557 lines
22 KiB
Rust
557 lines
22 KiB
Rust
use crate::infer::{InferCtxt, ShallowResolver};
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use rustc::ty::error::ExpectedFound;
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use rustc::ty::{self, ToPolyTraitRef, Ty, TypeFoldable};
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use rustc_data_structures::obligation_forest::ProcessResult;
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use rustc_data_structures::obligation_forest::{DoCompleted, Error, ForestObligation};
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use rustc_data_structures::obligation_forest::{ObligationForest, ObligationProcessor};
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use std::marker::PhantomData;
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use super::engine::{TraitEngine, TraitEngineExt};
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use super::project;
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use super::select::SelectionContext;
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use super::wf;
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use super::CodeAmbiguity;
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use super::CodeProjectionError;
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use super::CodeSelectionError;
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use super::{ConstEvalFailure, Unimplemented};
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use super::{FulfillmentError, FulfillmentErrorCode};
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use super::{ObligationCause, PredicateObligation};
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impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
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/// Note that we include both the `ParamEnv` and the `Predicate`,
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/// as the `ParamEnv` can influence whether fulfillment succeeds
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/// or fails.
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type CacheKey = ty::ParamEnvAnd<'tcx, ty::Predicate<'tcx>>;
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fn as_cache_key(&self) -> Self::CacheKey {
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self.obligation.param_env.and(self.obligation.predicate)
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}
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}
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/// The fulfillment context is used to drive trait resolution. It
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/// consists of a list of obligations that must be (eventually)
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/// satisfied. The job is to track which are satisfied, which yielded
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/// errors, and which are still pending. At any point, users can call
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/// `select_where_possible`, and the fulfillment context will try to do
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/// selection, retaining only those obligations that remain
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/// ambiguous. This may be helpful in pushing type inference
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/// along. Once all type inference constraints have been generated, the
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/// method `select_all_or_error` can be used to report any remaining
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/// ambiguous cases as errors.
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pub struct FulfillmentContext<'tcx> {
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// A list of all obligations that have been registered with this
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// fulfillment context.
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predicates: ObligationForest<PendingPredicateObligation<'tcx>>,
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// Should this fulfillment context register type-lives-for-region
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// obligations on its parent infcx? In some cases, region
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// obligations are either already known to hold (normalization) or
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// hopefully verifed elsewhere (type-impls-bound), and therefore
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// should not be checked.
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//
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// Note that if we are normalizing a type that we already
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// know is well-formed, there should be no harm setting this
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// to true - all the region variables should be determinable
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// using the RFC 447 rules, which don't depend on
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// type-lives-for-region constraints, and because the type
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// is well-formed, the constraints should hold.
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register_region_obligations: bool,
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// Is it OK to register obligations into this infcx inside
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// an infcx snapshot?
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//
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// The "primary fulfillment" in many cases in typeck lives
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// outside of any snapshot, so any use of it inside a snapshot
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// will lead to trouble and therefore is checked against, but
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// other fulfillment contexts sometimes do live inside of
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// a snapshot (they don't *straddle* a snapshot, so there
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// is no trouble there).
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usable_in_snapshot: bool,
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}
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#[derive(Clone, Debug)]
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pub struct PendingPredicateObligation<'tcx> {
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pub obligation: PredicateObligation<'tcx>,
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pub stalled_on: Vec<ty::InferTy>,
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}
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// `PendingPredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
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#[cfg(target_arch = "x86_64")]
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static_assert_size!(PendingPredicateObligation<'_>, 136);
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impl<'a, 'tcx> FulfillmentContext<'tcx> {
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/// Creates a new fulfillment context.
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pub fn new() -> FulfillmentContext<'tcx> {
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FulfillmentContext {
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predicates: ObligationForest::new(),
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register_region_obligations: true,
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usable_in_snapshot: false,
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}
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}
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pub fn new_in_snapshot() -> FulfillmentContext<'tcx> {
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FulfillmentContext {
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predicates: ObligationForest::new(),
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register_region_obligations: true,
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usable_in_snapshot: true,
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}
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}
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pub fn new_ignoring_regions() -> FulfillmentContext<'tcx> {
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FulfillmentContext {
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predicates: ObligationForest::new(),
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register_region_obligations: false,
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usable_in_snapshot: false,
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}
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}
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/// Attempts to select obligations using `selcx`.
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fn select(
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&mut self,
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selcx: &mut SelectionContext<'a, 'tcx>,
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) -> Result<(), Vec<FulfillmentError<'tcx>>> {
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debug!("select(obligation-forest-size={})", self.predicates.len());
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let mut errors = Vec::new();
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loop {
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debug!("select: starting another iteration");
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// Process pending obligations.
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let outcome = self.predicates.process_obligations(
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&mut FulfillProcessor {
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selcx,
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register_region_obligations: self.register_region_obligations,
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},
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DoCompleted::No,
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);
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debug!("select: outcome={:#?}", outcome);
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// FIXME: if we kept the original cache key, we could mark projection
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// obligations as complete for the projection cache here.
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errors.extend(outcome.errors.into_iter().map(|e| to_fulfillment_error(e)));
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// If nothing new was added, no need to keep looping.
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if outcome.stalled {
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break;
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}
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}
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debug!(
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"select({} predicates remaining, {} errors) done",
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self.predicates.len(),
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errors.len()
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);
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if errors.is_empty() { Ok(()) } else { Err(errors) }
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}
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}
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impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
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/// "Normalize" a projection type `<SomeType as SomeTrait>::X` by
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/// creating a fresh type variable `$0` as well as a projection
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/// predicate `<SomeType as SomeTrait>::X == $0`. When the
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/// inference engine runs, it will attempt to find an impl of
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/// `SomeTrait` or a where-clause that lets us unify `$0` with
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/// something concrete. If this fails, we'll unify `$0` with
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/// `projection_ty` again.
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fn normalize_projection_type(
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&mut self,
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infcx: &InferCtxt<'_, 'tcx>,
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param_env: ty::ParamEnv<'tcx>,
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projection_ty: ty::ProjectionTy<'tcx>,
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cause: ObligationCause<'tcx>,
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) -> Ty<'tcx> {
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debug!("normalize_projection_type(projection_ty={:?})", projection_ty);
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debug_assert!(!projection_ty.has_escaping_bound_vars());
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// FIXME(#20304) -- cache
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let mut selcx = SelectionContext::new(infcx);
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let mut obligations = vec![];
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let normalized_ty = project::normalize_projection_type(
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&mut selcx,
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param_env,
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projection_ty,
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cause,
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0,
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&mut obligations,
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);
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self.register_predicate_obligations(infcx, obligations);
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debug!("normalize_projection_type: result={:?}", normalized_ty);
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normalized_ty
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}
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fn register_predicate_obligation(
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&mut self,
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infcx: &InferCtxt<'_, 'tcx>,
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obligation: PredicateObligation<'tcx>,
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) {
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// this helps to reduce duplicate errors, as well as making
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// debug output much nicer to read and so on.
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let obligation = infcx.resolve_vars_if_possible(&obligation);
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debug!("register_predicate_obligation(obligation={:?})", obligation);
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assert!(!infcx.is_in_snapshot() || self.usable_in_snapshot);
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self.predicates
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.register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
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}
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fn select_all_or_error(
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&mut self,
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infcx: &InferCtxt<'_, 'tcx>,
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) -> Result<(), Vec<FulfillmentError<'tcx>>> {
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self.select_where_possible(infcx)?;
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let errors: Vec<_> = self
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.predicates
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.to_errors(CodeAmbiguity)
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.into_iter()
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.map(|e| to_fulfillment_error(e))
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.collect();
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if errors.is_empty() { Ok(()) } else { Err(errors) }
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}
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fn select_where_possible(
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&mut self,
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infcx: &InferCtxt<'_, 'tcx>,
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) -> Result<(), Vec<FulfillmentError<'tcx>>> {
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let mut selcx = SelectionContext::new(infcx);
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self.select(&mut selcx)
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}
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fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
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self.predicates.map_pending_obligations(|o| o.obligation.clone())
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}
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}
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struct FulfillProcessor<'a, 'b, 'tcx> {
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selcx: &'a mut SelectionContext<'b, 'tcx>,
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register_region_obligations: bool,
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}
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fn mk_pending(os: Vec<PredicateObligation<'tcx>>) -> Vec<PendingPredicateObligation<'tcx>> {
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os.into_iter()
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.map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
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.collect()
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}
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impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
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type Obligation = PendingPredicateObligation<'tcx>;
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type Error = FulfillmentErrorCode<'tcx>;
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/// Processes a predicate obligation and returns either:
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/// - `Changed(v)` if the predicate is true, presuming that `v` are also true
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/// - `Unchanged` if we don't have enough info to be sure
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/// - `Error(e)` if the predicate does not hold
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///
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/// This is always inlined, despite its size, because it has a single
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/// callsite and it is called *very* frequently.
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#[inline(always)]
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fn process_obligation(
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&mut self,
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pending_obligation: &mut Self::Obligation,
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) -> ProcessResult<Self::Obligation, Self::Error> {
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// If we were stalled on some unresolved variables, first check whether
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// any of them have been resolved; if not, don't bother doing more work
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// yet.
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let change = match pending_obligation.stalled_on.len() {
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// Match arms are in order of frequency, which matters because this
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// code is so hot. 1 and 0 dominate; 2+ is fairly rare.
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1 => {
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let infer = pending_obligation.stalled_on[0];
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ShallowResolver::new(self.selcx.infcx()).shallow_resolve_changed(infer)
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}
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0 => {
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// In this case we haven't changed, but wish to make a change.
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true
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}
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_ => {
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// This `for` loop was once a call to `all()`, but this lower-level
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// form was a perf win. See #64545 for details.
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(|| {
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for &infer in &pending_obligation.stalled_on {
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if ShallowResolver::new(self.selcx.infcx()).shallow_resolve_changed(infer) {
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return true;
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}
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}
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false
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})()
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}
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};
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if !change {
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debug!(
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"process_predicate: pending obligation {:?} still stalled on {:?}",
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self.selcx.infcx().resolve_vars_if_possible(&pending_obligation.obligation),
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pending_obligation.stalled_on
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);
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return ProcessResult::Unchanged;
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}
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// This part of the code is much colder.
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pending_obligation.stalled_on.truncate(0);
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let obligation = &mut pending_obligation.obligation;
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if obligation.predicate.has_infer_types_or_consts() {
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obligation.predicate =
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self.selcx.infcx().resolve_vars_if_possible(&obligation.predicate);
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}
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debug!("process_obligation: obligation = {:?} cause = {:?}", obligation, obligation.cause);
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fn infer_ty(ty: Ty<'tcx>) -> ty::InferTy {
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match ty.kind {
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ty::Infer(infer) => infer,
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_ => panic!(),
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}
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}
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match obligation.predicate {
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ty::Predicate::Trait(ref data, _) => {
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let trait_obligation = obligation.with(data.clone());
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if data.is_global() {
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// no type variables present, can use evaluation for better caching.
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// FIXME: consider caching errors too.
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if self.selcx.infcx().predicate_must_hold_considering_regions(&obligation) {
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debug!(
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"selecting trait `{:?}` at depth {} evaluated to holds",
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data, obligation.recursion_depth
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);
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return ProcessResult::Changed(vec![]);
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}
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}
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match self.selcx.select(&trait_obligation) {
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Ok(Some(vtable)) => {
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debug!(
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"selecting trait `{:?}` at depth {} yielded Ok(Some)",
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data, obligation.recursion_depth
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);
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ProcessResult::Changed(mk_pending(vtable.nested_obligations()))
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}
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Ok(None) => {
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debug!(
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"selecting trait `{:?}` at depth {} yielded Ok(None)",
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data, obligation.recursion_depth
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);
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// This is a bit subtle: for the most part, the
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// only reason we can fail to make progress on
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// trait selection is because we don't have enough
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// information about the types in the trait.
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pending_obligation.stalled_on =
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trait_ref_type_vars(self.selcx, data.to_poly_trait_ref());
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debug!(
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"process_predicate: pending obligation {:?} now stalled on {:?}",
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self.selcx.infcx().resolve_vars_if_possible(obligation),
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pending_obligation.stalled_on
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);
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ProcessResult::Unchanged
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}
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Err(selection_err) => {
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info!(
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"selecting trait `{:?}` at depth {} yielded Err",
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data, obligation.recursion_depth
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);
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ProcessResult::Error(CodeSelectionError(selection_err))
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}
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}
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}
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ty::Predicate::RegionOutlives(ref binder) => {
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match self.selcx.infcx().region_outlives_predicate(&obligation.cause, binder) {
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Ok(()) => ProcessResult::Changed(vec![]),
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Err(_) => ProcessResult::Error(CodeSelectionError(Unimplemented)),
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}
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}
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ty::Predicate::TypeOutlives(ref binder) => {
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// Check if there are higher-ranked vars.
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match binder.no_bound_vars() {
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// If there are, inspect the underlying type further.
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None => {
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// Convert from `Binder<OutlivesPredicate<Ty, Region>>` to `Binder<Ty>`.
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let binder = binder.map_bound_ref(|pred| pred.0);
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// Check if the type has any bound vars.
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match binder.no_bound_vars() {
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// If so, this obligation is an error (for now). Eventually we should be
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// able to support additional cases here, like `for<'a> &'a str: 'a`.
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// NOTE: this is duplicate-implemented between here and fulfillment.
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None => ProcessResult::Error(CodeSelectionError(Unimplemented)),
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// Otherwise, we have something of the form
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// `for<'a> T: 'a where 'a not in T`, which we can treat as
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// `T: 'static`.
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Some(t_a) => {
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let r_static = self.selcx.tcx().lifetimes.re_static;
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if self.register_region_obligations {
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self.selcx.infcx().register_region_obligation_with_cause(
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t_a,
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r_static,
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&obligation.cause,
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);
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}
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ProcessResult::Changed(vec![])
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}
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}
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}
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// If there aren't, register the obligation.
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Some(ty::OutlivesPredicate(t_a, r_b)) => {
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if self.register_region_obligations {
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self.selcx.infcx().register_region_obligation_with_cause(
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t_a,
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r_b,
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&obligation.cause,
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);
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}
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ProcessResult::Changed(vec![])
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}
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}
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}
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ty::Predicate::Projection(ref data) => {
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let project_obligation = obligation.with(data.clone());
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match project::poly_project_and_unify_type(self.selcx, &project_obligation) {
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Ok(None) => {
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let tcx = self.selcx.tcx();
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pending_obligation.stalled_on =
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trait_ref_type_vars(self.selcx, data.to_poly_trait_ref(tcx));
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ProcessResult::Unchanged
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}
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Ok(Some(os)) => ProcessResult::Changed(mk_pending(os)),
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Err(e) => ProcessResult::Error(CodeProjectionError(e)),
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}
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}
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ty::Predicate::ObjectSafe(trait_def_id) => {
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if !self.selcx.tcx().is_object_safe(trait_def_id) {
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ProcessResult::Error(CodeSelectionError(Unimplemented))
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} else {
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ProcessResult::Changed(vec![])
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}
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}
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ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
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match self.selcx.infcx().closure_kind(closure_def_id, closure_substs) {
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Some(closure_kind) => {
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if closure_kind.extends(kind) {
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ProcessResult::Changed(vec![])
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} else {
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ProcessResult::Error(CodeSelectionError(Unimplemented))
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}
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}
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None => ProcessResult::Unchanged,
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}
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}
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ty::Predicate::WellFormed(ty) => {
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match wf::obligations(
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self.selcx.infcx(),
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obligation.param_env,
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obligation.cause.body_id,
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ty,
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obligation.cause.span,
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) {
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None => {
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pending_obligation.stalled_on = vec![infer_ty(ty)];
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ProcessResult::Unchanged
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}
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Some(os) => ProcessResult::Changed(mk_pending(os)),
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}
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}
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ty::Predicate::Subtype(ref subtype) => {
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match self.selcx.infcx().subtype_predicate(
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&obligation.cause,
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obligation.param_env,
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subtype,
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) {
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None => {
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// None means that both are unresolved.
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pending_obligation.stalled_on = vec![
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infer_ty(subtype.skip_binder().a),
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infer_ty(subtype.skip_binder().b),
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];
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ProcessResult::Unchanged
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}
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Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
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|
Some(Err(err)) => {
|
|
let expected_found = ExpectedFound::new(
|
|
subtype.skip_binder().a_is_expected,
|
|
subtype.skip_binder().a,
|
|
subtype.skip_binder().b,
|
|
);
|
|
ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
|
|
expected_found,
|
|
err,
|
|
))
|
|
}
|
|
}
|
|
}
|
|
|
|
ty::Predicate::ConstEvaluatable(def_id, substs) => {
|
|
match self.selcx.infcx().const_eval_resolve(
|
|
obligation.param_env,
|
|
def_id,
|
|
substs,
|
|
None,
|
|
Some(obligation.cause.span),
|
|
) {
|
|
Ok(_) => ProcessResult::Changed(vec![]),
|
|
Err(err) => ProcessResult::Error(CodeSelectionError(ConstEvalFailure(err))),
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
fn process_backedge<'c, I>(
|
|
&mut self,
|
|
cycle: I,
|
|
_marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
|
|
) where
|
|
I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
|
|
{
|
|
if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
|
|
debug!("process_child_obligations: coinductive match");
|
|
} else {
|
|
let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
|
|
self.selcx.infcx().report_overflow_error_cycle(&cycle);
|
|
}
|
|
}
|
|
}
|
|
|
|
/// Returns the set of type variables contained in a trait ref
|
|
fn trait_ref_type_vars<'a, 'tcx>(
|
|
selcx: &mut SelectionContext<'a, 'tcx>,
|
|
t: ty::PolyTraitRef<'tcx>,
|
|
) -> Vec<ty::InferTy> {
|
|
t.skip_binder() // ok b/c this check doesn't care about regions
|
|
.input_types()
|
|
.map(|t| selcx.infcx().resolve_vars_if_possible(&t))
|
|
.filter(|t| t.has_infer_types())
|
|
.flat_map(|t| t.walk())
|
|
.filter_map(|t| match t.kind {
|
|
ty::Infer(infer) => Some(infer),
|
|
_ => None,
|
|
})
|
|
.collect()
|
|
}
|
|
|
|
fn to_fulfillment_error<'tcx>(
|
|
error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
|
|
) -> FulfillmentError<'tcx> {
|
|
let obligation = error.backtrace.into_iter().next().unwrap().obligation;
|
|
FulfillmentError::new(obligation, error.error)
|
|
}
|