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Diffstat (limited to 'compiler/rustc_infer/src/infer/opaque_types.rs')
| -rw-r--r-- | compiler/rustc_infer/src/infer/opaque_types.rs | 649 |
1 files changed, 649 insertions, 0 deletions
diff --git a/compiler/rustc_infer/src/infer/opaque_types.rs b/compiler/rustc_infer/src/infer/opaque_types.rs new file mode 100644 index 000000000..e579afbf3 --- /dev/null +++ b/compiler/rustc_infer/src/infer/opaque_types.rs @@ -0,0 +1,649 @@ +use crate::infer::{DefiningAnchor, InferCtxt, InferOk}; +use crate::traits; +use hir::def_id::{DefId, LocalDefId}; +use hir::{HirId, OpaqueTyOrigin}; +use rustc_data_structures::sync::Lrc; +use rustc_data_structures::vec_map::VecMap; +use rustc_hir as hir; +use rustc_middle::traits::ObligationCause; +use rustc_middle::ty::fold::BottomUpFolder; +use rustc_middle::ty::subst::{GenericArgKind, Subst}; +use rustc_middle::ty::{ + self, OpaqueHiddenType, OpaqueTypeKey, Ty, TyCtxt, TypeFoldable, TypeSuperVisitable, + TypeVisitable, TypeVisitor, +}; +use rustc_span::Span; + +use std::ops::ControlFlow; + +pub type OpaqueTypeMap<'tcx> = VecMap<OpaqueTypeKey<'tcx>, OpaqueTypeDecl<'tcx>>; + +mod table; + +pub use table::{OpaqueTypeStorage, OpaqueTypeTable}; + +use super::type_variable::{TypeVariableOrigin, TypeVariableOriginKind}; +use super::InferResult; + +/// Information about the opaque types whose values we +/// are inferring in this function (these are the `impl Trait` that +/// appear in the return type). +#[derive(Clone, Debug)] +pub struct OpaqueTypeDecl<'tcx> { + /// The hidden types that have been inferred for this opaque type. + /// There can be multiple, but they are all `lub`ed together at the end + /// to obtain the canonical hidden type. + pub hidden_type: OpaqueHiddenType<'tcx>, + + /// The origin of the opaque type. + pub origin: hir::OpaqueTyOrigin, +} + +impl<'a, 'tcx> InferCtxt<'a, 'tcx> { + /// This is a backwards compatibility hack to prevent breaking changes from + /// lazy TAIT around RPIT handling. + pub fn replace_opaque_types_with_inference_vars<T: TypeFoldable<'tcx>>( + &self, + value: T, + body_id: HirId, + span: Span, + param_env: ty::ParamEnv<'tcx>, + ) -> InferOk<'tcx, T> { + if !value.has_opaque_types() { + return InferOk { value, obligations: vec![] }; + } + let mut obligations = vec![]; + let replace_opaque_type = |def_id: DefId| { + def_id + .as_local() + .map_or(false, |def_id| self.opaque_type_origin(def_id, span).is_some()) + }; + let value = value.fold_with(&mut ty::fold::BottomUpFolder { + tcx: self.tcx, + lt_op: |lt| lt, + ct_op: |ct| ct, + ty_op: |ty| match *ty.kind() { + ty::Opaque(def_id, _substs) if replace_opaque_type(def_id) => { + let def_span = self.tcx.def_span(def_id); + let span = if span.contains(def_span) { def_span } else { span }; + let code = traits::ObligationCauseCode::OpaqueReturnType(None); + let cause = ObligationCause::new(span, body_id, code); + // FIXME(compiler-errors): We probably should add a new TypeVariableOriginKind + // for opaque types, and then use that kind to fix the spans for type errors + // that we see later on. + let ty_var = self.next_ty_var(TypeVariableOrigin { + kind: TypeVariableOriginKind::TypeInference, + span, + }); + obligations.extend( + self.handle_opaque_type(ty, ty_var, true, &cause, param_env) + .unwrap() + .obligations, + ); + ty_var + } + _ => ty, + }, + }); + InferOk { value, obligations } + } + + pub fn handle_opaque_type( + &self, + a: Ty<'tcx>, + b: Ty<'tcx>, + a_is_expected: bool, + cause: &ObligationCause<'tcx>, + param_env: ty::ParamEnv<'tcx>, + ) -> InferResult<'tcx, ()> { + if a.references_error() || b.references_error() { + return Ok(InferOk { value: (), obligations: vec![] }); + } + let (a, b) = if a_is_expected { (a, b) } else { (b, a) }; + let process = |a: Ty<'tcx>, b: Ty<'tcx>| match *a.kind() { + ty::Opaque(def_id, substs) if def_id.is_local() => { + let def_id = def_id.expect_local(); + let origin = match self.defining_use_anchor { + DefiningAnchor::Bind(_) => { + // Check that this is `impl Trait` type is + // declared by `parent_def_id` -- i.e., one whose + // value we are inferring. At present, this is + // always true during the first phase of + // type-check, but not always true later on during + // NLL. Once we support named opaque types more fully, + // this same scenario will be able to arise during all phases. + // + // Here is an example using type alias `impl Trait` + // that indicates the distinction we are checking for: + // + // ```rust + // mod a { + // pub type Foo = impl Iterator; + // pub fn make_foo() -> Foo { .. } + // } + // + // mod b { + // fn foo() -> a::Foo { a::make_foo() } + // } + // ``` + // + // Here, the return type of `foo` references an + // `Opaque` indeed, but not one whose value is + // presently being inferred. You can get into a + // similar situation with closure return types + // today: + // + // ```rust + // fn foo() -> impl Iterator { .. } + // fn bar() { + // let x = || foo(); // returns the Opaque assoc with `foo` + // } + // ``` + self.opaque_type_origin(def_id, cause.span)? + } + DefiningAnchor::Bubble => self.opaque_ty_origin_unchecked(def_id, cause.span), + DefiningAnchor::Error => return None, + }; + if let ty::Opaque(did2, _) = *b.kind() { + // We could accept this, but there are various ways to handle this situation, and we don't + // want to make a decision on it right now. Likely this case is so super rare anyway, that + // no one encounters it in practice. + // It does occur however in `fn fut() -> impl Future<Output = i32> { async { 42 } }`, + // where it is of no concern, so we only check for TAITs. + if let Some(OpaqueTyOrigin::TyAlias) = + did2.as_local().and_then(|did2| self.opaque_type_origin(did2, cause.span)) + { + self.tcx + .sess + .struct_span_err( + cause.span, + "opaque type's hidden type cannot be another opaque type from the same scope", + ) + .span_label(cause.span, "one of the two opaque types used here has to be outside its defining scope") + .span_note( + self.tcx.def_span(def_id), + "opaque type whose hidden type is being assigned", + ) + .span_note( + self.tcx.def_span(did2), + "opaque type being used as hidden type", + ) + .emit(); + } + } + Some(self.register_hidden_type( + OpaqueTypeKey { def_id, substs }, + cause.clone(), + param_env, + b, + origin, + )) + } + _ => None, + }; + if let Some(res) = process(a, b) { + res + } else if let Some(res) = process(b, a) { + res + } else { + // Rerun equality check, but this time error out due to + // different types. + match self.at(cause, param_env).define_opaque_types(false).eq(a, b) { + Ok(_) => span_bug!( + cause.span, + "opaque types are never equal to anything but themselves: {:#?}", + (a.kind(), b.kind()) + ), + Err(e) => Err(e), + } + } + } + + /// Given the map `opaque_types` containing the opaque + /// `impl Trait` types whose underlying, hidden types are being + /// inferred, this method adds constraints to the regions + /// appearing in those underlying hidden types to ensure that they + /// at least do not refer to random scopes within the current + /// function. These constraints are not (quite) sufficient to + /// guarantee that the regions are actually legal values; that + /// final condition is imposed after region inference is done. + /// + /// # The Problem + /// + /// Let's work through an example to explain how it works. Assume + /// the current function is as follows: + /// + /// ```text + /// fn foo<'a, 'b>(..) -> (impl Bar<'a>, impl Bar<'b>) + /// ``` + /// + /// Here, we have two `impl Trait` types whose values are being + /// inferred (the `impl Bar<'a>` and the `impl + /// Bar<'b>`). Conceptually, this is sugar for a setup where we + /// define underlying opaque types (`Foo1`, `Foo2`) and then, in + /// the return type of `foo`, we *reference* those definitions: + /// + /// ```text + /// type Foo1<'x> = impl Bar<'x>; + /// type Foo2<'x> = impl Bar<'x>; + /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. } + /// // ^^^^ ^^ + /// // | | + /// // | substs + /// // def_id + /// ``` + /// + /// As indicating in the comments above, each of those references + /// is (in the compiler) basically a substitution (`substs`) + /// applied to the type of a suitable `def_id` (which identifies + /// `Foo1` or `Foo2`). + /// + /// Now, at this point in compilation, what we have done is to + /// replace each of the references (`Foo1<'a>`, `Foo2<'b>`) with + /// fresh inference variables C1 and C2. We wish to use the values + /// of these variables to infer the underlying types of `Foo1` and + /// `Foo2`. That is, this gives rise to higher-order (pattern) unification + /// constraints like: + /// + /// ```text + /// for<'a> (Foo1<'a> = C1) + /// for<'b> (Foo1<'b> = C2) + /// ``` + /// + /// For these equation to be satisfiable, the types `C1` and `C2` + /// can only refer to a limited set of regions. For example, `C1` + /// can only refer to `'static` and `'a`, and `C2` can only refer + /// to `'static` and `'b`. The job of this function is to impose that + /// constraint. + /// + /// Up to this point, C1 and C2 are basically just random type + /// inference variables, and hence they may contain arbitrary + /// regions. In fact, it is fairly likely that they do! Consider + /// this possible definition of `foo`: + /// + /// ```text + /// fn foo<'a, 'b>(x: &'a i32, y: &'b i32) -> (impl Bar<'a>, impl Bar<'b>) { + /// (&*x, &*y) + /// } + /// ``` + /// + /// Here, the values for the concrete types of the two impl + /// traits will include inference variables: + /// + /// ```text + /// &'0 i32 + /// &'1 i32 + /// ``` + /// + /// Ordinarily, the subtyping rules would ensure that these are + /// sufficiently large. But since `impl Bar<'a>` isn't a specific + /// type per se, we don't get such constraints by default. This + /// is where this function comes into play. It adds extra + /// constraints to ensure that all the regions which appear in the + /// inferred type are regions that could validly appear. + /// + /// This is actually a bit of a tricky constraint in general. We + /// want to say that each variable (e.g., `'0`) can only take on + /// values that were supplied as arguments to the opaque type + /// (e.g., `'a` for `Foo1<'a>`) or `'static`, which is always in + /// scope. We don't have a constraint quite of this kind in the current + /// region checker. + /// + /// # The Solution + /// + /// We generally prefer to make `<=` constraints, since they + /// integrate best into the region solver. To do that, we find the + /// "minimum" of all the arguments that appear in the substs: that + /// is, some region which is less than all the others. In the case + /// of `Foo1<'a>`, that would be `'a` (it's the only choice, after + /// all). Then we apply that as a least bound to the variables + /// (e.g., `'a <= '0`). + /// + /// In some cases, there is no minimum. Consider this example: + /// + /// ```text + /// fn baz<'a, 'b>() -> impl Trait<'a, 'b> { ... } + /// ``` + /// + /// Here we would report a more complex "in constraint", like `'r + /// in ['a, 'b, 'static]` (where `'r` is some region appearing in + /// the hidden type). + /// + /// # Constrain regions, not the hidden concrete type + /// + /// Note that generating constraints on each region `Rc` is *not* + /// the same as generating an outlives constraint on `Tc` itself. + /// For example, if we had a function like this: + /// + /// ``` + /// # #![feature(type_alias_impl_trait)] + /// # fn main() {} + /// # trait Foo<'a> {} + /// # impl<'a, T> Foo<'a> for (&'a u32, T) {} + /// fn foo<'a, T>(x: &'a u32, y: T) -> impl Foo<'a> { + /// (x, y) + /// } + /// + /// // Equivalent to: + /// # mod dummy { use super::*; + /// type FooReturn<'a, T> = impl Foo<'a>; + /// fn foo<'a, T>(x: &'a u32, y: T) -> FooReturn<'a, T> { + /// (x, y) + /// } + /// # } + /// ``` + /// + /// then the hidden type `Tc` would be `(&'0 u32, T)` (where `'0` + /// is an inference variable). If we generated a constraint that + /// `Tc: 'a`, then this would incorrectly require that `T: 'a` -- + /// but this is not necessary, because the opaque type we + /// create will be allowed to reference `T`. So we only generate a + /// constraint that `'0: 'a`. + #[instrument(level = "debug", skip(self))] + pub fn register_member_constraints( + &self, + param_env: ty::ParamEnv<'tcx>, + opaque_type_key: OpaqueTypeKey<'tcx>, + concrete_ty: Ty<'tcx>, + span: Span, + ) { + let def_id = opaque_type_key.def_id; + + let tcx = self.tcx; + + let concrete_ty = self.resolve_vars_if_possible(concrete_ty); + + debug!(?concrete_ty); + + let first_own_region = match self.opaque_ty_origin_unchecked(def_id, span) { + hir::OpaqueTyOrigin::FnReturn(..) | hir::OpaqueTyOrigin::AsyncFn(..) => { + // We lower + // + // fn foo<'l0..'ln>() -> impl Trait<'l0..'lm> + // + // into + // + // type foo::<'p0..'pn>::Foo<'q0..'qm> + // fn foo<l0..'ln>() -> foo::<'static..'static>::Foo<'l0..'lm>. + // + // For these types we only iterate over `'l0..lm` below. + tcx.generics_of(def_id).parent_count + } + // These opaque type inherit all lifetime parameters from their + // parent, so we have to check them all. + hir::OpaqueTyOrigin::TyAlias => 0, + }; + + // For a case like `impl Foo<'a, 'b>`, we would generate a constraint + // `'r in ['a, 'b, 'static]` for each region `'r` that appears in the + // hidden type (i.e., it must be equal to `'a`, `'b`, or `'static`). + // + // `conflict1` and `conflict2` are the two region bounds that we + // detected which were unrelated. They are used for diagnostics. + + // Create the set of choice regions: each region in the hidden + // type can be equal to any of the region parameters of the + // opaque type definition. + let choice_regions: Lrc<Vec<ty::Region<'tcx>>> = Lrc::new( + opaque_type_key.substs[first_own_region..] + .iter() + .filter_map(|arg| match arg.unpack() { + GenericArgKind::Lifetime(r) => Some(r), + GenericArgKind::Type(_) | GenericArgKind::Const(_) => None, + }) + .chain(std::iter::once(self.tcx.lifetimes.re_static)) + .collect(), + ); + + concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor { + op: |r| self.member_constraint(opaque_type_key, span, concrete_ty, r, &choice_regions), + }); + } + + #[instrument(skip(self), level = "trace")] + pub fn opaque_type_origin(&self, def_id: LocalDefId, span: Span) -> Option<OpaqueTyOrigin> { + let opaque_hir_id = self.tcx.hir().local_def_id_to_hir_id(def_id); + let parent_def_id = match self.defining_use_anchor { + DefiningAnchor::Bubble | DefiningAnchor::Error => return None, + DefiningAnchor::Bind(bind) => bind, + }; + let item_kind = &self.tcx.hir().expect_item(def_id).kind; + + let hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, .. }) = item_kind else { + span_bug!( + span, + "weird opaque type: {:#?}, {:#?}", + def_id, + item_kind + ) + }; + let in_definition_scope = match *origin { + // Async `impl Trait` + hir::OpaqueTyOrigin::AsyncFn(parent) => parent == parent_def_id, + // Anonymous `impl Trait` + hir::OpaqueTyOrigin::FnReturn(parent) => parent == parent_def_id, + // Named `type Foo = impl Bar;` + hir::OpaqueTyOrigin::TyAlias => { + may_define_opaque_type(self.tcx, parent_def_id, opaque_hir_id) + } + }; + trace!(?origin); + in_definition_scope.then_some(*origin) + } + + #[instrument(skip(self), level = "trace")] + fn opaque_ty_origin_unchecked(&self, def_id: LocalDefId, span: Span) -> OpaqueTyOrigin { + let origin = match self.tcx.hir().expect_item(def_id).kind { + hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, .. }) => origin, + ref itemkind => { + span_bug!(span, "weird opaque type: {:?}, {:#?}", def_id, itemkind) + } + }; + trace!(?origin); + origin + } +} + +// Visitor that requires that (almost) all regions in the type visited outlive +// `least_region`. We cannot use `push_outlives_components` because regions in +// closure signatures are not included in their outlives components. We need to +// ensure all regions outlive the given bound so that we don't end up with, +// say, `ReVar` appearing in a return type and causing ICEs when other +// functions end up with region constraints involving regions from other +// functions. +// +// We also cannot use `for_each_free_region` because for closures it includes +// the regions parameters from the enclosing item. +// +// We ignore any type parameters because impl trait values are assumed to +// capture all the in-scope type parameters. +struct ConstrainOpaqueTypeRegionVisitor<OP> { + op: OP, +} + +impl<'tcx, OP> TypeVisitor<'tcx> for ConstrainOpaqueTypeRegionVisitor<OP> +where + OP: FnMut(ty::Region<'tcx>), +{ + fn visit_binder<T: TypeVisitable<'tcx>>( + &mut self, + t: &ty::Binder<'tcx, T>, + ) -> ControlFlow<Self::BreakTy> { + t.super_visit_with(self); + ControlFlow::CONTINUE + } + + fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> { + match *r { + // ignore bound regions, keep visiting + ty::ReLateBound(_, _) => ControlFlow::CONTINUE, + _ => { + (self.op)(r); + ControlFlow::CONTINUE + } + } + } + + fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> { + // We're only interested in types involving regions + if !ty.flags().intersects(ty::TypeFlags::HAS_FREE_REGIONS) { + return ControlFlow::CONTINUE; + } + + match ty.kind() { + ty::Closure(_, ref substs) => { + // Skip lifetime parameters of the enclosing item(s) + + substs.as_closure().tupled_upvars_ty().visit_with(self); + substs.as_closure().sig_as_fn_ptr_ty().visit_with(self); + } + + ty::Generator(_, ref substs, _) => { + // Skip lifetime parameters of the enclosing item(s) + // Also skip the witness type, because that has no free regions. + + substs.as_generator().tupled_upvars_ty().visit_with(self); + substs.as_generator().return_ty().visit_with(self); + substs.as_generator().yield_ty().visit_with(self); + substs.as_generator().resume_ty().visit_with(self); + } + _ => { + ty.super_visit_with(self); + } + } + + ControlFlow::CONTINUE + } +} + +pub enum UseKind { + DefiningUse, + OpaqueUse, +} + +impl UseKind { + pub fn is_defining(self) -> bool { + match self { + UseKind::DefiningUse => true, + UseKind::OpaqueUse => false, + } + } +} + +impl<'a, 'tcx> InferCtxt<'a, 'tcx> { + #[instrument(skip(self), level = "debug")] + pub fn register_hidden_type( + &self, + opaque_type_key: OpaqueTypeKey<'tcx>, + cause: ObligationCause<'tcx>, + param_env: ty::ParamEnv<'tcx>, + hidden_ty: Ty<'tcx>, + origin: hir::OpaqueTyOrigin, + ) -> InferResult<'tcx, ()> { + let tcx = self.tcx; + let OpaqueTypeKey { def_id, substs } = opaque_type_key; + + // Ideally, we'd get the span where *this specific `ty` came + // from*, but right now we just use the span from the overall + // value being folded. In simple cases like `-> impl Foo`, + // these are the same span, but not in cases like `-> (impl + // Foo, impl Bar)`. + let span = cause.span; + + let mut obligations = vec![]; + let prev = self.inner.borrow_mut().opaque_types().register( + OpaqueTypeKey { def_id, substs }, + OpaqueHiddenType { ty: hidden_ty, span }, + origin, + ); + if let Some(prev) = prev { + obligations = self.at(&cause, param_env).eq(prev, hidden_ty)?.obligations; + } + + let item_bounds = tcx.bound_explicit_item_bounds(def_id.to_def_id()); + + for predicate in item_bounds.transpose_iter().map(|e| e.map_bound(|(p, _)| *p)) { + debug!(?predicate); + let predicate = predicate.subst(tcx, substs); + + let predicate = predicate.fold_with(&mut BottomUpFolder { + tcx, + ty_op: |ty| match *ty.kind() { + // We can't normalize associated types from `rustc_infer`, + // but we can eagerly register inference variables for them. + ty::Projection(projection_ty) if !projection_ty.has_escaping_bound_vars() => { + self.infer_projection( + param_env, + projection_ty, + cause.clone(), + 0, + &mut obligations, + ) + } + // Replace all other mentions of the same opaque type with the hidden type, + // as the bounds must hold on the hidden type after all. + ty::Opaque(def_id2, substs2) + if def_id.to_def_id() == def_id2 && substs == substs2 => + { + hidden_ty + } + _ => ty, + }, + lt_op: |lt| lt, + ct_op: |ct| ct, + }); + + if let ty::PredicateKind::Projection(projection) = predicate.kind().skip_binder() { + if projection.term.references_error() { + // No point on adding these obligations since there's a type error involved. + return Ok(InferOk { value: (), obligations: vec![] }); + } + trace!("{:#?}", projection.term); + } + // Require that the predicate holds for the concrete type. + debug!(?predicate); + obligations.push(traits::Obligation::new(cause.clone(), param_env, predicate)); + } + Ok(InferOk { value: (), obligations }) + } +} + +/// Returns `true` if `opaque_hir_id` is a sibling or a child of a sibling of `def_id`. +/// +/// Example: +/// ```ignore UNSOLVED (is this a bug?) +/// # #![feature(type_alias_impl_trait)] +/// pub mod foo { +/// pub mod bar { +/// pub trait Bar { /* ... */ } +/// pub type Baz = impl Bar; +/// +/// # impl Bar for () {} +/// fn f1() -> Baz { /* ... */ } +/// } +/// fn f2() -> bar::Baz { /* ... */ } +/// } +/// ``` +/// +/// Here, `def_id` is the `LocalDefId` of the defining use of the opaque type (e.g., `f1` or `f2`), +/// and `opaque_hir_id` is the `HirId` of the definition of the opaque type `Baz`. +/// For the above example, this function returns `true` for `f1` and `false` for `f2`. +fn may_define_opaque_type(tcx: TyCtxt<'_>, def_id: LocalDefId, opaque_hir_id: hir::HirId) -> bool { + let mut hir_id = tcx.hir().local_def_id_to_hir_id(def_id); + + // Named opaque types can be defined by any siblings or children of siblings. + let scope = tcx.hir().get_defining_scope(opaque_hir_id); + // We walk up the node tree until we hit the root or the scope of the opaque type. + while hir_id != scope && hir_id != hir::CRATE_HIR_ID { + hir_id = tcx.hir().local_def_id_to_hir_id(tcx.hir().get_parent_item(hir_id)); + } + // Syntactically, we are allowed to define the concrete type if: + let res = hir_id == scope; + trace!( + "may_define_opaque_type(def={:?}, opaque_node={:?}) = {}", + tcx.hir().find(hir_id), + tcx.hir().get(opaque_hir_id), + res + ); + res +} |