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<h1 id="candidate-preference"><a class="header" href="#candidate-preference">Candidate preference</a></h1>
<p>There are multiple ways to prove <code>Trait</code> and <code>NormalizesTo</code> goals. Each such option is called a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_next_trait_solver/solve/assembly/struct.Candidate.html"><code>Candidate</code></a>. If there are multiple applicable candidates, we prefer some candidates over others. We store the relevant information in their <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_next_trait_solver/solve/enum.CandidateSource.html"><code>CandidateSource</code></a>.</p>
<p>This preference may result in incorrect inference or region constraints and would therefore be unsound during coherence. Because of this, we simply try to merge all candidates in coherence.</p>
<h2 id="trait-goals"><a class="header" href="#trait-goals"><code>Trait</code> goals</a></h2>
<p>Trait goals merge their applicable candidates in <a href="https://github.com/rust-lang/rust/blob/e3ee7f7aea5b45af3b42b5e4713da43876a65ac9/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs#L1342-L1424"><code>fn merge_trait_candidates</code></a>. This document provides additional details and references to explain <em>why</em> we’ve got the current preference rules.</p>
<h3 id="candidatesourcebuiltinimplbuiltinimplsourcetrivial"><a class="header" href="#candidatesourcebuiltinimplbuiltinimplsourcetrivial"><code>CandidateSource::BuiltinImpl(BuiltinImplSource::Trivial))</code></a></h3>
<p>Trivial builtin impls are builtin impls which are known to be always applicable for well-formed types. This means that if one exists, using another candidate should never have fewer constraints. We currently only consider <code>Sized</code> - and <code>MetaSized</code> - impls to be trivial.</p>
<p>This is necessary to prevent a lifetime error for the following pattern</p>
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>trait Trait&lt;T&gt;: Sized {}
impl&lt;'a&gt; Trait&lt;u32&gt; for &amp;'a str {}
impl&lt;'a&gt; Trait&lt;i32&gt; for &amp;'a str {}
fn is_sized&lt;T: Sized&gt;(_: T) {}
fn foo&lt;'a, 'b, T&gt;(x: &amp;'b str)
where
&amp;'a str: Trait&lt;T&gt;,
{
// Elaborating the `&amp;'a str: Trait&lt;T&gt;` where-bound results in a
// `&amp;'a str: Sized` where-bound. We do not want to prefer this
// over the builtin impl.
is_sized(x);
}
<span class="boring">}</span></code></pre>
<p>This preference is incorrect in case the builtin impl has a nested goal which relies on a non-param where-clause</p>
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>struct MyType&lt;'a, T: ?Sized&gt;(&amp;'a (), T);
fn is_sized&lt;T&gt;() {}
fn foo&lt;'a, T: ?Sized&gt;()
where
(MyType&lt;'a, T&gt;,): Sized,
MyType&lt;'static, T&gt;: Sized,
{
// The where-bound is trivial while the builtin `Sized` impl for tuples
// requires proving `MyType&lt;'a, T&gt;: Sized` which can only be proven by
// using the where-clause, adding an unnecessary `'static` constraint.
is_sized::&lt;(MyType&lt;'a, T&gt;,)&gt;();
//~^ ERROR lifetime may not live long enough
}
<span class="boring">}</span></code></pre>
<h3 id="candidatesourceparamenv"><a class="header" href="#candidatesourceparamenv"><code>CandidateSource::ParamEnv</code></a></h3>
<p>Once there’s at least one <em>non-global</em> <code>ParamEnv</code> candidate, we prefer <em>all</em> <code>ParamEnv</code> candidates over other candidate kinds.
A where-bound is global if it is not higher-ranked and doesn’t contain any generic parameters. It may contain <code>'static</code>.</p>
<p>We try to apply where-bounds over other candidates as users tends to have the most control over them, so they can most easily
adjust them in case our candidate preference is incorrect.</p>
<h4 id="preference-over-impl-candidates"><a class="header" href="#preference-over-impl-candidates">Preference over <code>Impl</code> candidates</a></h4>
<p>This is necessary to avoid region errors in the following example</p>
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>trait Trait&lt;'a&gt; {}
impl&lt;T&gt; Trait&lt;'static&gt; for T {}
fn impls_trait&lt;'a, T: Trait&lt;'a&gt;&gt;() {}
fn foo&lt;'a, T: Trait&lt;'a&gt;&gt;() {
impls_trait::&lt;'a, T&gt;();
}
<span class="boring">}</span></code></pre>
<p>We also need this as shadowed impls can result in currently ambiguous solver cycles: <a href="https://github.com/rust-lang/trait-system-refactor-initiative/issues/76">trait-system-refactor-initiative#76</a>. Without preference we’d be forced to fail with ambiguity
errors if the where-bound results in region constraints to avoid incompleteness.</p>
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>trait Super {
type SuperAssoc;
}
trait Trait: Super&lt;SuperAssoc = Self::TraitAssoc&gt; {
type TraitAssoc;
}
impl&lt;T, U&gt; Trait for T
where
T: Super&lt;SuperAssoc = U&gt;,
{
type TraitAssoc = U;
}
fn overflow&lt;T: Trait&gt;() {
// We can use the elaborated `Super&lt;SuperAssoc = Self::TraitAssoc&gt;` where-bound
// to prove the where-bound of the `T: Trait` implementation. This currently results in
// overflow.
let x: &lt;T as Trait&gt;::TraitAssoc;
}
<span class="boring">}</span></code></pre>
<p>This preference causes a lot of issues. See <a href="https://github.com/rust-lang/rust/issues/24066">#24066</a>. Most of the
issues are caused by preferring where-bounds over impls even if the where-bound guides type inference:</p>
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>trait Trait&lt;T&gt; {
fn call_me(&amp;self, x: T) {}
}
impl&lt;T&gt; Trait&lt;u32&gt; for T {}
impl&lt;T&gt; Trait&lt;i32&gt; for T {}
fn bug&lt;T: Trait&lt;U&gt;, U&gt;(x: T) {
x.call_me(1u32);
//~^ ERROR mismatched types
}
<span class="boring">}</span></code></pre>
<p>However, even if we only apply this preference if the where-bound doesn’t guide inference, it may still result
in incorrect lifetime constraints:</p>
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>trait Trait&lt;'a&gt; {}
impl&lt;'a&gt; Trait&lt;'a&gt; for &amp;'a str {}
fn impls_trait&lt;'a, T: Trait&lt;'a&gt;&gt;(_: T) {}
fn foo&lt;'a, 'b&gt;(x: &amp;'b str)
where
&amp;'a str: Trait&lt;'b&gt;
{
// Need to prove `&amp;'x str: Trait&lt;'b&gt;` with `'b: 'x`.
impls_trait::&lt;'b, _&gt;(x);
//~^ ERROR lifetime may not live long enough
}
<span class="boring">}</span></code></pre>
<h4 id="preference-over-aliasbound-candidates"><a class="header" href="#preference-over-aliasbound-candidates">Preference over <code>AliasBound</code> candidates</a></h4>
<p>This is necessary to avoid region errors in the following example</p>
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>trait Bound&lt;'a&gt; {}
trait Trait&lt;'a&gt; {
type Assoc: Bound&lt;'a&gt;;
}
fn impls_bound&lt;'b, T: Bound&lt;'b&gt;&gt;() {}
fn foo&lt;'a, 'b, 'c, T&gt;()
where
T: Trait&lt;'a&gt;,
for&lt;'hr&gt; T::Assoc: Bound&lt;'hr&gt;,
{
impls_bound::&lt;'b, T::Assoc&gt;();
impls_bound::&lt;'c, T::Assoc&gt;();
}
<span class="boring">}</span></code></pre>
<p>It can also result in unnecessary constraints</p>
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>trait Bound&lt;'a&gt; {}
trait Trait&lt;'a&gt; {
type Assoc: Bound&lt;'a&gt;;
}
fn impls_bound&lt;'b, T: Bound&lt;'b&gt;&gt;() {}
fn foo&lt;'a, 'b, T&gt;()
where
T: for&lt;'hr&gt; Trait&lt;'hr&gt;,
&lt;T as Trait&lt;'b&gt;&gt;::Assoc: Bound&lt;'a&gt;,
{
// Using the where-bound for `&lt;T as Trait&lt;'a&gt;&gt;::Assoc: Bound&lt;'a&gt;`
// unnecessarily equates `&lt;T as Trait&lt;'a&gt;&gt;::Assoc` with the
// `&lt;T as Trait&lt;'b&gt;&gt;::Assoc` from the env.
impls_bound::&lt;'a, &lt;T as Trait&lt;'a&gt;&gt;::Assoc&gt;();
// For a `&lt;T as Trait&lt;'b&gt;&gt;::Assoc: Bound&lt;'b&gt;` the self type of the
// where-bound matches, but the arguments of the trait bound don't.
impls_bound::&lt;'b, &lt;T as Trait&lt;'b&gt;&gt;::Assoc&gt;();
}
<span class="boring">}</span></code></pre>
<h4 id="why-no-preference-for-global-where-bounds"><a class="header" href="#why-no-preference-for-global-where-bounds">Why no preference for global where-bounds</a></h4>
<p>Global where-bounds are either fully implied by an impl or unsatisfiable. If they are unsatisfiable, we don’t really care what happens. If a where-bound is fully implied then using the impl to prove the trait goal cannot result in additional constraints. For trait goals this is only useful for where-bounds which use <code>'static</code>:</p>
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>trait A {
fn test(&amp;self);
}
fn foo(x: &amp;dyn A)
where
dyn A + 'static: A, // Using this bound would lead to a lifetime error.
{
x.test();
}
<span class="boring">}</span></code></pre>
<p>More importantly, by using impls here we prevent global where-bounds from shadowing impls when normalizing associated types. There are no known issues from preferring impls over global where-bounds.</p>
<h4 id="why-still-consider-global-where-bounds"><a class="header" href="#why-still-consider-global-where-bounds">Why still consider global where-bounds</a></h4>
<p>Given that we just use impls even if there exists a global where-bounds, you may ask why we don’t just ignore these global where-bounds entirely: we use them to weaken the inference guidance from non-global where-bounds.</p>
<p>Without a global where-bound, we currently prefer non-global where bounds even though there would be an applicable impl as well. By adding a non-global where-bound, this unnecessary inference guidance is disabled, allowing the following to compile:</p>
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>fn check&lt;Color&gt;(color: Color)
where
Vec: Into&lt;Color&gt; + Into&lt;f32&gt;,
{
let _: f32 = Vec.into();
// Without the global `Vec: Into&lt;f32&gt;` bound we'd
// eagerly use the non-global `Vec: Into&lt;Color&gt;` bound
// here, causing this to fail.
}
struct Vec;
impl From&lt;Vec&gt; for f32 {
fn from(_: Vec) -&gt; Self {
loop {}
}
}
<span class="boring">}</span></code></pre>
<h3 id="candidatesourcealiasbound"><a class="header" href="#candidatesourcealiasbound"><code>CandidateSource::AliasBound</code></a></h3>
<p>We prefer alias-bound candidates over impls. We currently use this preference to guide type inference, causing the following to compile. I personally don’t think this preference is desirable 🤷</p>
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub trait Dyn {
type Word: Into&lt;u64&gt;;
fn d_tag(&amp;self) -&gt; Self::Word;
fn tag32(&amp;self) -&gt; Option&lt;u32&gt; {
self.d_tag().into().try_into().ok()
// prove `Self::Word: Into&lt;?0&gt;` and then select a method
// on `?0`, needs eager inference.
}
}
<span class="boring">}</span></code></pre>
<pre class="playground"><code class="language-rust">fn impl_trait() -&gt; impl Into&lt;u32&gt; {
0u16
}
fn main() {
// There are two possible types for `x`:
// - `u32` by using the "alias bound" of `impl Into&lt;u32&gt;`
// - `impl Into&lt;u32&gt;`, i.e. `u16`, by using `impl&lt;T&gt; From&lt;T&gt; for T`
//
// We infer the type of `x` to be `u32` even though this is not
// strictly necessary and can even lead to surprising errors.
let x = impl_trait().into();
println!("{}", std::mem::size_of_val(&amp;x));
}</code></pre>
<p>This preference also avoids ambiguity due to region constraints, I don’t know whether people rely on this in practice.</p>
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>trait Bound&lt;'a&gt; {}
impl&lt;T&gt; Bound&lt;'static&gt; for T {}
trait Trait&lt;'a&gt; {
type Assoc: Bound&lt;'a&gt;;
}
fn impls_bound&lt;'b, T: Bound&lt;'b&gt;&gt;() {}
fn foo&lt;'a, T: Trait&lt;'a&gt;&gt;() {
// Should we infer this to `'a` or `'static`.
impls_bound::&lt;'_, T::Assoc&gt;();
}
<span class="boring">}</span></code></pre>
<h3 id="candidatesourcebuiltinimplbuiltinimplsourceobject_"><a class="header" href="#candidatesourcebuiltinimplbuiltinimplsourceobject_"><code>CandidateSource::BuiltinImpl(BuiltinImplSource::Object(_))</code></a></h3>
<p>We prefer builtin trait object impls over user-written impls. This is <strong>unsound</strong> and should be remoed in the future. See <a href="https://github.com/rust-lang/rust/issues/57893">#57893</a> and <a href="https://github.com/rust-lang/rust/pull/141347">#141347</a> for more details.</p>
<h2 id="normalizesto-goals"><a class="header" href="#normalizesto-goals"><code>NormalizesTo</code> goals</a></h2>
<p>The candidate preference behavior during normalization is implemented in <a href="https://github.com/rust-lang/rust/blob/e3ee7f7aea5b45af3b42b5e4713da43876a65ac9/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs#L920-L1003"><code>fn assemble_and_merge_candidates</code></a>.</p>
<h3 id="we-always-consider-aliasbound-candidates"><a class="header" href="#we-always-consider-aliasbound-candidates">We always consider <code>AliasBound</code> candidates</a></h3>
<p>In case the where-bound does not specify the associated item, we consider <code>AliasBound</code> candidates instead of treating the alias as rigid, even though the trait goal was proven via a <code>ParamEnv</code> candidate.</p>
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>trait Super {
type Assoc;
}
trait Bound {
type Assoc: Super&lt;Assoc = u32&gt;;
}
trait Trait: Super {}
// Elaborating the environment results in a `T::Assoc: Super` where-bound.
// This where-bound must not prevent normalization via the `Super&lt;Assoc = u32&gt;`
// item bound.
fn heck&lt;T: Bound&lt;Assoc: Trait&gt;&gt;(x: &lt;T::Assoc as Super&gt;::Assoc) -&gt; u32 {
x
}
<span class="boring">}</span></code></pre>
<p>Using such an alias can result in additional region constraints, cc <a href="https://github.com/rust-lang/rust/issues/133044">#133044</a>.</p>
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>trait Bound&lt;'a&gt; {
type Assoc;
}
trait Trait {
type Assoc: Bound&lt;'static, Assoc = u32&gt;;
}
fn heck&lt;'a, T: Trait&lt;Assoc: Bound&lt;'a&gt;&gt;&gt;(x: &lt;T::Assoc as Bound&lt;'a&gt;&gt;::Assoc) {
// Normalizing the associated type requires `T::Assoc: Bound&lt;'static&gt;` as it
// uses the `Bound&lt;'static&gt;` alias-bound instead of keeping the alias rigid.
drop(x);
}
<span class="boring">}</span></code></pre>
<h3 id="we-prefer-paramenv-candidates-over-aliasbound"><a class="header" href="#we-prefer-paramenv-candidates-over-aliasbound">We prefer <code>ParamEnv</code> candidates over <code>AliasBound</code></a></h3>
<p>While we use <code>AliasBound</code> candidates if the where-bound does not specify the associated type, in case it does, we prefer the where-bound.
This is necessary for the following example:</p>
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>// Make sure we prefer the `I::IntoIterator: Iterator&lt;Item = ()&gt;`
// where-bound over the `I::Intoiterator: Iterator&lt;Item = I::Item&gt;`
// alias-bound.
trait Iterator {
type Item;
}
trait IntoIterator {
type Item;
type IntoIter: Iterator&lt;Item = Self::Item&gt;;
}
fn normalize&lt;I: Iterator&lt;Item = ()&gt;&gt;() {}
fn foo&lt;I&gt;()
where
I: IntoIterator,
I::IntoIter: Iterator&lt;Item = ()&gt;,
{
// We need to prefer the `I::IntoIterator: Iterator&lt;Item = ()&gt;`
// where-bound over the `I::Intoiterator: Iterator&lt;Item = I::Item&gt;`
// alias-bound.
normalize::&lt;I::IntoIter&gt;();
}
<span class="boring">}</span></code></pre>
<h3 id="we-always-consider-where-bounds"><a class="header" href="#we-always-consider-where-bounds">We always consider where-bounds</a></h3>
<p>Even if the trait goal was proven via an impl, we still prefer <code>ParamEnv</code> candidates, if any exist.</p>
<h4 id="we-prefer-orphaned-where-bounds"><a class="header" href="#we-prefer-orphaned-where-bounds">We prefer “orphaned” where-bounds</a></h4>
<p>We add “orphaned” <code>Projection</code> clauses into the <code>ParamEnv</code> when normalizing item bounds of GATs and RPITIT in <code>fn check_type_bounds</code>.
We need to prefer these <code>ParamEnv</code> candidates over impls and other where-bounds.</p>
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span>#![feature(associated_type_defaults)]
<span class="boring">fn main() {
</span>trait Foo {
// We should be able to prove that `i32: Baz&lt;Self&gt;` because of
// the impl below, which requires that `Self::Bar&lt;()&gt;: Eq&lt;i32&gt;`
// which is true, because we assume `for&lt;T&gt; Self::Bar&lt;T&gt; = i32`.
type Bar&lt;T&gt;: Baz&lt;Self&gt; = i32;
}
trait Baz&lt;T: ?Sized&gt; {}
impl&lt;T: Foo + ?Sized&gt; Baz&lt;T&gt; for i32 where T::Bar&lt;()&gt;: Eq&lt;i32&gt; {}
trait Eq&lt;T&gt; {}
impl&lt;T&gt; Eq&lt;T&gt; for T {}
<span class="boring">}</span></code></pre>
<p>I don’t fully understand the cases where this preference is actually necessary and haven’t been able to exploit this in fun ways yet, but 🤷</p>
<h4 id="we-prefer-global-where-bounds-over-impls"><a class="header" href="#we-prefer-global-where-bounds-over-impls">We prefer global where-bounds over impls</a></h4>
<p>This is necessary for the following to compile. I don’t know whether anything relies on it in practice 🤷</p>
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>trait Id {
type This;
}
impl&lt;T&gt; Id for T {
type This = T;
}
fn foo&lt;T&gt;(x: T) -&gt; &lt;u32 as Id&gt;::This
where
u32: Id&lt;This = T&gt;,
{
x
}
<span class="boring">}</span></code></pre>
<p>This means normalization can result in additional region constraints, cc <a href="https://github.com/rust-lang/rust/issues/133044">#133044</a>.</p>
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>trait Trait {
type Assoc;
}
impl Trait for &amp;u32 {
type Assoc = u32;
}
fn trait_bound&lt;T: Trait&gt;() {}
fn normalize&lt;T: Trait&lt;Assoc = u32&gt;&gt;() {}
fn foo&lt;'a&gt;()
where
&amp;'static u32: Trait&lt;Assoc = u32&gt;,
{
trait_bound::&lt;&amp;'a u32&gt;(); // ok, proven via impl
normalize::&lt;&amp;'a u32&gt;(); // error, proven via where-bound
}
<span class="boring">}</span></code></pre>
<h3 id="trait-where-bounds-shadow-impls"><a class="header" href="#trait-where-bounds-shadow-impls">Trait where-bounds shadow impls</a></h3>
<p>Normalization of associated items does not consider impls if the corresponding trait goal has been proven via a <code>ParamEnv</code> or <code>AliasBound</code> candidate.
This means that for where-bounds which do not constrain associated types, the associated types remain <em>rigid</em>.</p>
<h4 id="using-impls-results-in-different-region-constraints"><a class="header" href="#using-impls-results-in-different-region-constraints">Using impls results in different region constraints</a></h4>
<p>This is necessary to avoid unnecessary region constraints from applying impls.</p>
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>trait Trait&lt;'a&gt; {
type Assoc;
}
impl Trait&lt;'static&gt; for u32 {
type Assoc = u32;
}
fn bar&lt;'b, T: Trait&lt;'b&gt;&gt;() -&gt; T::Assoc { todo!() }
fn foo&lt;'a&gt;()
where
u32: Trait&lt;'a&gt;,
{
// Normalizing the return type would use the impl, proving
// the `T: Trait` where-bound would use the where-bound, resulting
// in different region constraints.
bar::&lt;'_, u32&gt;();
}
<span class="boring">}</span></code></pre>
<h4 id="rpitit-type_of-cycles"><a class="header" href="#rpitit-type_of-cycles">RPITIT <code>type_of</code> cycles</a></h4>
<p>We currently have to avoid impl candidates if there are where-bounds to avoid query cycles for RPITIT, see <a href="https://github.com/rust-lang/rust/pull/139762">#139762</a>. It feels desirable to me to stop relying on auto-trait leakage of during RPITIT computation to remove this issue, see <a href="https://github.com/rust-lang/rust/issues/139788">#139788</a>.</p>
<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use std::future::Future;
pub trait ReactiveFunction: Send {
type Output;
fn invoke(self) -&gt; Self::Output;
}
trait AttributeValue {
fn resolve(self) -&gt; impl Future&lt;Output = ()&gt; + Send;
}
impl&lt;F, V&gt; AttributeValue for F
where
F: ReactiveFunction&lt;Output = V&gt;,
V: AttributeValue,
{
async fn resolve(self) {
// We're awaiting `&lt;V as AttributeValue&gt;::{synthetic#0}` here.
// Normalizing that one via the the impl we're currently in
// relies on `collect_return_position_impl_trait_in_trait_tys` which
// ends up relying on auto-trait leakage when checking that the
// opaque return type of this function implements the `Send` item
// bound of the trait definition.
self.invoke().resolve().await
}
}
<span class="boring">}</span></code></pre>
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