rust/src/librustc/traits

TRAIT RESOLUTION

This document describes the general process and points out some non-obvious things.

Major concepts

Trait resolution is the process of pairing up an impl with each reference to a trait. So, for example, if there is a generic function like:

fn clone_slice<T:Clone>(x: &[T]) -> Vec<T> { /*...*/ }

and then a call to that function:

let v: Vec<isize> = clone_slice(&[1, 2, 3])

it is the job of trait resolution to figure out (in which case) whether there exists an impl of isize : Clone

Note that in some cases, like generic functions, we may not be able to find a specific impl, but we can figure out that the caller must provide an impl. To see what I mean, consider the body of clone_slice:

fn clone_slice<T:Clone>(x: &[T]) -> Vec<T> {
    let mut v = Vec::new();
    for e in &x {
        v.push((*e).clone()); // (*)
    }
}

The line marked (*) is only legal if T (the type of *e) implements the Clone trait. Naturally, since we don't know what T is, we can't find the specific impl; but based on the bound T:Clone, we can say that there exists an impl which the caller must provide.

We use the term obligation to refer to a trait reference in need of an impl.

Overview

Trait resolution consists of three major parts:

• SELECTION: Deciding how to resolve a specific obligation. For example, selection might decide that a specific obligation can be resolved by employing an impl which matches the self type, or by using a parameter bound. In the case of an impl, Selecting one obligation can create nested obligations because of where clauses on the impl itself. It may also require evaluating those nested obligations to resolve ambiguities.

• FULFILLMENT: The fulfillment code is what tracks that obligations are completely fulfilled. Basically it is a worklist of obligations to be selected: once selection is successful, the obligation is removed from the worklist and any nested obligations are enqueued.

• COHERENCE: The coherence checks are intended to ensure that there are never overlapping impls, where two impls could be used with equal precedence.

Selection

Selection is the process of deciding whether an obligation can be resolved and, if so, how it is to be resolved (via impl, where clause, etc). The main interface is the select() function, which takes an obligation and returns a SelectionResult. There are three possible outcomes:

• Ok(Some(selection)) -- yes, the obligation can be resolved, and selection indicates how. If the impl was resolved via an impl, then selection may also indicate nested obligations that are required by the impl.

• Ok(None) -- we are not yet sure whether the obligation can be resolved or not. This happens most commonly when the obligation contains unbound type variables.

• Err(err) -- the obligation definitely cannot be resolved due to a type error, or because there are no impls that could possibly apply, etc.

The basic algorithm for selection is broken into two big phases: candidate assembly and confirmation.

Candidate assembly

Searches for impls/where-clauses/etc that might possibly be used to satisfy the obligation. Each of those is called a candidate. To avoid ambiguity, we want to find exactly one candidate that is definitively applicable. In some cases, we may not know whether an impl/where-clause applies or not -- this occurs when the obligation contains unbound inference variables.

The basic idea for candidate assembly is to do a first pass in which we identify all possible candidates. During this pass, all that we do is try and unify the type parameters. (In particular, we ignore any nested where clauses.) Presuming that this unification succeeds, the impl is added as a candidate.

Once this first pass is done, we can examine the set of candidates. If it is a singleton set, then we are done: this is the only impl in scope that could possibly apply. Otherwise, we can winnow down the set of candidates by using where clauses and other conditions. If this reduced set yields a single, unambiguous entry, we're good to go, otherwise the result is considered ambiguous.

The basic process: Inferring based on the impls we see

This process is easier if we work through some examples. Consider the following trait:

trait Convert<Target> {
    fn convert(&self) -> Target;
}

This trait just has one method. It's about as simple as it gets. It converts from the (implicit) Self type to the Target type. If we wanted to permit conversion between isize and usize, we might implement Convert like so:

impl Convert<usize> for isize { /*...*/ } // isize -> usize
impl Convert<isize> for usize { /*...*/ } // usize -> isize

Now imagine there is some code like the following:

let x: isize = ...;
let y = x.convert();

The call to convert will generate a trait reference Convert<$Y> for isize, where $Y is the type variable representing the type of y. When we match this against the two impls we can see, we will find that only one remains: Convert<usize> for isize. Therefore, we can select this impl, which will cause the type of $Y to be unified to usize. (Note that while assembling candidates, we do the initial unifications in a transaction, so that they don't affect one another.)

There are tests to this effect in src/test/run-pass:

traits-multidispatch-infer-convert-source-and-target.rs traits-multidispatch-infer-convert-target.rs

Winnowing: Resolving ambiguities

But what happens if there are multiple impls where all the types unify? Consider this example:

trait Get {
    fn get(&self) -> Self;
}

impl<T:Copy> Get for T {
    fn get(&self) -> T { *self }
}

impl<T:Get> Get for Box<T> {
    fn get(&self) -> Box<T> { box get_it(&**self) }
}

What happens when we invoke get_it(&box 1_u16), for example? In this case, the Self type is Box<u16> -- that unifies with both impls, because the first applies to all types, and the second to all boxes. In the olden days we'd have called this ambiguous. But what we do now is do a second winnowing pass that considers where clauses and attempts to remove candidates -- in this case, the first impl only applies if Box<u16> : Copy, which doesn't hold. After winnowing, then, we are left with just one candidate, so we can proceed. There is a test of this in src/test/run-pass/traits-conditional-dispatch.rs.

Matching

The subroutines that decide whether a particular impl/where-clause/etc applies to a particular obligation. At the moment, this amounts to unifying the self types, but in the future we may also recursively consider some of the nested obligations, in the case of an impl.

Lifetimes and selection

Because of how that lifetime inference works, it is not possible to give back immediate feedback as to whether a unification or subtype relationship between lifetimes holds or not. Therefore, lifetime matching is not considered during selection. This is reflected in the fact that subregion assignment is infallible. This may yield lifetime constraints that will later be found to be in error (in contrast, the non-lifetime-constraints have already been checked during selection and can never cause an error, though naturally they may lead to other errors downstream).

Where clauses

Besides an impl, the other major way to resolve an obligation is via a where clause. The selection process is always given a parameter environment which contains a list of where clauses, which are basically obligations that can assume are satisfiable. We will iterate over that list and check whether our current obligation can be found in that list, and if so it is considered satisfied. More precisely, we want to check whether there is a where-clause obligation that is for the same trait (or some subtrait) and for which the self types match, using the definition of matching given above.

Consider this simple example:

trait A1 { /*...*/ }
trait A2 : A1 { /*...*/ }

trait B { /*...*/ }

fn foo<X:A2+B> { /*...*/ }

Clearly we can use methods offered by A1, A2, or B within the body of foo. In each case, that will incur an obligation like X : A1 or X : A2. The parameter environment will contain two where-clauses, X : A2 and X : B. For each obligation, then, we search this list of where-clauses. To resolve an obligation X:A1, we would note that X:A2 implies that X:A1.

Confirmation

Confirmation unifies the output type parameters of the trait with the values found in the obligation, possibly yielding a type error. If we return to our example of the Convert trait from the previous section, confirmation is where an error would be reported, because the impl specified that T would be usize, but the obligation reported char. Hence the result of selection would be an error.

Selection during translation

During type checking, we do not store the results of trait selection. We simply wish to verify that trait selection will succeed. Then later, at trans time, when we have all concrete types available, we can repeat the trait selection. In this case, we do not consider any where-clauses to be in scope. We know that therefore each resolution will resolve to a particular impl.

One interesting twist has to do with nested obligations. In general, in trans, we only need to do a "shallow" selection for an obligation. That is, we wish to identify which impl applies, but we do not (yet) need to decide how to select any nested obligations. Nonetheless, we do currently do a complete resolution, and that is because it can sometimes inform the results of type inference. That is, we do not have the full substitutions in terms of the type variables of the impl available to us, so we must run trait selection to figure everything out.

Here is an example:

trait Foo { /*...*/ }
impl<U,T:Bar<U>> Foo for Vec<T> { /*...*/ }

impl Bar<usize> for isize { /*...*/ }

After one shallow round of selection for an obligation like `Vec

Foo, we would know which impl we want, and we would know that T=isize, but we do not know the type of U. We must select the nested obligation isize : Barto find out thatU=usize`.

It would be good to only do just as much nested resolution as necessary. Currently, though, we just do a full resolution.

Higher-ranked trait bounds

One of the more subtle concepts at work are higher-ranked trait bounds. An example of such a bound is for<'a> MyTrait<&'a isize>. Let's walk through how selection on higher-ranked trait references works.

Basic matching and skolemization leaks

Let's walk through the test compile-fail/hrtb-just-for-static.rs to see how it works. The test starts with the trait Foo:

trait Foo<X> {
    fn foo(&self, x: X) { }
}

Let's say we have a function want_hrtb that wants a type which implements Foo<&'a isize> for any 'a:

fn want_hrtb<T>() where T : for<'a> Foo<&'a isize> { ... }

Now we have a struct AnyInt that implements Foo<&'a isize> for any 'a:

struct AnyInt;
impl<'a> Foo<&'a isize> for AnyInt { }

And the question is, does AnyInt : for<'a> Foo<&'a isize>? We want the answer to be yes. The algorithm for figuring it out is closely related to the subtyping for higher-ranked types (which is described in middle::infer::higher_ranked::doc, but also in a paper by SPJ that I recommend you read).

1. Skolemize the obligation.
2. Match the impl against the skolemized obligation.
3. Check for skolemization leaks.

So let's work through our example. The first thing we would do is to skolemize the obligation, yielding AnyInt : Foo<&'0 isize> (here '0 represents skolemized region #0). Note that now have no quantifiers; in terms of the compiler type, this changes from a ty::PolyTraitRef to a TraitRef. We would then create the TraitRef from the impl, using fresh variables for it's bound regions (and thus getting Foo<&'$a isize>, where '$a is the inference variable for 'a). Next we relate the two trait refs, yielding a graph with the constraint that '0 == '$a. Finally, we check for skolemization "leaks" -- a leak is basically any attempt to relate a skolemized region to another skolemized region, or to any region that pre-existed the impl match. The leak check is done by searching from the skolemized region to find the set of regions that it is related to in any way. This is called the "taint" set. To pass the check, that set must consist solely of itself and region variables from the impl. If the taint set includes any other region, then the match is a failure. In this case, the taint set for '0 is {'0, '$a}, and hence the check will succeed.

Let's consider a failure case. Imagine we also have a struct

struct StaticInt;
impl Foo<&'static isize> for StaticInt;

We want the obligation StaticInt : for<'a> Foo<&'a isize> to be considered unsatisfied. The check begins just as before. 'a is skolemized to '0 and the impl trait reference is instantiated to Foo<&'static isize>. When we relate those two, we get a constraint like 'static == '0. This means that the taint set for '0 is {'0, 'static}, which fails the leak check.

Higher-ranked trait obligations

Once the basic matching is done, we get to another interesting topic: how to deal with impl obligations. I'll work through a simple example here. Imagine we have the traits Foo and Bar and an associated impl:

trait Foo<X> {
    fn foo(&self, x: X) { }
}

trait Bar<X> {
    fn bar(&self, x: X) { }
}

impl<X,F> Foo<X> for F
    where F : Bar<X>
{
}

Now let's say we have a obligation for<'a> Foo<&'a isize> and we match this impl. What obligation is generated as a result? We want to get for<'a> Bar<&'a isize>, but how does that happen?

After the matching, we are in a position where we have a skolemized substitution like X => &'0 isize. If we apply this substitution to the impl obligations, we get F : Bar<&'0 isize>. Obviously this is not directly usable because the skolemized region '0 cannot leak out of our computation.

What we do is to create an inverse mapping from the taint set of '0 back to the original bound region ('a, here) that '0 resulted from. (This is done in higher_ranked::plug_leaks). We know that the leak check passed, so this taint set consists solely of the skolemized region itself plus various intermediate region variables. We then walk the trait-reference and convert every region in that taint set back to a late-bound region, so in this case we'd wind up with for<'a> F : Bar<&'a isize>.

Caching and subtle considerations therewith

In general we attempt to cache the results of trait selection. This is a somewhat complex process. Part of the reason for this is that we want to be able to cache results even when all the types in the trait reference are not fully known. In that case, it may happen that the trait selection process is also influencing type variables, so we have to be able to not only cache the result of the selection process, but replay its effects on the type variables.

An example

The high-level idea of how the cache works is that we first replace all unbound inference variables with skolemized versions. Therefore, if we had a trait reference usize : Foo<$1>, where $n is an unbound inference variable, we might replace it with usize : Foo<%0>, where %n is a skolemized type. We would then look this up in the cache. If we found a hit, the hit would tell us the immediate next step to take in the selection process: i.e., apply impl #22, or apply where clause X : Foo<Y>. Let's say in this case there is no hit. Therefore, we search through impls and where clauses and so forth, and we come to the conclusion that the only possible impl is this one, with def-id 22:

impl Foo<isize> for usize { ... } // Impl #22

We would then record in the cache usize : Foo<%0> ==> ImplCandidate(22). Next we would confirm ImplCandidate(22), which would (as a side-effect) unify $1 with isize.

Now, at some later time, we might come along and see a usize : Foo<$3>. When skolemized, this would yield usize : Foo<%0>, just as before, and hence the cache lookup would succeed, yielding ImplCandidate(22). We would confirm ImplCandidate(22) which would (as a side-effect) unify $3 with isize.

Where clauses and the local vs global cache

One subtle interaction is that the results of trait lookup will vary depending on what where clauses are in scope. Therefore, we actually have two caches, a local and a global cache. The local cache is attached to the ParamEnv and the global cache attached to the tcx. We use the local cache whenever the result might depend on the where clauses that are in scope. The determination of which cache to use is done by the method pick_candidate_cache in select.rs. At the moment, we use a very simple, conservative rule: if there are any where-clauses in scope, then we use the local cache. We used to try and draw finer-grained distinctions, but that led to a serious of annoying and weird bugs like #22019 and #18290. This simple rule seems to be pretty clearly safe and also still retains a very high hit rate (~95% when compiling rustc).

Specialization

Defined in the specialize module.

The basic strategy is to build up a specialization graph during coherence checking. Insertion into the graph locates the right place to put an impl in the specialization hierarchy; if there is no right place (due to partial overlap but no containment), you get an overlap error. Specialization is consulted when selecting an impl (of course), and the graph is consulted when propagating defaults down the specialization hierarchy.

You might expect that the specialization graph would be used during selection -- i.e., when actually performing specialization. This is not done for two reasons:

• It's merely an optimization: given a set of candidates that apply, we can determine the most specialized one by comparing them directly for specialization, rather than consulting the graph. Given that we also cache the results of selection, the benefit of this optimization is questionable.

• To build the specialization graph in the first place, we need to use selection (because we need to determine whether one impl specializes another). Dealing with this reentrancy would require some additional mode switch for selection. Given that there seems to be no strong reason to use the graph anyway, we stick with a simpler approach in selection, and use the graph only for propagating default implementations.

Trait impl selection can succeed even when multiple impls can apply, as long as they are part of the same specialization family. In that case, it returns a single impl on success -- this is the most specialized impl known to apply. However, if there are any inference variables in play, the returned impl may not be the actual impl we will use at trans time. Thus, we take special care to avoid projecting associated types unless either (1) the associated type does not use default and thus cannot be overridden or (2) all input types are known concretely.