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| <h1 id="inference-of-opaque-types-impl-trait"><a class="header" href="#inference-of-opaque-types-impl-trait">Inference of opaque types (<code>impl Trait</code>)</a></h1> |
| <p>This page describes how the compiler infers the <a href="./borrow-check/region-inference/member-constraints.html?highlight=%22hidden%20type%22#member-constraints">hidden type</a> for an <a href="./opaque-types-type-alias-impl-trait.html">opaque type</a>. |
| This kind of type inference is particularly complex because, |
| unlike other kinds of type inference, |
| it can work across functions and function bodies.</p> |
| <h2 id="running-example"><a class="header" href="#running-example">Running example</a></h2> |
| <p>To help explain how it works, let’s consider an example.</p> |
| <pre class="playground"><code class="language-rust">#![feature(type_alias_impl_trait)] |
| mod m { |
| pub type Seq<T> = impl IntoIterator<Item = T>; |
| |
| #[define_opaque(Seq)] |
| pub fn produce_singleton<T>(t: T) -> Seq<T> { |
| vec![t] |
| } |
| |
| #[define_opaque(Seq)] |
| pub fn produce_doubleton<T>(t: T, u: T) -> Seq<T> { |
| vec![t, u] |
| } |
| } |
| |
| fn is_send<T: Send>(_: &T) {} |
| |
| pub fn main() { |
| let elems = m::produce_singleton(22); |
| |
| is_send(&elems); |
| |
| for elem in elems { |
| println!("elem = {:?}", elem); |
| } |
| }</code></pre> |
| <p>In this code, the <em>opaque type</em> is <code>Seq<T></code>. |
| Its defining scope is the module <code>m</code>. |
| Its <em>hidden type</em> is <code>Vec<T></code>, |
| which is inferred from <code>m::produce_singleton</code> and <code>m::produce_doubleton</code>.</p> |
| <p>In the <code>main</code> function, the opaque type is out of its defining scope. |
| When <code>main</code> calls <code>m::produce_singleton</code>, it gets back a reference to the opaque type <code>Seq<i32></code>. |
| The <code>is_send</code> call checks that <code>Seq<i32>: Send</code>. |
| <code>Send</code> is not listed amongst the bounds of the impl trait, |
| but because of auto-trait leakage, we are able to infer that it holds. |
| The <code>for</code> loop desugaring requires that <code>Seq<T>: IntoIterator</code>, |
| which is provable from the bounds declared on <code>Seq<T></code>.</p> |
| <h3 id="type-checking-main"><a class="header" href="#type-checking-main">Type-checking <code>main</code></a></h3> |
| <p>Let’s start by looking what happens when we type-check <code>main</code>. |
| Initially we invoke <code>produce_singleton</code> and the return type is an opaque type |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/enum.ItemKind.html#variant.OpaqueTy"><code>OpaqueTy</code></a>.</p> |
| <h4 id="type-checking-the-for-loop"><a class="header" href="#type-checking-the-for-loop">Type-checking the for loop</a></h4> |
| <p>The for loop desugars the <code>in elems</code> part to <code>IntoIterator::into_iter(elems)</code>. |
| <code>elems</code> is of type <code>Seq<T></code>, so the type checker registers a <code>Seq<T>: IntoIterator</code> obligation. |
| This obligation is trivially satisfied, |
| because <code>Seq<T></code> is an opaque type (<code>impl IntoIterator<Item = T></code>) that has a bound for the trait. |
| Similar to how a <code>U: Foo</code> where bound allows <code>U</code> to trivially satisfy <code>Foo</code>, |
| opaque types’ bounds are available to the type checker and are used to fulfill obligations.</p> |
| <p>The type of <code>elem</code> in the for loop is inferred to be <code><Seq<T> as IntoIterator>::Item</code>, which is <code>T</code>. |
| At no point is the type checker interested in the hidden type.</p> |
| <h4 id="type-checking-the-is_send-call"><a class="header" href="#type-checking-the-is_send-call">Type-checking the <code>is_send</code> call</a></h4> |
| <p>When trying to prove auto trait bounds, |
| we first repeat the process as above, |
| to see if the auto trait is in the bound list of the opaque type. |
| If that fails, we reveal the hidden type of the opaque type, |
| but only to prove this specific trait bound, not in general. |
| Revealing is done by invoking the <code>type_of</code> query on the <code>DefId</code> of the opaque type. |
| The query will internally request the hidden types from the defining function(s) |
| and return that (see <a href="#within-the-type_of-query">the section on <code>type_of</code></a> for more details).</p> |
| <h4 id="flowchart-of-type-checking-steps"><a class="header" href="#flowchart-of-type-checking-steps">Flowchart of type checking steps</a></h4> |
| <pre class="mermaid">flowchart TD |
| TypeChecking["type checking `main`"] |
| subgraph TypeOfSeq["type_of(Seq<T>) query"] |
| WalkModuleHir["Walk the HIR for the module `m`\nto find the hidden types from each\nfunction/const/static within"] |
| VisitProduceSingleton["visit `produce_singleton`"] |
| InterimType["`produce_singleton` hidden type is `Vec<T>`\nkeep searching"] |
| VisitProduceDoubleton["visit `produce_doubleton`"] |
| CompareType["`produce_doubleton` hidden type is also Vec<T>\nthis matches what we saw before ✅"] |
| Done["No more items to look at in scope\nReturn `Vec<T>`"] |
| end |
| |
| BorrowCheckProduceSingleton["`borrow_check(produce_singleton)`"] |
| TypeCheckProduceSingleton["`type_check(produce_singleton)`"] |
| |
| BorrowCheckProduceDoubleton["`borrow_check(produce_doubleton)`"] |
| TypeCheckProduceDoubleton["`type_check(produce_doubleton)`"] |
| |
| Substitute["Substitute `T => u32`,\nyielding `Vec<i32>` as the hidden type"] |
| CheckSend["Check that `Vec<i32>: Send` ✅"] |
| |
| TypeChecking -- trait code for auto traits --> TypeOfSeq |
| TypeOfSeq --> WalkModuleHir |
| WalkModuleHir --> VisitProduceSingleton |
| VisitProduceSingleton --> BorrowCheckProduceSingleton |
| BorrowCheckProduceSingleton --> TypeCheckProduceSingleton |
| TypeCheckProduceSingleton --> InterimType |
| InterimType --> VisitProduceDoubleton |
| VisitProduceDoubleton --> BorrowCheckProduceDoubleton |
| BorrowCheckProduceDoubleton --> TypeCheckProduceDoubleton |
| TypeCheckProduceDoubleton --> CompareType --> Done |
| Done --> Substitute --> CheckSend |
| </pre> |
| |
| <h3 id="within-the-type_of-query"><a class="header" href="#within-the-type_of-query">Within the <code>type_of</code> query</a></h3> |
| <p>The <code>type_of</code> query, when applied to an opaque type O, returns the hidden type. |
| That hidden type is computed by combining the results |
| from each constraining function within the defining scope of O.</p> |
| <pre class="mermaid">flowchart TD |
| TypeOf["type_of query"] |
| TypeOf -- find_opaque_ty_constraints --> FindOpaqueTyConstraints |
| FindOpaqueTyConstraints --> Iterate |
| Iterate["Iterate over each item in defining scope"] |
| Iterate -- For each item --> TypeCheck |
| TypeCheck["Check typeck(I) to see if it constraints O"] |
| TypeCheck -- I does not\nconstrain O --> Iterate |
| TypeCheck -- I constrains O --> BorrowCheck |
| BorrowCheck["Invoke mir_borrowck(I) to get hidden type\nfor O computed by I"] |
| BorrowCheck --> PreviousType |
| PreviousType["Hidden type from I\nsame as any previous hidden type\nfound so far?"] |
| PreviousType -- Yes --> Complete |
| PreviousType -- No --> ReportError |
| ReportError["Report an error"] |
| ReportError --> Complete["Item I complete"] |
| Complete --> Iterate |
| |
| FindOpaqueTyConstraints -- All constraints found --> Done |
| Done["Done"] |
| </pre> |
| |
| <h3 id="relating-an-opaque-type-to-another-type"><a class="header" href="#relating-an-opaque-type-to-another-type">Relating an opaque type to another type</a></h3> |
| <p>There is one central place where an opaque type gets its hidden type constrained, |
| and that is the <code>handle_opaque_type</code> function. |
| Amusingly it takes two types, so you can pass any two types, |
| but one of them should be an opaque type. |
| The order is only important for diagnostics.</p> |
| <pre class="mermaid">flowchart TD |
| subgraph typecheck["type check comparison routines"] |
| equate.rs |
| sub.rs |
| lub.rs |
| end |
| |
| typecheck --> TwoSimul |
| |
| subgraph handleopaquetype["infcx.handle_opaque_type"] |
| |
| TwoSimul["Defining two opaque types simultaneously?"] |
| |
| TwoSimul -- Yes --> ReportError["Report error"] |
| |
| TwoSimul -- No --> MayDefine -- Yes --> RegisterOpaqueType --> AlreadyHasValue |
| |
| MayDefine -- No --> ReportError |
| |
| MayDefine["In defining scope OR in query?"] |
| |
| AlreadyHasValue["Opaque type X already has\na registered value?"] |
| |
| AlreadyHasValue -- No --> Obligations["Register opaque type bounds\nas obligations for hidden type"] |
| |
| RegisterOpaqueType["Register opaque type with\nother type as value"] |
| |
| AlreadyHasValue -- Yes --> EquateOpaqueTypes["Equate new hidden type\nwith old hidden type"] |
| end |
| </pre> |
| |
| <h3 id="interactions-with-queries"><a class="header" href="#interactions-with-queries">Interactions with queries</a></h3> |
| <p>When queries handle opaque types, |
| they cannot figure out whether they are in a defining scope, |
| so they just assume they are.</p> |
| <p>The registered hidden types are stored into the <code>QueryResponse</code> struct |
| in the <code>opaque_types</code> field (the function |
| <code>take_opaque_types_for_query_response</code> reads them out).</p> |
| <p>When the <code>QueryResponse</code> is instantiated into the surrounding infcx in |
| <code>query_response_substitution_guess</code>, |
| we convert each hidden type constraint by invoking <code>handle_opaque_type</code> (as above).</p> |
| <p>There is one bit of “weirdness”. |
| The instantiated opaque types have an order |
| (if one opaque type was compared with another, |
| and we have to pick one opaque type to use as the one that gets its hidden type assigned). |
| We use the one that is considered “expected”. |
| But really both of the opaque types may have defining uses. |
| When the query result is instantiated, |
| that will be re-evaluated from the context that is using the query. |
| The final context (typeck of a function, mir borrowck or wf-checks) |
| will know which opaque type can actually be instantiated |
| and then handle it correctly.</p> |
| <h3 id="within-the-mir-borrow-checker"><a class="header" href="#within-the-mir-borrow-checker">Within the MIR borrow checker</a></h3> |
| <p>The MIR borrow checker relates things via <code>nll_relate</code> and only cares about regions. |
| Any type relation will trigger the binding of hidden types, |
| so the borrow checker is doing the same thing as the type checker, |
| but ignores obviously dead code (e.g. after a panic). |
| The borrow checker is also the source of truth when it comes to hidden types, |
| as it is the only one who can properly figure out what lifetimes on the hidden type correspond |
| to which lifetimes on the opaque type declaration.</p> |
| <h2 id="backwards-compatibility-hacks"><a class="header" href="#backwards-compatibility-hacks">Backwards compatibility hacks</a></h2> |
| <p><code>impl Trait</code> in return position has various quirks that were not part |
| of any RFCs and are likely accidental stabilization. |
| To support these, |
| the <code>replace_opaque_types_with_inference_vars</code> is being used to reintroduce the previous behaviour.</p> |
| <p>There are three backwards compatibility hacks:</p> |
| <ol> |
| <li> |
| <p>All return sites share the same inference variable, |
| so some return sites may only compile if another return site uses a concrete type.</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>fn foo() -> impl Debug { |
| if false { |
| return std::iter::empty().collect(); |
| } |
| vec![42] |
| } |
| <span class="boring">}</span></code></pre> |
| </li> |
| <li> |
| <p>Associated type equality constraints for <code>impl Trait</code> can be used |
| as long as the hidden type satisfies the trait bounds on the associated type. |
| The opaque <code>impl Trait</code> signature does not need to satisfy them.</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>trait Duh {} |
| |
| impl Duh for i32 {} |
| |
| trait Trait { |
| type Assoc: Duh; |
| } |
| |
| // the fact that `R` is the `::Output` projection on `F` causes |
| // an intermediate inference var to be generated which is then later |
| // compared against the actually found `Assoc` type. |
| impl<R: Duh, F: FnMut() -> R> Trait for F { |
| type Assoc = R; |
| } |
| |
| // The `impl Send` here is then later compared against the inference var |
| // created, causing the inference var to be set to `impl Send` instead of |
| // the hidden type. We already have obligations registered on the inference |
| // var to make it uphold the `: Duh` bound on `Trait::Assoc`. The opaque |
| // type does not implement `Duh`, even if its hidden type does. |
| // Lazy TAIT would error out, but we inserted a hack to make it work again, |
| // keeping backwards compatibility. |
| fn foo() -> impl Trait<Assoc = impl Send> { |
| || 42 |
| } |
| <span class="boring">}</span></code></pre> |
| </li> |
| <li> |
| <p>Closures cannot create hidden types for their parent function’s <code>impl Trait</code>. |
| This point is mostly moot, |
| because of point 1 introducing inference vars, |
| so the closure only ever sees the inference var, but should we fix 1, this will become a problem.</p> |
| </li> |
| </ol> |
| |
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