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<h1 id="invariants-of-the-type-system"><a class="header" href="#invariants-of-the-type-system">Invariants of the type system</a></h1>
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<p>FIXME: This file talks about invariants of the type system as a whole, not only the solver</p>
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<p>There are a lot of invariants - things the type system guarantees to be true at all times -
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which are desirable or expected from other languages and type systems. Unfortunately, quite
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a few of them do not hold in Rust right now. This is either a fundamental to its design or
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caused by bugs and something that may change in the future.</p>
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<p>It is important to know about the things you can assume while working on - and with - the
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type system, so here's an incomplete and unofficial list of invariants of
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the core type system:</p>
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<ul>
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<li>✅: this invariant mostly holds, with some weird exceptions, you can rely on it outside
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of these cases</li>
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<li>❌: this invariant does not hold, either due to bugs or by design, you must not rely on
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it for soundness or have to be incredibly careful when doing so</li>
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</ul>
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<h3 id="wfx-implies-wfnormalizex-"><a class="header" href="#wfx-implies-wfnormalizex-"><code>wf(X)</code> implies <code>wf(normalize(X))</code> ✅</a></h3>
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<p>If a type containing aliases is well-formed, it should also be
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well-formed after normalizing said aliases. We rely on this as
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otherwise we would have to re-check for well-formedness for these
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types.</p>
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<h3 id="structural-equality-modulo-regions-implies-semantic-equality-"><a class="header" href="#structural-equality-modulo-regions-implies-semantic-equality-">Structural equality modulo regions implies semantic equality ✅</a></h3>
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<p>If you have a some type and equate it to itself after replacing any regions with unique
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inference variables in both the lhs and rhs, the now potentially structurally different
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types should still be equal to each other.</p>
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<p>Needed to prevent goals from succeeding in HIR typeck and then failing in MIR borrowck.
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If this invariant is broken MIR typeck ends up failing with an ICE.</p>
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<h3 id="applying-inference-results-from-a-goal-does-not-change-its-result-"><a class="header" href="#applying-inference-results-from-a-goal-does-not-change-its-result-">Applying inference results from a goal does not change its result ❌</a></h3>
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<p>TODO: this invariant is formulated in a weird way and needs to be elaborated.
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Pretty much: I would like this check to only fail if there's a solver bug:
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https://github.com/rust-lang/rust/blob/2ffeb4636b4ae376f716dc4378a7efb37632dc2d/compiler/rustc_trait_selection/src/solve/eval_ctxt.rs#L391-L407</p>
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<p>If we prove some goal/equate types/whatever, apply the resulting inference constraints,
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and then redo the original action, the result should be the same.</p>
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<p>This unfortunately does not hold - at least in the new solver - due to a few annoying reasons.</p>
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<h3 id="the-trait-solver-has-to-be-locally-sound-"><a class="header" href="#the-trait-solver-has-to-be-locally-sound-">The trait solver has to be <em>locally sound</em> ✅</a></h3>
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<p>This means that we must never return <em>success</em> for goals for which no <code>impl</code> exists. That would
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mean we assume a trait is implemented even though it is not, which is very likely to result in
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actual unsoundness. When using <code>where</code>-bounds to prove a goal, the <code>impl</code> will be provided by the
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user of the item.</p>
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<p>This invariant only holds if we check region constraints. As we do not check region constraints
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during implicit negative overlap check in coherence, this invariant is broken there. As this check
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relies on <em>completeness</em> of the trait solver, it is not able to use the current region constraints
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check - <code>InferCtxt::resolve_regions</code> - as its handling of type outlives goals is incomplete.</p>
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<h3 id="normalization-of-semantically-equal-aliases-in-empty-environments-results-in-a-unique-type-"><a class="header" href="#normalization-of-semantically-equal-aliases-in-empty-environments-results-in-a-unique-type-">Normalization of semantically equal aliases in empty environments results in a unique type ✅</a></h3>
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<p>Normalization for alias types/consts has to have a unique result. Otherwise we can easily
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implement transmute in safe code. Given the following function, we have to make sure that
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the input and output types always get normalized to the same concrete type.</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>fn foo<T: Trait>(
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x: <T as Trait>::Assoc
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) -> <T as Trait>::Assoc {
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x
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}
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<span class="boring">}</span></code></pre></pre>
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<p>Many of the currently known unsound issues end up relying on this invariant being broken.
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It is however very difficult to imagine a sound type system without this invariant, so
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the issue is that the invariant is broken, not that we incorrectly rely on it.</p>
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<h3 id="generic-goals-and-their-instantiations-have-the-same-result-"><a class="header" href="#generic-goals-and-their-instantiations-have-the-same-result-">Generic goals and their instantiations have the same result ✅</a></h3>
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<p>Pretty much: If we successfully typecheck a generic function concrete instantiations
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of that function should also typeck. We should not get errors post-monomorphization.
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We can however get overflow errors at that point.</p>
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<p>TODO: example for overflow error post-monomorphization</p>
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<p>This invariant is relied on to allow the normalization of generic aliases. Breaking
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it can easily result in unsoundness, e.g. <a href="https://github.com/rust-lang/rust/issues/57893">#57893</a></p>
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<h3 id="trait-goals-in-empty-environments-are-proven-by-a-unique-impl-"><a class="header" href="#trait-goals-in-empty-environments-are-proven-by-a-unique-impl-">Trait goals in empty environments are proven by a unique impl ✅</a></h3>
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<p>If a trait goal holds with an empty environment, there should be a unique <code>impl</code>,
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either user-defined or builtin, which is used to prove that goal. This is
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necessary to select a unique method.</p>
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<p>We do however break this invariant in few cases, some of which are due to bugs,
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some by design:</p>
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<ul>
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<li><em>marker traits</em> are allowed to overlap as they do not have associated items</li>
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<li><em>specialization</em> allows specializing impls to overlap with their parent</li>
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<li>the builtin trait object trait implementation can overlap with a user-defined impl:
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<a href="https://github.com/rust-lang/rust/issues/57893">#57893</a></li>
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</ul>
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<h3 id="the-type-system-is-complete-"><a class="header" href="#the-type-system-is-complete-">The type system is complete ❌</a></h3>
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<p>The type system is not complete, it often adds unnecessary inference constraints, and errors
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even though the goal could hold.</p>
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<ul>
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<li>method selection</li>
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<li>opaque type inference</li>
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<li>handling type outlives constraints</li>
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<li>preferring <code>ParamEnv</code> candidates over <code>Impl</code> candidates during candidate selection
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in the trait solver</li>
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</ul>
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<h4 id="the-type-system-is-complete-during-the-implicit-negative-overlap-check-in-coherence-"><a class="header" href="#the-type-system-is-complete-during-the-implicit-negative-overlap-check-in-coherence-">The type system is complete during the implicit negative overlap check in coherence ✅</a></h4>
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<p>For more on overlap checking: <a href="../coherence.html">coherence</a></p>
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<p>During the implicit negative overlap check in coherence we must never return <em>error</em> for
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goals which can be proven. This would allow for overlapping impls with potentially different
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associated items, breaking a bunch of other invariants.</p>
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<p>This invariant is currently broken in many different ways while actually something we rely on.
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We have to be careful as it is quite easy to break:</p>
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<ul>
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<li>generalization of aliases</li>
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<li>generalization during subtyping binders (luckily not exploitable in coherence)</li>
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</ul>
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<h3 id="trait-solving-must-be-free-lifetime-agnostic-"><a class="header" href="#trait-solving-must-be-free-lifetime-agnostic-">Trait solving must be (free) lifetime agnostic ✅</a></h3>
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<p>Trait solving during codegen should have the same result as during typeck. As we erase
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all free regions during codegen we must not rely on them during typeck. A noteworthy example
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is special behavior for <code>'static</code>.</p>
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<p>We also have to be careful with relying on equality of regions in the trait solver.
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This is fine for codegen, as we treat all erased regions as equal. We can however
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lose equality information from HIR to MIR typeck.</p>
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<p>The new solver "uniquifies regions" during canonicalization, canonicalizing <code>u32: Trait<'x, 'x></code>
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as <code>exists<'0, '1> u32: Trait<'0, '1></code>, to make it harder to rely on this property.</p>
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<h3 id="removing-ambiguity-makes-strictly-more-things-compile-"><a class="header" href="#removing-ambiguity-makes-strictly-more-things-compile-">Removing ambiguity makes strictly more things compile ❌</a></h3>
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<p>Ideally we <em>should</em> not rely on ambiguity for things to compile.
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Not doing that will cause future improvements to be breaking changes.</p>
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<p>Due to <em>incompleteness</em> this is not the case and improving inference can result in inference
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changes, breaking existing projects.</p>
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<h3 id="semantic-equality-implies-structural-equality-"><a class="header" href="#semantic-equality-implies-structural-equality-">Semantic equality implies structural equality ✅</a></h3>
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<p>Two types being equal in the type system must mean that they have the
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same <code>TypeId</code> after instantiating their generic parameters with concrete
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arguments. This currently does not hold: <a href="https://github.com/rust-lang/rust/issues/97156">#97156</a>.</p>
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