, Z>(x: T, y: Z) -> X<T, Z> { y }


the “hidden” type would be B. Type alias can be treated as type X<A, B> = B.

If the defining use is:

fn foo<T>(x: T) -> X<T, T> { x }


that would be ambiguous, it could either be A, B or a combination of both.

So, the code is rejected by the compiler.

There’s a similar problem with the following example:

type X<A> = impl Sized;
fn foo() -> X<u32> { 22_u32 }


in this case, the type of X<A> could either be u32 or just A. So, the compiler again rejects the code.

So in order to avoid this ambiguity, when we have a “defining use” of a TAIT, the generic arguments to that TAIT must all be unique generic arguments from the surrounding scope.

In our previous examples we saw that we used u32 which is not generic so the compiler rejected the code and in the other example we’ve used <T, T> which is generic but the generic arguments are not unique, so the compiler rejected it too.

## Back to the reported issue

type X<A, B> = impl Into<&'static A>;

fn f<A, B: 'static>(a: &'static A, _b: B) -> (X<A, B>, X<B, A>) {
(a, a)
}


As we see in this example, there are two defining uses of X, X<A, B> and X<B, A> and each of those uses follow the rules. Both reference only generic parameters from f and are both unique. The bug happens because we do not recognize that there are two different uses of X. What we do today is that we create just one inference variable ?X for both uses and we ignore one of the uses. What we should do is treat each use separately, by creating two inference variables, let’s say ?X1 and ?X2.

In the following post we’ll see how we’ve implemented this change that allowed the compiler to create different inference variable for each different defining use.

## Implementing the idea in the compiler

I’ve placed a PR #86118 which implements the previously discussed idea.

During type check, for each defining use of a TAIT, we replace those uses with inference variables in instantiate_opaque_types. Following down the path taken by instantiate_opaque_types, we will be calling fold_opaque_ty for each opaque type that shows up meanwhile type checking. That’s going to replace each of the opaque types with an inference variable.

The problem we’ve is due this line. Using def_id as the key, we would be getting the same inference variable for both X<A, B> and X<B, A> defining uses. So, as stated previously we need to use def_id and substs as part of the key in order to differentiate things like X<A, B> from X<B, A>.

The first naive thing to try to do is to change OpaqueTypeMap to be, instead of a DefId -> OpaqueTypeDecl map, be just DefId, SubstsRef -> OpaqueTypeDecl map. I’ve tried that path and it gives a lot of troubles because the new key pair needs to implement HashStable and stuff like that, so I’m not taking this path in the blog post :).

What we’ve done instead is to implement a VecMap structure which will sort the previous issue, but also, we got rid of HashMap which was an overkill for our use case. This “map” would not have a lot of elements in it.

Considering all this, please check out the provided PR, which is already merged. Most of the changes in the PR that are not explained in this post, are fallouts produced by this mentioned key change, where we substituted DefId as the key of the map with DefId and SubstsRef and also I’ve removed SubstsRef from the produced values of OpaqueTypeMap given that SubstsRef is now part of the key.