mv compiler to compiler/
This commit is contained in:
mark
881
compiler/rustc_trait_selection/src/traits/auto_trait.rs
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881
compiler/rustc_trait_selection/src/traits/auto_trait.rs
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@@ -0,0 +1,881 @@
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//! 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::infer::region_constraints::{Constraint, RegionConstraintData};
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use crate::infer::InferCtxt;
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use rustc_middle::ty::fold::TypeFolder;
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use rustc_middle::ty::{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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// 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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impl<A> AutoTraitResult<A> {
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fn is_auto(&self) -> bool {
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match *self {
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AutoTraitResult::PositiveImpl(_) | AutoTraitResult::NegativeImpl => true,
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_ => false,
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}
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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 type).
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/// The analysis attempts to account for custom impls as well as other complex cases. This
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/// 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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auto_trait_callback: impl Fn(&InferCtxt<'_, 'tcx>, 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::bind(trait_ref);
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let bail_out = tcx.infer_ctxt().enter(|infcx| {
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let mut selcx = SelectionContext::with_negative(&infcx, true);
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let result = selcx.select(&Obligation::new(
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ObligationCause::dummy(),
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orig_env,
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trait_pred.to_poly_trait_predicate(),
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));
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match result {
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Ok(Some(ImplSource::ImplSourceUserDefined(_))) => {
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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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true
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}
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_ => false,
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}
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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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if bail_out {
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return AutoTraitResult::ExplicitImpl;
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}
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tcx.infer_ctxt().enter(|infcx| {
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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 (new_env, user_env) = match 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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) {
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Some(e) => e,
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None => return AutoTraitResult::NegativeImpl,
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};
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let (full_env, full_user_env) = self
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.evaluate_predicates(
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&infcx,
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trait_did,
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ty,
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new_env,
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user_env,
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&mut fresh_preds,
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true,
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)
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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 mut fulfill = FulfillmentContext::new();
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fulfill.register_bound(&infcx, full_env, ty, trait_did, ObligationCause::dummy());
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fulfill.select_all_or_error(&infcx).unwrap_or_else(|e| {
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panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, e)
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});
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let body_id_map: FxHashMap<_, _> = infcx
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.inner
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.borrow()
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.region_obligations()
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.iter()
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.map(|&(id, _)| (id, vec![]))
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.collect();
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infcx.process_registered_region_obligations(&body_id_map, None, full_env);
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let region_data = infcx
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.inner
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.borrow_mut()
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.unwrap_region_constraints()
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.region_constraint_data()
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.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(&infcx, info))
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})
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}
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}
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impl 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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///
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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 proess 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::with_negative(&infcx, true);
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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::bind(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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}));
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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 = infcx.resolve_vars_if_possible(&Obligation::new(
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dummy_cause.clone(),
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new_env,
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pred,
|
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));
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let result = select.select(&obligation);
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|
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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::ImplSourceUserDefined(ImplSourceUserDefinedData {
|
||||
impl_def_id,
|
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..
|
||||
}) = impl_source
|
||||
{
|
||||
// Blame 'tidy' for the weird bracket placement.
|
||||
if infcx.tcx.impl_polarity(*impl_def_id) == ty::ImplPolarity::Negative {
|
||||
debug!(
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"evaluate_nested_obligations: found explicit negative impl\
|
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{:?}, bailing out",
|
||||
impl_def_id
|
||||
);
|
||||
return None;
|
||||
}
|
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}
|
||||
|
||||
let obligations = impl_source.clone().nested_obligations().into_iter();
|
||||
|
||||
if !self.evaluate_nested_obligations(
|
||||
ty,
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obligations,
|
||||
&mut user_computed_preds,
|
||||
fresh_preds,
|
||||
&mut predicates,
|
||||
&mut select,
|
||||
only_projections,
|
||||
) {
|
||||
return None;
|
||||
}
|
||||
}
|
||||
&Ok(None) => {}
|
||||
&Err(SelectionError::Unimplemented) => {
|
||||
if self.is_param_no_infer(pred.skip_binder().trait_ref.substs) {
|
||||
already_visited.remove(&pred);
|
||||
self.add_user_pred(
|
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&mut user_computed_preds,
|
||||
pred.without_const().to_predicate(self.tcx),
|
||||
);
|
||||
predicates.push_back(pred);
|
||||
} else {
|
||||
debug!(
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||||
"evaluate_nested_obligations: `Unimplemented` found, bailing: \
|
||||
{:?} {:?} {:?}",
|
||||
ty,
|
||||
pred,
|
||||
pred.skip_binder().trait_ref.substs
|
||||
);
|
||||
return None;
|
||||
}
|
||||
}
|
||||
_ => panic!("Unexpected error for '{:?}': {:?}", ty, result),
|
||||
};
|
||||
|
||||
let normalized_preds = elaborate_predicates(
|
||||
tcx,
|
||||
computed_preds.clone().chain(user_computed_preds.iter().cloned()),
|
||||
)
|
||||
.map(|o| o.predicate);
|
||||
new_env =
|
||||
ty::ParamEnv::new(tcx.mk_predicates(normalized_preds), param_env.reveal(), None);
|
||||
}
|
||||
|
||||
let final_user_env = ty::ParamEnv::new(
|
||||
tcx.mk_predicates(user_computed_preds.into_iter()),
|
||||
user_env.reveal(),
|
||||
None,
|
||||
);
|
||||
debug!(
|
||||
"evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \
|
||||
'{:?}'",
|
||||
ty, trait_did, new_env, final_user_env
|
||||
);
|
||||
|
||||
Some((new_env, final_user_env))
|
||||
}
|
||||
|
||||
/// This method is designed to work around the following issue:
|
||||
/// When we compute auto trait bounds, we repeatedly call `SelectionContext.select`,
|
||||
/// progressively building a `ParamEnv` based on the results we get.
|
||||
/// However, our usage of `SelectionContext` differs from its normal use within the compiler,
|
||||
/// in that we capture and re-reprocess predicates from `Unimplemented` errors.
|
||||
///
|
||||
/// This can lead to a corner case when dealing with region parameters.
|
||||
/// During our selection loop in `evaluate_predicates`, we might end up with
|
||||
/// two trait predicates that differ only in their region parameters:
|
||||
/// one containing a HRTB lifetime parameter, and one containing a 'normal'
|
||||
/// lifetime parameter. For example:
|
||||
///
|
||||
/// T as MyTrait<'a>
|
||||
/// T as MyTrait<'static>
|
||||
///
|
||||
/// If we put both of these predicates in our computed `ParamEnv`, we'll
|
||||
/// confuse `SelectionContext`, since it will (correctly) view both as being applicable.
|
||||
///
|
||||
/// To solve this, we pick the 'more strict' lifetime bound -- i.e., the HRTB
|
||||
/// Our end goal is to generate a user-visible description of the conditions
|
||||
/// under which a type implements an auto trait. A trait predicate involving
|
||||
/// a HRTB means that the type needs to work with any choice of lifetime,
|
||||
/// not just one specific lifetime (e.g., `'static`).
|
||||
fn add_user_pred(
|
||||
&self,
|
||||
user_computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
|
||||
new_pred: ty::Predicate<'tcx>,
|
||||
) {
|
||||
let mut should_add_new = true;
|
||||
user_computed_preds.retain(|&old_pred| {
|
||||
if let (
|
||||
ty::PredicateAtom::Trait(new_trait, _),
|
||||
ty::PredicateAtom::Trait(old_trait, _),
|
||||
) = (new_pred.skip_binders(), old_pred.skip_binders())
|
||||
{
|
||||
if new_trait.def_id() == old_trait.def_id() {
|
||||
let new_substs = new_trait.trait_ref.substs;
|
||||
let old_substs = old_trait.trait_ref.substs;
|
||||
|
||||
if !new_substs.types().eq(old_substs.types()) {
|
||||
// We can't compare lifetimes if the types are different,
|
||||
// so skip checking `old_pred`.
|
||||
return true;
|
||||
}
|
||||
|
||||
for (new_region, old_region) in new_substs.regions().zip(old_substs.regions()) {
|
||||
match (new_region, old_region) {
|
||||
// If both predicates have an `ReLateBound` (a HRTB) in the
|
||||
// same spot, we do nothing.
|
||||
(
|
||||
ty::RegionKind::ReLateBound(_, _),
|
||||
ty::RegionKind::ReLateBound(_, _),
|
||||
) => {}
|
||||
|
||||
(ty::RegionKind::ReLateBound(_, _), _)
|
||||
| (_, ty::RegionKind::ReVar(_)) => {
|
||||
// One of these is true:
|
||||
// The new predicate has a HRTB in a spot where the old
|
||||
// predicate does not (if they both had a HRTB, the previous
|
||||
// match arm would have executed). A HRBT is a 'stricter'
|
||||
// bound than anything else, so we want to keep the newer
|
||||
// predicate (with the HRBT) in place of the old predicate.
|
||||
//
|
||||
// OR
|
||||
//
|
||||
// The old predicate has a region variable where the new
|
||||
// predicate has some other kind of region. An region
|
||||
// variable isn't something we can actually display to a user,
|
||||
// so we choose their new predicate (which doesn't have a region
|
||||
// variable).
|
||||
//
|
||||
// In both cases, we want to remove the old predicate,
|
||||
// from `user_computed_preds`, and replace it with the new
|
||||
// one. Having both the old and the new
|
||||
// predicate in a `ParamEnv` would confuse `SelectionContext`.
|
||||
//
|
||||
// We're currently in the predicate passed to 'retain',
|
||||
// so we return `false` to remove the old predicate from
|
||||
// `user_computed_preds`.
|
||||
return false;
|
||||
}
|
||||
(_, ty::RegionKind::ReLateBound(_, _))
|
||||
| (ty::RegionKind::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 {
|
||||
match p.ty().skip_binder().kind {
|
||||
ty::Projection(proj) if proj == p.skip_binder().projection_ty => true,
|
||||
_ => 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
|
||||
|
||||
match predicate.skip_binders() {
|
||||
ty::PredicateAtom::Trait(p, _) => {
|
||||
if self.is_param_no_infer(p.trait_ref.substs)
|
||||
&& !only_projections
|
||||
&& is_new_pred
|
||||
{
|
||||
self.add_user_pred(computed_preds, predicate);
|
||||
}
|
||||
predicates.push_back(ty::Binder::bind(p));
|
||||
}
|
||||
ty::PredicateAtom::Projection(p) => {
|
||||
let p = ty::Binder::bind(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.ty().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-refeential
|
||||
// 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)) {
|
||||
Err(e) => {
|
||||
debug!(
|
||||
"evaluate_nested_obligations: Unable to unify predicate \
|
||||
'{:?}' '{:?}', bailing out",
|
||||
ty, e
|
||||
);
|
||||
return false;
|
||||
}
|
||||
Ok(Err(project::InProgress)) => {
|
||||
debug!("evaluate_nested_obligations: recursive projection predicate");
|
||||
return false;
|
||||
}
|
||||
Ok(Ok(Some(v))) => {
|
||||
// We only care about sub-obligations
|
||||
// when we started out trying to unify
|
||||
// some inference variables. See the comment above
|
||||
// for more infomration
|
||||
if p.ty().skip_binder().has_infer_types() {
|
||||
if !self.evaluate_nested_obligations(
|
||||
ty,
|
||||
v.into_iter(),
|
||||
computed_preds,
|
||||
fresh_preds,
|
||||
predicates,
|
||||
select,
|
||||
only_projections,
|
||||
) {
|
||||
return false;
|
||||
}
|
||||
}
|
||||
}
|
||||
Ok(Ok(None)) => {
|
||||
// It's ok not to make progress when have no inference variables -
|
||||
// in that case, we were only performing unifcation 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.ty().skip_binder().has_infer_types() {
|
||||
panic!("Unexpected result when selecting {:?} {:?}", ty, obligation)
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
ty::PredicateAtom::RegionOutlives(binder) => {
|
||||
let binder = ty::Binder::bind(binder);
|
||||
if select.infcx().region_outlives_predicate(&dummy_cause, binder).is_err() {
|
||||
return false;
|
||||
}
|
||||
}
|
||||
ty::PredicateAtom::TypeOutlives(binder) => {
|
||||
let binder = ty::Binder::bind(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::PredicateAtom::ConstEquate(c1, c2) => {
|
||||
let evaluate = |c: &'tcx ty::Const<'tcx>| {
|
||||
if let ty::ConstKind::Unevaluated(def, substs, promoted) = c.val {
|
||||
match select.infcx().const_eval_resolve(
|
||||
obligation.param_env,
|
||||
def,
|
||||
substs,
|
||||
promoted,
|
||||
Some(obligation.cause.span),
|
||||
) {
|
||||
Ok(val) => Ok(ty::Const::from_value(select.tcx(), val, c.ty)),
|
||||
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,
|
||||
}
|
||||
}
|
||||
_ => panic!("Unexpected predicate {:?} {:?}", ty, predicate),
|
||||
};
|
||||
}
|
||||
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))
|
||||
}
|
||||
}
|
||||
Reference in New Issue
Block a user