bjoernager/rust
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Move some functions into rustc_infer.

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They don't depend on trait selection anymore, so there is no need for an extension trait.
This commit is contained in:
Oli Scherer 2021-10-28 13:54:11 +00:00
parent 849b73b8d9
commit a8f06b249b
2 changed files with 316 additions and 318 deletions
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318
compiler/rustc_trait_selection/src/opaque_types.rs
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@ -1,19 +1,16 @@
use crate::traits::{self, ObligationCause, PredicateObligation};
use rustc_data_structures::fx::FxHashMap;
use rustc_data_structures::sync::Lrc;
use rustc_hir as hir;
use rustc_hir::def_id::{DefId, LocalDefId};
use rustc_infer::infer::error_reporting::unexpected_hidden_region_diagnostic;
use rustc_infer::infer::opaque_types::OpaqueTypeDecl;
use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
use rustc_infer::infer::{InferCtxt, InferOk};
use rustc_middle::ty::fold::{BottomUpFolder, TypeFoldable, TypeFolder, TypeVisitor};
use rustc_middle::ty::fold::{BottomUpFolder, TypeFoldable, TypeFolder};
use rustc_middle::ty::subst::{GenericArg, GenericArgKind, InternalSubsts, Subst};
use rustc_middle::ty::{self, OpaqueTypeKey, Ty, TyCtxt};
use rustc_span::Span;
use std::ops::ControlFlow;
pub trait InferCtxtExt<'tcx> {
fn instantiate_opaque_types<T: TypeFoldable<'tcx>>(
&self,
@ -23,12 +20,6 @@ pub trait InferCtxtExt<'tcx> {
value_span: Span,
) -> InferOk<'tcx, T>;
fn constrain_opaque_type(
&self,
opaque_type_key: OpaqueTypeKey<'tcx>,
opaque_defn: &OpaqueTypeDecl<'tcx>,
);
fn infer_opaque_definition_from_instantiation(
&self,
opaque_type_key: OpaqueTypeKey<'tcx>,
@ -81,236 +72,6 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
InferOk { value, obligations: instantiator.obligations }
}
/// 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))]
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,
)
},
});
}
/// Given the fully resolved, instantiated type for an opaque
/// type, i.e., the value of an inference variable like C1 or C2
/// (*), computes the "definition type" for an opaque type
@ -372,83 +133,6 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
}
}
// 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
}
}
struct ReverseMapper<'tcx> {
tcx: TyCtxt<'tcx>,