mirror of
https://github.com/rust-lang/rust.git
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868 lines
41 KiB
Rust
868 lines
41 KiB
Rust
//! Support code for rustdoc and external tools.
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//! You really don't want to be using this unless you need to.
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use super::*;
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use crate::errors::UnableToConstructConstantValue;
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use crate::infer::region_constraints::{Constraint, RegionConstraintData};
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use crate::infer::InferCtxt;
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use crate::traits::project::ProjectAndUnifyResult;
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use rustc_middle::mir::interpret::ErrorHandled;
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use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
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use rustc_middle::ty::visit::TypeVisitable;
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use rustc_middle::ty::{PolyTraitRef, Region, RegionVid};
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use rustc_data_structures::fx::{FxHashMap, FxHashSet};
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use std::collections::hash_map::Entry;
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use std::collections::VecDeque;
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use std::iter;
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// FIXME(twk): this is obviously not nice to duplicate like that
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#[derive(Eq, PartialEq, Hash, Copy, Clone, Debug)]
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pub enum RegionTarget<'tcx> {
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Region(Region<'tcx>),
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RegionVid(RegionVid),
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}
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#[derive(Default, Debug, Clone)]
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pub struct RegionDeps<'tcx> {
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larger: FxHashSet<RegionTarget<'tcx>>,
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smaller: FxHashSet<RegionTarget<'tcx>>,
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}
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pub enum AutoTraitResult<A> {
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ExplicitImpl,
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PositiveImpl(A),
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NegativeImpl,
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}
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#[allow(dead_code)]
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impl<A> AutoTraitResult<A> {
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fn is_auto(&self) -> bool {
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matches!(self, AutoTraitResult::PositiveImpl(_) | AutoTraitResult::NegativeImpl)
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}
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}
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pub struct AutoTraitInfo<'cx> {
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pub full_user_env: ty::ParamEnv<'cx>,
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pub region_data: RegionConstraintData<'cx>,
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pub vid_to_region: FxHashMap<ty::RegionVid, ty::Region<'cx>>,
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}
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pub struct AutoTraitFinder<'tcx> {
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tcx: TyCtxt<'tcx>,
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}
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impl<'tcx> AutoTraitFinder<'tcx> {
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pub fn new(tcx: TyCtxt<'tcx>) -> Self {
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AutoTraitFinder { tcx }
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}
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/// Makes a best effort to determine whether and under which conditions an auto trait is
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/// implemented for a type. For example, if you have
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///
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/// ```
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/// struct Foo<T> { data: Box<T> }
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/// ```
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///
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/// then this might return that `Foo<T>: Send` if `T: Send` (encoded in the AutoTraitResult
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/// type). The analysis attempts to account for custom impls as well as other complex cases.
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/// This result is intended for use by rustdoc and other such consumers.
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///
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/// (Note that due to the coinductive nature of Send, the full and correct result is actually
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/// quite simple to generate. That is, when a type has no custom impl, it is Send iff its field
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/// types are all Send. So, in our example, we might have that `Foo<T>: Send` if `Box<T>: Send`.
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/// But this is often not the best way to present to the user.)
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///
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/// Warning: The API should be considered highly unstable, and it may be refactored or removed
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/// in the future.
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pub fn find_auto_trait_generics<A>(
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&self,
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ty: Ty<'tcx>,
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orig_env: ty::ParamEnv<'tcx>,
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trait_did: DefId,
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mut auto_trait_callback: impl FnMut(AutoTraitInfo<'tcx>) -> A,
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) -> AutoTraitResult<A> {
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let tcx = self.tcx;
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let trait_ref = ty::TraitRef { def_id: trait_did, substs: tcx.mk_substs_trait(ty, &[]) };
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let trait_pred = ty::Binder::dummy(trait_ref);
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let infcx = tcx.infer_ctxt().build();
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let mut selcx = SelectionContext::new(&infcx);
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for f in [
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PolyTraitRef::to_poly_trait_predicate,
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PolyTraitRef::to_poly_trait_predicate_negative_polarity,
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] {
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let result =
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selcx.select(&Obligation::new(ObligationCause::dummy(), orig_env, f(&trait_pred)));
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if let Ok(Some(ImplSource::UserDefined(_))) = result {
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debug!(
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"find_auto_trait_generics({:?}): \
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manual impl found, bailing out",
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trait_ref
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);
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// If an explicit impl exists, it always takes priority over an auto impl
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return AutoTraitResult::ExplicitImpl;
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}
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}
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let infcx = tcx.infer_ctxt().build();
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let mut fresh_preds = FxHashSet::default();
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// Due to the way projections are handled by SelectionContext, we need to run
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// evaluate_predicates twice: once on the original param env, and once on the result of
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// the first evaluate_predicates call.
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//
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// The problem is this: most of rustc, including SelectionContext and traits::project,
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// are designed to work with a concrete usage of a type (e.g., Vec<u8>
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// fn<T>() { Vec<T> }. This information will generally never change - given
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// the 'T' in fn<T>() { ... }, we'll never know anything else about 'T'.
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// If we're unable to prove that 'T' implements a particular trait, we're done -
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// there's nothing left to do but error out.
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//
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// However, synthesizing an auto trait impl works differently. Here, we start out with
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// a set of initial conditions - the ParamEnv of the struct/enum/union we're dealing
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// with - and progressively discover the conditions we need to fulfill for it to
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// implement a certain auto trait. This ends up breaking two assumptions made by trait
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// selection and projection:
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//
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// * We can always cache the result of a particular trait selection for the lifetime of
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// an InfCtxt
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// * Given a projection bound such as '<T as SomeTrait>::SomeItem = K', if 'T:
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// SomeTrait' doesn't hold, then we don't need to care about the 'SomeItem = K'
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//
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// We fix the first assumption by manually clearing out all of the InferCtxt's caches
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// in between calls to SelectionContext.select. This allows us to keep all of the
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// intermediate types we create bound to the 'tcx lifetime, rather than needing to lift
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// them between calls.
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//
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// We fix the second assumption by reprocessing the result of our first call to
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// evaluate_predicates. Using the example of '<T as SomeTrait>::SomeItem = K', our first
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// pass will pick up 'T: SomeTrait', but not 'SomeItem = K'. On our second pass,
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// traits::project will see that 'T: SomeTrait' is in our ParamEnv, allowing
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// SelectionContext to return it back to us.
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let Some((new_env, user_env)) = self.evaluate_predicates(
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&infcx,
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trait_did,
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ty,
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orig_env,
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orig_env,
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&mut fresh_preds,
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false,
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) else {
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return AutoTraitResult::NegativeImpl;
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};
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let (full_env, full_user_env) = self
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.evaluate_predicates(&infcx, trait_did, ty, new_env, user_env, &mut fresh_preds, true)
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.unwrap_or_else(|| {
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panic!("Failed to fully process: {:?} {:?} {:?}", ty, trait_did, orig_env)
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});
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debug!(
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"find_auto_trait_generics({:?}): fulfilling \
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with {:?}",
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trait_ref, full_env
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);
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infcx.clear_caches();
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// At this point, we already have all of the bounds we need. FulfillmentContext is used
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// to store all of the necessary region/lifetime bounds in the InferContext, as well as
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// an additional sanity check.
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let errors =
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super::fully_solve_bound(&infcx, ObligationCause::dummy(), full_env, ty, trait_did);
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if !errors.is_empty() {
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panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, errors);
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}
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infcx.process_registered_region_obligations(&Default::default(), full_env);
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let region_data =
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infcx.inner.borrow_mut().unwrap_region_constraints().region_constraint_data().clone();
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let vid_to_region = self.map_vid_to_region(®ion_data);
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let info = AutoTraitInfo { full_user_env, region_data, vid_to_region };
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AutoTraitResult::PositiveImpl(auto_trait_callback(info))
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}
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}
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impl<'tcx> AutoTraitFinder<'tcx> {
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/// The core logic responsible for computing the bounds for our synthesized impl.
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///
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/// To calculate the bounds, we call `SelectionContext.select` in a loop. Like
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/// `FulfillmentContext`, we recursively select the nested obligations of predicates we
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/// encounter. However, whenever we encounter an `UnimplementedError` involving a type
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/// parameter, we add it to our `ParamEnv`. Since our goal is to determine when a particular
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/// type implements an auto trait, Unimplemented errors tell us what conditions need to be met.
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///
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/// This method ends up working somewhat similarly to `FulfillmentContext`, but with a few key
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/// differences. `FulfillmentContext` works under the assumption that it's dealing with concrete
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/// user code. According, it considers all possible ways that a `Predicate` could be met, which
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/// isn't always what we want for a synthesized impl. For example, given the predicate `T:
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/// Iterator`, `FulfillmentContext` can end up reporting an Unimplemented error for `T:
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/// IntoIterator` -- since there's an implementation of `Iterator` where `T: IntoIterator`,
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/// `FulfillmentContext` will drive `SelectionContext` to consider that impl before giving up.
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/// If we were to rely on `FulfillmentContext`s decision, we might end up synthesizing an impl
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/// like this:
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/// ```ignore (illustrative)
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/// impl<T> Send for Foo<T> where T: IntoIterator
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/// ```
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/// While it might be technically true that Foo implements Send where `T: IntoIterator`,
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/// the bound is overly restrictive - it's really only necessary that `T: Iterator`.
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///
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/// For this reason, `evaluate_predicates` handles predicates with type variables specially.
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/// When we encounter an `Unimplemented` error for a bound such as `T: Iterator`, we immediately
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/// add it to our `ParamEnv`, and add it to our stack for recursive evaluation. When we later
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/// select it, we'll pick up any nested bounds, without ever inferring that `T: IntoIterator`
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/// needs to hold.
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///
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/// One additional consideration is supertrait bounds. Normally, a `ParamEnv` is only ever
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/// constructed once for a given type. As part of the construction process, the `ParamEnv` will
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/// have any supertrait bounds normalized -- e.g., if we have a type `struct Foo<T: Copy>`, the
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/// `ParamEnv` will contain `T: Copy` and `T: Clone`, since `Copy: Clone`. When we construct our
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/// own `ParamEnv`, we need to do this ourselves, through `traits::elaborate_predicates`, or
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/// else `SelectionContext` will choke on the missing predicates. However, this should never
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/// show up in the final synthesized generics: we don't want our generated docs page to contain
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/// something like `T: Copy + Clone`, as that's redundant. Therefore, we keep track of a
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/// separate `user_env`, which only holds the predicates that will actually be displayed to the
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/// user.
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fn evaluate_predicates(
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&self,
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infcx: &InferCtxt<'tcx>,
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trait_did: DefId,
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ty: Ty<'tcx>,
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param_env: ty::ParamEnv<'tcx>,
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user_env: ty::ParamEnv<'tcx>,
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fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
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only_projections: bool,
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) -> Option<(ty::ParamEnv<'tcx>, ty::ParamEnv<'tcx>)> {
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let tcx = infcx.tcx;
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// Don't try to process any nested obligations involving predicates
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// that are already in the `ParamEnv` (modulo regions): we already
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// know that they must hold.
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for predicate in param_env.caller_bounds() {
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fresh_preds.insert(self.clean_pred(infcx, predicate));
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}
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let mut select = SelectionContext::new(&infcx);
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let mut already_visited = FxHashSet::default();
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let mut predicates = VecDeque::new();
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predicates.push_back(ty::Binder::dummy(ty::TraitPredicate {
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trait_ref: ty::TraitRef {
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def_id: trait_did,
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substs: infcx.tcx.mk_substs_trait(ty, &[]),
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},
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constness: ty::BoundConstness::NotConst,
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// Auto traits are positive
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polarity: ty::ImplPolarity::Positive,
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}));
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let computed_preds = param_env.caller_bounds().iter();
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let mut user_computed_preds: FxHashSet<_> = user_env.caller_bounds().iter().collect();
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let mut new_env = param_env;
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let dummy_cause = ObligationCause::dummy();
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while let Some(pred) = predicates.pop_front() {
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infcx.clear_caches();
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if !already_visited.insert(pred) {
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continue;
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}
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// Call `infcx.resolve_vars_if_possible` to see if we can
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// get rid of any inference variables.
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let obligation =
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infcx.resolve_vars_if_possible(Obligation::new(dummy_cause.clone(), new_env, pred));
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let result = select.select(&obligation);
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match result {
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Ok(Some(ref impl_source)) => {
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// If we see an explicit negative impl (e.g., `impl !Send for MyStruct`),
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// we immediately bail out, since it's impossible for us to continue.
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if let ImplSource::UserDefined(ImplSourceUserDefinedData {
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impl_def_id, ..
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}) = impl_source
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{
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// Blame 'tidy' for the weird bracket placement.
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if infcx.tcx.impl_polarity(*impl_def_id) == ty::ImplPolarity::Negative {
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debug!(
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"evaluate_nested_obligations: found explicit negative impl\
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{:?}, bailing out",
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impl_def_id
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);
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return None;
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}
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}
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let obligations = impl_source.borrow_nested_obligations().iter().cloned();
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if !self.evaluate_nested_obligations(
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ty,
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obligations,
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&mut user_computed_preds,
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fresh_preds,
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&mut predicates,
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&mut select,
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only_projections,
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) {
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return None;
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}
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}
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Ok(None) => {}
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Err(SelectionError::Unimplemented) => {
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if self.is_param_no_infer(pred.skip_binder().trait_ref.substs) {
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already_visited.remove(&pred);
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self.add_user_pred(&mut user_computed_preds, pred.to_predicate(self.tcx));
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predicates.push_back(pred);
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} else {
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debug!(
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"evaluate_nested_obligations: `Unimplemented` found, bailing: \
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{:?} {:?} {:?}",
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ty,
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pred,
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pred.skip_binder().trait_ref.substs
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);
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return None;
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}
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}
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_ => panic!("Unexpected error for '{:?}': {:?}", ty, result),
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};
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let normalized_preds = elaborate_predicates(
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tcx,
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computed_preds.clone().chain(user_computed_preds.iter().cloned()),
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)
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.map(|o| o.predicate);
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new_env = ty::ParamEnv::new(
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tcx.mk_predicates(normalized_preds),
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param_env.reveal(),
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param_env.constness(),
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);
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}
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let final_user_env = ty::ParamEnv::new(
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tcx.mk_predicates(user_computed_preds.into_iter()),
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user_env.reveal(),
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user_env.constness(),
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);
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debug!(
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"evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \
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'{:?}'",
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ty, trait_did, new_env, final_user_env
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);
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Some((new_env, final_user_env))
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}
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/// This method is designed to work around the following issue:
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/// When we compute auto trait bounds, we repeatedly call `SelectionContext.select`,
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/// progressively building a `ParamEnv` based on the results we get.
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/// However, our usage of `SelectionContext` differs from its normal use within the compiler,
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/// in that we capture and re-reprocess predicates from `Unimplemented` errors.
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///
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/// This can lead to a corner case when dealing with region parameters.
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/// During our selection loop in `evaluate_predicates`, we might end up with
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/// two trait predicates that differ only in their region parameters:
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/// one containing a HRTB lifetime parameter, and one containing a 'normal'
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/// lifetime parameter. For example:
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/// ```ignore (illustrative)
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/// T as MyTrait<'a>
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/// T as MyTrait<'static>
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/// ```
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/// If we put both of these predicates in our computed `ParamEnv`, we'll
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/// confuse `SelectionContext`, since it will (correctly) view both as being applicable.
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///
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/// To solve this, we pick the 'more strict' lifetime bound -- i.e., the HRTB
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/// Our end goal is to generate a user-visible description of the conditions
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/// under which a type implements an auto trait. A trait predicate involving
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/// a HRTB means that the type needs to work with any choice of lifetime,
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/// not just one specific lifetime (e.g., `'static`).
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fn add_user_pred(
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&self,
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user_computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
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new_pred: ty::Predicate<'tcx>,
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) {
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let mut should_add_new = true;
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user_computed_preds.retain(|&old_pred| {
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if let (ty::PredicateKind::Trait(new_trait), ty::PredicateKind::Trait(old_trait)) =
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(new_pred.kind().skip_binder(), old_pred.kind().skip_binder())
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{
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if new_trait.def_id() == old_trait.def_id() {
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let new_substs = new_trait.trait_ref.substs;
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let old_substs = old_trait.trait_ref.substs;
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if !new_substs.types().eq(old_substs.types()) {
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// We can't compare lifetimes if the types are different,
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// so skip checking `old_pred`.
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return true;
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}
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for (new_region, old_region) in
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iter::zip(new_substs.regions(), old_substs.regions())
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{
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match (*new_region, *old_region) {
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// If both predicates have an `ReLateBound` (a HRTB) in the
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// same spot, we do nothing.
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(ty::ReLateBound(_, _), ty::ReLateBound(_, _)) => {}
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(ty::ReLateBound(_, _), _) | (_, ty::ReVar(_)) => {
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// One of these is true:
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// The new predicate has a HRTB in a spot where the old
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// predicate does not (if they both had a HRTB, the previous
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// match arm would have executed). A HRBT is a 'stricter'
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// bound than anything else, so we want to keep the newer
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// predicate (with the HRBT) in place of the old predicate.
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//
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// OR
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//
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// The old predicate has a region variable where the new
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// predicate has some other kind of region. An region
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// variable isn't something we can actually display to a user,
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// so we choose their new predicate (which doesn't have a region
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// variable).
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//
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// In both cases, we want to remove the old predicate,
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// from `user_computed_preds`, and replace it with the new
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// one. Having both the old and the new
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// predicate in a `ParamEnv` would confuse `SelectionContext`.
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//
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// We're currently in the predicate passed to 'retain',
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// so we return `false` to remove the old predicate from
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// `user_computed_preds`.
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return false;
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|
}
|
|
(_, ty::ReLateBound(_, _)) | (ty::ReVar(_), _) => {
|
|
// This is the opposite situation as the previous arm.
|
|
// One of these is true:
|
|
//
|
|
// The old predicate has a HRTB lifetime in a place where the
|
|
// new predicate does not.
|
|
//
|
|
// OR
|
|
//
|
|
// The new predicate has a region variable where the old
|
|
// predicate has some other type of region.
|
|
//
|
|
// We want to leave the old
|
|
// predicate in `user_computed_preds`, and skip adding
|
|
// new_pred to `user_computed_params`.
|
|
should_add_new = false
|
|
}
|
|
_ => {}
|
|
}
|
|
}
|
|
}
|
|
}
|
|
true
|
|
});
|
|
|
|
if should_add_new {
|
|
user_computed_preds.insert(new_pred);
|
|
}
|
|
}
|
|
|
|
/// This is very similar to `handle_lifetimes`. However, instead of matching `ty::Region`s
|
|
/// to each other, we match `ty::RegionVid`s to `ty::Region`s.
|
|
fn map_vid_to_region<'cx>(
|
|
&self,
|
|
regions: &RegionConstraintData<'cx>,
|
|
) -> FxHashMap<ty::RegionVid, ty::Region<'cx>> {
|
|
let mut vid_map: FxHashMap<RegionTarget<'cx>, RegionDeps<'cx>> = FxHashMap::default();
|
|
let mut finished_map = FxHashMap::default();
|
|
|
|
for constraint in regions.constraints.keys() {
|
|
match constraint {
|
|
&Constraint::VarSubVar(r1, r2) => {
|
|
{
|
|
let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
|
|
deps1.larger.insert(RegionTarget::RegionVid(r2));
|
|
}
|
|
|
|
let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
|
|
deps2.smaller.insert(RegionTarget::RegionVid(r1));
|
|
}
|
|
&Constraint::RegSubVar(region, vid) => {
|
|
{
|
|
let deps1 = vid_map.entry(RegionTarget::Region(region)).or_default();
|
|
deps1.larger.insert(RegionTarget::RegionVid(vid));
|
|
}
|
|
|
|
let deps2 = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
|
|
deps2.smaller.insert(RegionTarget::Region(region));
|
|
}
|
|
&Constraint::VarSubReg(vid, region) => {
|
|
finished_map.insert(vid, region);
|
|
}
|
|
&Constraint::RegSubReg(r1, r2) => {
|
|
{
|
|
let deps1 = vid_map.entry(RegionTarget::Region(r1)).or_default();
|
|
deps1.larger.insert(RegionTarget::Region(r2));
|
|
}
|
|
|
|
let deps2 = vid_map.entry(RegionTarget::Region(r2)).or_default();
|
|
deps2.smaller.insert(RegionTarget::Region(r1));
|
|
}
|
|
}
|
|
}
|
|
|
|
while !vid_map.is_empty() {
|
|
let target = *vid_map.keys().next().expect("Keys somehow empty");
|
|
let deps = vid_map.remove(&target).expect("Entry somehow missing");
|
|
|
|
for smaller in deps.smaller.iter() {
|
|
for larger in deps.larger.iter() {
|
|
match (smaller, larger) {
|
|
(&RegionTarget::Region(_), &RegionTarget::Region(_)) => {
|
|
if let Entry::Occupied(v) = vid_map.entry(*smaller) {
|
|
let smaller_deps = v.into_mut();
|
|
smaller_deps.larger.insert(*larger);
|
|
smaller_deps.larger.remove(&target);
|
|
}
|
|
|
|
if let Entry::Occupied(v) = vid_map.entry(*larger) {
|
|
let larger_deps = v.into_mut();
|
|
larger_deps.smaller.insert(*smaller);
|
|
larger_deps.smaller.remove(&target);
|
|
}
|
|
}
|
|
(&RegionTarget::RegionVid(v1), &RegionTarget::Region(r1)) => {
|
|
finished_map.insert(v1, r1);
|
|
}
|
|
(&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
|
|
// Do nothing; we don't care about regions that are smaller than vids.
|
|
}
|
|
(&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
|
|
if let Entry::Occupied(v) = vid_map.entry(*smaller) {
|
|
let smaller_deps = v.into_mut();
|
|
smaller_deps.larger.insert(*larger);
|
|
smaller_deps.larger.remove(&target);
|
|
}
|
|
|
|
if let Entry::Occupied(v) = vid_map.entry(*larger) {
|
|
let larger_deps = v.into_mut();
|
|
larger_deps.smaller.insert(*smaller);
|
|
larger_deps.smaller.remove(&target);
|
|
}
|
|
}
|
|
}
|
|
}
|
|
}
|
|
}
|
|
finished_map
|
|
}
|
|
|
|
fn is_param_no_infer(&self, substs: SubstsRef<'_>) -> bool {
|
|
self.is_of_param(substs.type_at(0)) && !substs.types().any(|t| t.has_infer_types())
|
|
}
|
|
|
|
pub fn is_of_param(&self, ty: Ty<'_>) -> bool {
|
|
match ty.kind() {
|
|
ty::Param(_) => true,
|
|
ty::Projection(p) => self.is_of_param(p.self_ty()),
|
|
_ => false,
|
|
}
|
|
}
|
|
|
|
fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool {
|
|
if let Some(ty) = p.term().skip_binder().ty() {
|
|
matches!(ty.kind(), ty::Projection(proj) if proj == &p.skip_binder().projection_ty)
|
|
} else {
|
|
false
|
|
}
|
|
}
|
|
|
|
fn evaluate_nested_obligations(
|
|
&self,
|
|
ty: Ty<'_>,
|
|
nested: impl Iterator<Item = Obligation<'tcx, ty::Predicate<'tcx>>>,
|
|
computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
|
|
fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
|
|
predicates: &mut VecDeque<ty::PolyTraitPredicate<'tcx>>,
|
|
select: &mut SelectionContext<'_, 'tcx>,
|
|
only_projections: bool,
|
|
) -> bool {
|
|
let dummy_cause = ObligationCause::dummy();
|
|
|
|
for obligation in nested {
|
|
let is_new_pred =
|
|
fresh_preds.insert(self.clean_pred(select.infcx(), obligation.predicate));
|
|
|
|
// Resolve any inference variables that we can, to help selection succeed
|
|
let predicate = select.infcx().resolve_vars_if_possible(obligation.predicate);
|
|
|
|
// We only add a predicate as a user-displayable bound if
|
|
// it involves a generic parameter, and doesn't contain
|
|
// any inference variables.
|
|
//
|
|
// Displaying a bound involving a concrete type (instead of a generic
|
|
// parameter) would be pointless, since it's always true
|
|
// (e.g. u8: Copy)
|
|
// Displaying an inference variable is impossible, since they're
|
|
// an internal compiler detail without a defined visual representation
|
|
//
|
|
// We check this by calling is_of_param on the relevant types
|
|
// from the various possible predicates
|
|
|
|
let bound_predicate = predicate.kind();
|
|
match bound_predicate.skip_binder() {
|
|
ty::PredicateKind::Trait(p) => {
|
|
// Add this to `predicates` so that we end up calling `select`
|
|
// with it. If this predicate ends up being unimplemented,
|
|
// then `evaluate_predicates` will handle adding it the `ParamEnv`
|
|
// if possible.
|
|
predicates.push_back(bound_predicate.rebind(p));
|
|
}
|
|
ty::PredicateKind::Projection(p) => {
|
|
let p = bound_predicate.rebind(p);
|
|
debug!(
|
|
"evaluate_nested_obligations: examining projection predicate {:?}",
|
|
predicate
|
|
);
|
|
|
|
// As described above, we only want to display
|
|
// bounds which include a generic parameter but don't include
|
|
// an inference variable.
|
|
// Additionally, we check if we've seen this predicate before,
|
|
// to avoid rendering duplicate bounds to the user.
|
|
if self.is_param_no_infer(p.skip_binder().projection_ty.substs)
|
|
&& !p.term().skip_binder().has_infer_types()
|
|
&& is_new_pred
|
|
{
|
|
debug!(
|
|
"evaluate_nested_obligations: adding projection predicate \
|
|
to computed_preds: {:?}",
|
|
predicate
|
|
);
|
|
|
|
// Under unusual circumstances, we can end up with a self-referential
|
|
// projection predicate. For example:
|
|
// <T as MyType>::Value == <T as MyType>::Value
|
|
// Not only is displaying this to the user pointless,
|
|
// having it in the ParamEnv will cause an issue if we try to call
|
|
// poly_project_and_unify_type on the predicate, since this kind of
|
|
// predicate will normally never end up in a ParamEnv.
|
|
//
|
|
// For these reasons, we ignore these weird predicates,
|
|
// ensuring that we're able to properly synthesize an auto trait impl
|
|
if self.is_self_referential_projection(p) {
|
|
debug!(
|
|
"evaluate_nested_obligations: encountered a projection
|
|
predicate equating a type with itself! Skipping"
|
|
);
|
|
} else {
|
|
self.add_user_pred(computed_preds, predicate);
|
|
}
|
|
}
|
|
|
|
// There are three possible cases when we project a predicate:
|
|
//
|
|
// 1. We encounter an error. This means that it's impossible for
|
|
// our current type to implement the auto trait - there's bound
|
|
// that we could add to our ParamEnv that would 'fix' this kind
|
|
// of error, as it's not caused by an unimplemented type.
|
|
//
|
|
// 2. We successfully project the predicate (Ok(Some(_))), generating
|
|
// some subobligations. We then process these subobligations
|
|
// like any other generated sub-obligations.
|
|
//
|
|
// 3. We receive an 'ambiguous' result (Ok(None))
|
|
// If we were actually trying to compile a crate,
|
|
// we would need to re-process this obligation later.
|
|
// However, all we care about is finding out what bounds
|
|
// are needed for our type to implement a particular auto trait.
|
|
// We've already added this obligation to our computed ParamEnv
|
|
// above (if it was necessary). Therefore, we don't need
|
|
// to do any further processing of the obligation.
|
|
//
|
|
// Note that we *must* try to project *all* projection predicates
|
|
// we encounter, even ones without inference variable.
|
|
// This ensures that we detect any projection errors,
|
|
// which indicate that our type can *never* implement the given
|
|
// auto trait. In that case, we will generate an explicit negative
|
|
// impl (e.g. 'impl !Send for MyType'). However, we don't
|
|
// try to process any of the generated subobligations -
|
|
// they contain no new information, since we already know
|
|
// that our type implements the projected-through trait,
|
|
// and can lead to weird region issues.
|
|
//
|
|
// Normally, we'll generate a negative impl as a result of encountering
|
|
// a type with an explicit negative impl of an auto trait
|
|
// (for example, raw pointers have !Send and !Sync impls)
|
|
// However, through some **interesting** manipulations of the type
|
|
// system, it's actually possible to write a type that never
|
|
// implements an auto trait due to a projection error, not a normal
|
|
// negative impl error. To properly handle this case, we need
|
|
// to ensure that we catch any potential projection errors,
|
|
// and turn them into an explicit negative impl for our type.
|
|
debug!("Projecting and unifying projection predicate {:?}", predicate);
|
|
|
|
match project::poly_project_and_unify_type(select, &obligation.with(p)) {
|
|
ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
|
|
debug!(
|
|
"evaluate_nested_obligations: Unable to unify predicate \
|
|
'{:?}' '{:?}', bailing out",
|
|
ty, e
|
|
);
|
|
return false;
|
|
}
|
|
ProjectAndUnifyResult::Recursive => {
|
|
debug!("evaluate_nested_obligations: recursive projection predicate");
|
|
return false;
|
|
}
|
|
ProjectAndUnifyResult::Holds(v) => {
|
|
// We only care about sub-obligations
|
|
// when we started out trying to unify
|
|
// some inference variables. See the comment above
|
|
// for more information
|
|
if p.term().skip_binder().has_infer_types() {
|
|
if !self.evaluate_nested_obligations(
|
|
ty,
|
|
v.into_iter(),
|
|
computed_preds,
|
|
fresh_preds,
|
|
predicates,
|
|
select,
|
|
only_projections,
|
|
) {
|
|
return false;
|
|
}
|
|
}
|
|
}
|
|
ProjectAndUnifyResult::FailedNormalization => {
|
|
// It's ok not to make progress when have no inference variables -
|
|
// in that case, we were only performing unification to check if an
|
|
// error occurred (which would indicate that it's impossible for our
|
|
// type to implement the auto trait).
|
|
// However, we should always make progress (either by generating
|
|
// subobligations or getting an error) when we started off with
|
|
// inference variables
|
|
if p.term().skip_binder().has_infer_types() {
|
|
panic!("Unexpected result when selecting {:?} {:?}", ty, obligation)
|
|
}
|
|
}
|
|
}
|
|
}
|
|
ty::PredicateKind::RegionOutlives(binder) => {
|
|
let binder = bound_predicate.rebind(binder);
|
|
select.infcx().region_outlives_predicate(&dummy_cause, binder)
|
|
}
|
|
ty::PredicateKind::TypeOutlives(binder) => {
|
|
let binder = bound_predicate.rebind(binder);
|
|
match (
|
|
binder.no_bound_vars(),
|
|
binder.map_bound_ref(|pred| pred.0).no_bound_vars(),
|
|
) {
|
|
(None, Some(t_a)) => {
|
|
select.infcx().register_region_obligation_with_cause(
|
|
t_a,
|
|
select.infcx().tcx.lifetimes.re_static,
|
|
&dummy_cause,
|
|
);
|
|
}
|
|
(Some(ty::OutlivesPredicate(t_a, r_b)), _) => {
|
|
select.infcx().register_region_obligation_with_cause(
|
|
t_a,
|
|
r_b,
|
|
&dummy_cause,
|
|
);
|
|
}
|
|
_ => {}
|
|
};
|
|
}
|
|
ty::PredicateKind::ConstEquate(c1, c2) => {
|
|
let evaluate = |c: ty::Const<'tcx>| {
|
|
if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
|
|
match select.infcx().const_eval_resolve(
|
|
obligation.param_env,
|
|
unevaluated,
|
|
Some(obligation.cause.span),
|
|
) {
|
|
Ok(Some(valtree)) => {
|
|
Ok(ty::Const::from_value(select.tcx(), valtree, c.ty()))
|
|
}
|
|
Ok(None) => {
|
|
let tcx = self.tcx;
|
|
let def_id = unevaluated.def.did;
|
|
let reported =
|
|
tcx.sess.emit_err(UnableToConstructConstantValue {
|
|
span: tcx.def_span(def_id),
|
|
unevaluated: unevaluated,
|
|
});
|
|
Err(ErrorHandled::Reported(reported))
|
|
}
|
|
Err(err) => Err(err),
|
|
}
|
|
} else {
|
|
Ok(c)
|
|
}
|
|
};
|
|
|
|
match (evaluate(c1), evaluate(c2)) {
|
|
(Ok(c1), Ok(c2)) => {
|
|
match select
|
|
.infcx()
|
|
.at(&obligation.cause, obligation.param_env)
|
|
.eq(c1, c2)
|
|
{
|
|
Ok(_) => (),
|
|
Err(_) => return false,
|
|
}
|
|
}
|
|
_ => return false,
|
|
}
|
|
}
|
|
// There's not really much we can do with these predicates -
|
|
// we start out with a `ParamEnv` with no inference variables,
|
|
// and these don't correspond to adding any new bounds to
|
|
// the `ParamEnv`.
|
|
ty::PredicateKind::WellFormed(..)
|
|
| ty::PredicateKind::ObjectSafe(..)
|
|
| ty::PredicateKind::ClosureKind(..)
|
|
| ty::PredicateKind::Subtype(..)
|
|
| ty::PredicateKind::ConstEvaluatable(..)
|
|
| ty::PredicateKind::Coerce(..)
|
|
| ty::PredicateKind::TypeWellFormedFromEnv(..) => {}
|
|
};
|
|
}
|
|
true
|
|
}
|
|
|
|
pub fn clean_pred(
|
|
&self,
|
|
infcx: &InferCtxt<'tcx>,
|
|
p: ty::Predicate<'tcx>,
|
|
) -> ty::Predicate<'tcx> {
|
|
infcx.freshen(p)
|
|
}
|
|
}
|
|
|
|
// Replaces all ReVars in a type with ty::Region's, using the provided map
|
|
pub struct RegionReplacer<'a, 'tcx> {
|
|
vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
|
|
tcx: TyCtxt<'tcx>,
|
|
}
|
|
|
|
impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
|
|
fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
|
|
self.tcx
|
|
}
|
|
|
|
fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
|
|
(match *r {
|
|
ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
|
|
_ => None,
|
|
})
|
|
.unwrap_or_else(|| r.super_fold_with(self))
|
|
}
|
|
}
|