Move some functions into rustc_infer.

They don't depend on trait selection anymore, so there is no need for an extension trait.

Author: oli-obk

compiler/rustc_infer/src/infer/opaque_types.rs

@@ -1,8 +1,13 @@
+use crate::infer::InferCtxt;
+use rustc_data_structures::sync::Lrc;
 use rustc_data_structures::vec_map::VecMap;
 use rustc_hir as hir;
-use rustc_middle::ty::{OpaqueTypeKey, Ty};
+use rustc_middle::ty::subst::GenericArgKind;
+use rustc_middle::ty::{self, OpaqueTypeKey, Ty, TyCtxt, TypeFoldable, TypeVisitor};
 use rustc_span::Span;
 
+use std::ops::ControlFlow;
+
 pub type OpaqueTypeMap<'tcx> = VecMap<OpaqueTypeKey<'tcx>, OpaqueTypeDecl<'tcx>>;
 
 /// Information about the opaque types whose values we
@@ -45,3 +50,312 @@ pub struct OpaqueTypeDecl<'tcx> {
     /// The origin of the opaque type.
     pub origin: hir::OpaqueTyOrigin,
 }
+
+impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
+    /// 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` iself.
+    /// For example, if we had a function like this:
+    ///
+    /// ```rust
+    /// fn foo<'a, T>(x: &'a u32, y: T) -> impl Foo<'a> {
+    ///   (x, y)
+    /// }
+    ///
+    /// // Equivalent to:
+    /// type FooReturn<'a, T> = impl Foo<'a>;
+    /// fn foo<'a, T>(..) -> FooReturn<'a, T> { .. }
+    /// ```
+    ///
+    /// 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`.
+    ///
+    /// # The `free_region_relations` parameter
+    ///
+    /// The `free_region_relations` argument is used to find the
+    /// "minimum" of the regions supplied to a given opaque type.
+    /// It must be a relation that can answer whether `'a <= 'b`,
+    /// where `'a` and `'b` are regions that appear in the "substs"
+    /// for the opaque type references (the `<'a>` in `Foo1<'a>`).
+    ///
+    /// Note that we do not impose the constraints based on the
+    /// generic regions from the `Foo1` definition (e.g., `'x`). This
+    /// is because the constraints we are imposing here is basically
+    /// the concern of the one generating the constraining type C1,
+    /// which is the current function. It also means that we can
+    /// take "implied bounds" into account in some cases:
+    ///
+    /// ```text
+    /// trait SomeTrait<'a, 'b> { }
+    /// fn foo<'a, 'b>(_: &'a &'b u32) -> impl SomeTrait<'a, 'b> { .. }
+    /// ```
+    ///
+    /// Here, the fact that `'b: 'a` is known only because of the
+    /// implied bounds from the `&'a &'b u32` parameter, and is not
+    /// "inherent" to the opaque type definition.
+    ///
+    /// # Parameters
+    ///
+    /// - `opaque_types` -- the map produced by `instantiate_opaque_types`
+    /// - `free_region_relations` -- something that can be used to relate
+    ///   the free regions (`'a`) that appear in the impl trait.
+    #[instrument(level = "debug", skip(self))]
+    pub fn constrain_opaque_type(
+        &self,
+        opaque_type_key: OpaqueTypeKey<'tcx>,
+        opaque_defn: &OpaqueTypeDecl<'tcx>,
+    ) {
+        let def_id = opaque_type_key.def_id;
+
+        let tcx = self.tcx;
+
+        let concrete_ty = self.resolve_vars_if_possible(opaque_defn.concrete_ty);
+
+        debug!(?concrete_ty);
+
+        let first_own_region = match opaque_defn.origin {
+            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 {
+            tcx: self.tcx,
+            op: |r| {
+                self.member_constraint(
+                    opaque_type_key.def_id,
+                    opaque_defn.definition_span,
+                    concrete_ty,
+                    r,
+                    &choice_regions,
+                )
+            },
+        });
+    }
+}
+
+// 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<'tcx, OP> {
+    tcx: TyCtxt<'tcx>,
+    op: OP,
+}
+
+impl<'tcx, OP> TypeVisitor<'tcx> for ConstrainOpaqueTypeRegionVisitor<'tcx, OP>
+where
+    OP: FnMut(ty::Region<'tcx>),
+{
+    fn tcx_for_anon_const_substs(&self) -> Option<TyCtxt<'tcx>> {
+        Some(self.tcx)
+    }
+
+    fn visit_binder<T: TypeFoldable<'tcx>>(
+        &mut self,
+        t: &ty::Binder<'tcx, T>,
+    ) -> ControlFlow<Self::BreakTy> {
+        t.as_ref().skip_binder().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_POTENTIAL_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
+    }
+}