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Clean up and reorganize traits chapter

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Mark Mansi 2018-02-12 15:22:38 -06:00
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3
src/SUMMARY.md
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@ -18,6 +18,9 @@
- [The `ty` module: representing types](./ty.md)
- [Type inference](./type-inference.md)
- [Trait resolution](./trait-resolution.md)
- [Higher-ranked trait bounds](./trait-hrtb.md)
- [Caching subtleties](./trait-caching.md)
- [Speciailization](./trait-specialization.md)
- [Type checking](./type-checking.md)
- [The MIR (Mid-level IR)](./mir.md)
- [MIR construction](./mir-construction.md)

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# 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:
```rust
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).

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# 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`:
```rust
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`:
```rust
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`:
```rust
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.
[paper by SPJ]: http://research.microsoft.com/en-us/um/people/simonpj/papers/higher-rank/
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
```rust
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:
```rust
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>`.

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@ -1,10 +1,7 @@
# Trait resolution
This document describes the general process and points out some non-obvious
things.
**WARNING:** This material was moved verbatim from a rustc README, so
it may not "fit" the style of the guide until it is adapted.
This chapter describes the general process of _trait resolution_ and points out
some non-obvious things.
## Major concepts
@ -21,12 +18,12 @@ 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`
it is the job of trait resolution to figure out whether there exists an impl of
(in this case) `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`:
provide an impl. For example, consider the body of `clone_slice`:
```rust
fn clone_slice<T:Clone>(x: &[T]) -> Vec<T> {
@ -43,26 +40,33 @@ 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.
an impl. Basically, the trait resolution system resolves an obligation
by proving that an appropriate impl does exist.
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 to choose an actual implementation, which
will then be generated in the output binary.
## Overview
Trait resolution consists of three major parts:
- SELECTION: Deciding how to resolve a specific obligation. For
- **Selection** is 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
resolved by employing an impl which matches the `Self` type, or by
using a parameter bound (e.g. `T: Trait`). 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
- **Fulfillment** is keeping track of which 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
- **Coherence** checks are intended to ensure that there
are never overlapping impls, where two impls could be used with
equal precedence.
@ -83,12 +87,21 @@ and returns a `SelectionResult`. There are three possible outcomes:
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.
type error or because there are no impls that could possibly apply.
The basic algorithm for selection is broken into two big phases:
candidate assembly and confirmation.
Note that because of how 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).
### Candidate assembly
Searches for impls/where-clauses/etc that might
@ -98,6 +111,15 @@ 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 subroutines that decide whether a particular impl/where-clause/etc
applies to a particular obligation are collectively refered to as the
process of _matching_. 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.
**TODO**: what does "unifying the `Self` types" mean? The `Self` of the
obligation with that of an impl?
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
@ -141,16 +163,14 @@ 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
`y`. Of the two impls we can see, the only one that matches is
`Convert<usize> for isize`. Therefore, we can
select this impl, which will cause the type of `$Y` to be unified to
`usize`. (Note that while assembling candidates, we do the initial
unifications in a transaction, so that they don't affect one another.)
There are tests to this effect in src/test/run-pass:
traits-multidispatch-infer-convert-source-and-target.rs
traits-multidispatch-infer-convert-target.rs
**TODO**: The example says we can "select" the impl, but this section is talking specifically about candidate assembly. Does this mean we can sometimes skip confirmation? Or is this poor wording?
**TODO**: Is the unification of `$Y` part of trait resolution or type inference? Or is this not the same type of "inference variable" as in type inference?
#### Winnowing: Resolving ambiguities
@ -167,94 +187,103 @@ impl<T:Copy> Get for T {
}
impl<T:Get> Get for Box<T> {
fn get(&self) -> Box<T> { box get_it(&**self) }
fn get(&self) -> Box<T> { Box::new(get_it(&**self)) }
}
```
What happens when we invoke `get_it(&box 1_u16)`, for example? In this
What happens when we invoke `get_it(&Box::new(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
boxes. In order for this to be unambiguous, the compiler does a *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`.
then, we are left with just one candidate, so we can proceed.
#### 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
#### `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
where clause. The selection process is always given a [parameter
environment] which contains a list of where clauses, which are
basically obligations that we 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
in that list. 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.
the same trait (or some subtrait) and which can match against the obligation.
[parameter environment]: ./param_env.html
Consider this simple example:
```rust
trait A1 { /*...*/ }
trait A1 {
fn do_a1(&self);
}
trait A2 : A1 { /*...*/ }
trait B { /*...*/ }
trait B {
fn do_b(&self);
}
fn foo<X:A2+B> { /*...*/ }
fn foo<X:A2+B>(x: X) {
x.do_a1(); // (*)
x.do_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`.
In the body of `foo`, clearly we can use methods of `A1`, `A2`, or `B`
on variable `x`. The line marked `(*)` will incur an obligation `X: A1`,
which the line marked `(#)` will incur an obligation `X: B`. Meanwhile,
the parameter environment will contain two where-clauses: `X : A2` and `X : B`.
For each obligation, then, we search this list of where-clauses. The
obligation `X: B` trivially matches against the where-clause `X: B`.
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.
_Confirmation_ unifies the output type parameters of the trait with the
values found in the obligation, possibly yielding a type error.
Suppose we have the following variation of the `Convert` example in the
previous section:
```rust
trait Convert<Target> {
fn convert(&self) -> Target;
}
impl Convert<usize> for isize { /*...*/ } // isize -> usize
impl Convert<isize> for usize { /*...*/ } // usize -> isize
let x: isize = ...;
let y: char = x.convert(); // NOTE: `y: char` now!
```
Confirmation is where an error would be reported because the impl specified
that `Target` would be `usize`, but the obligation reported `char`. Hence the
result of selection would be an error.
Note that the candidate impl is chosen base on the `Self` type, but
confirmation is done based on (in this case) the `Target` type parameter.
### 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.
As mentioned above, during type checking, we do not store the results of trait
selection. At trans time, repeat the trait selection to choose a particular
impl for each method call. In this second selection, we do not consider any
where-clauses to be in scope because we know that 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.
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 for the type
variables of the impl available to us, so we must run trait selection to figure
everything out.
**TODO**: is this still talking about trans?
Here is an example:
@ -272,214 +301,3 @@ nested obligation `isize : Bar<U>` to find out that `U=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`:
```rust
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`:
```rust
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`:
```rust
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.
[paper by SPJ]: http://research.microsoft.com/en-us/um/people/simonpj/papers/higher-rank/
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
```rust
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:
```rust
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:
```rust
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`](./param_env.html) 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.

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# 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.