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//! Trait Resolution. See the [rustc-dev-guide] for more information on how this works.
//!
//! [rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/traits/resolution.html
mod engine;
pub mod error_reporting;
mod project;
mod structural_impls;
pub mod util;
use rustc_hir as hir;
use rustc_middle::ty::error::{ExpectedFound, TypeError};
use rustc_middle::ty::{self, Const, ToPredicate, Ty, TyCtxt};
use rustc_span::Span;
pub use self::FulfillmentErrorCode::*;
pub use self::ImplSource::*;
pub use self::ObligationCauseCode::*;
pub use self::SelectionError::*;
pub use self::engine::{TraitEngine, TraitEngineExt};
pub use self::project::MismatchedProjectionTypes;
pub(crate) use self::project::UndoLog;
pub use self::project::{
Normalized, NormalizedTy, ProjectionCache, ProjectionCacheEntry, ProjectionCacheKey,
ProjectionCacheStorage, Reveal,
};
pub use rustc_middle::traits::*;
/// An `Obligation` represents some trait reference (e.g., `i32: Eq`) for
/// which the "impl_source" must be found. The process of finding an "impl_source" is
/// called "resolving" the `Obligation`. This process consists of
/// either identifying an `impl` (e.g., `impl Eq for i32`) that
/// satisfies the obligation, or else finding a bound that is in
/// scope. The eventual result is usually a `Selection` (defined below).
#[derive(Clone, PartialEq, Eq, Hash)]
pub struct Obligation<'tcx, T> {
/// The reason we have to prove this thing.
pub cause: ObligationCause<'tcx>,
/// The environment in which we should prove this thing.
pub param_env: ty::ParamEnv<'tcx>,
/// The thing we are trying to prove.
pub predicate: T,
/// If we started proving this as a result of trying to prove
/// something else, track the total depth to ensure termination.
/// If this goes over a certain threshold, we abort compilation --
/// in such cases, we can not say whether or not the predicate
/// holds for certain. Stupid halting problem; such a drag.
pub recursion_depth: usize,
}
pub type PredicateObligation<'tcx> = Obligation<'tcx, ty::Predicate<'tcx>>;
pub type TraitObligation<'tcx> = Obligation<'tcx, ty::PolyTraitPredicate<'tcx>>;
impl<'tcx> PredicateObligation<'tcx> {
/// Flips the polarity of the inner predicate.
///
/// Given `T: Trait` predicate it returns `T: !Trait` and given `T: !Trait` returns `T: Trait`.
pub fn flip_polarity(&self, tcx: TyCtxt<'tcx>) -> Option<PredicateObligation<'tcx>> {
Some(PredicateObligation {
cause: self.cause.clone(),
param_env: self.param_env,
predicate: self.predicate.flip_polarity(tcx)?,
recursion_depth: self.recursion_depth,
})
}
pub fn without_const(mut self, tcx: TyCtxt<'tcx>) -> PredicateObligation<'tcx> {
self.param_env = self.param_env.without_const();
if let ty::PredicateKind::Clause(ty::Clause::Trait(trait_pred)) = self.predicate.kind().skip_binder() && trait_pred.is_const_if_const() {
self.predicate = tcx.mk_predicate(self.predicate.kind().map_bound(|_| ty::PredicateKind::Clause(ty::Clause::Trait(trait_pred.without_const()))));
}
self
}
}
impl<'tcx> TraitObligation<'tcx> {
/// Returns `true` if the trait predicate is considered `const` in its ParamEnv.
pub fn is_const(&self) -> bool {
match (self.predicate.skip_binder().constness, self.param_env.constness()) {
(ty::BoundConstness::ConstIfConst, hir::Constness::Const) => true,
_ => false,
}
}
pub fn derived_cause(
&self,
variant: impl FnOnce(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>,
) -> ObligationCause<'tcx> {
self.cause.clone().derived_cause(self.predicate, variant)
}
}
// `PredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
static_assert_size!(PredicateObligation<'_>, 48);
pub type PredicateObligations<'tcx> = Vec<PredicateObligation<'tcx>>;
pub type Selection<'tcx> = ImplSource<'tcx, PredicateObligation<'tcx>>;
pub struct FulfillmentError<'tcx> {
pub obligation: PredicateObligation<'tcx>,
pub code: FulfillmentErrorCode<'tcx>,
/// Diagnostics only: the 'root' obligation which resulted in
/// the failure to process `obligation`. This is the obligation
/// that was initially passed to `register_predicate_obligation`
pub root_obligation: PredicateObligation<'tcx>,
}
#[derive(Clone)]
pub enum FulfillmentErrorCode<'tcx> {
/// Inherently impossible to fulfill; this trait is implemented if and only if it is already implemented.
CodeCycle(Vec<Obligation<'tcx, ty::Predicate<'tcx>>>),
CodeSelectionError(SelectionError<'tcx>),
CodeProjectionError(MismatchedProjectionTypes<'tcx>),
CodeSubtypeError(ExpectedFound<Ty<'tcx>>, TypeError<'tcx>), // always comes from a SubtypePredicate
CodeConstEquateError(ExpectedFound<Const<'tcx>>, TypeError<'tcx>),
CodeAmbiguity,
}
impl<'tcx, O> Obligation<'tcx, O> {
pub fn new(
tcx: TyCtxt<'tcx>,
cause: ObligationCause<'tcx>,
param_env: ty::ParamEnv<'tcx>,
predicate: impl ToPredicate<'tcx, O>,
) -> Obligation<'tcx, O> {
Self::with_depth(tcx, cause, 0, param_env, predicate)
}
pub fn with_depth(
tcx: TyCtxt<'tcx>,
cause: ObligationCause<'tcx>,
recursion_depth: usize,
param_env: ty::ParamEnv<'tcx>,
predicate: impl ToPredicate<'tcx, O>,
) -> Obligation<'tcx, O> {
let predicate = predicate.to_predicate(tcx);
Obligation { cause, param_env, recursion_depth, predicate }
}
pub fn misc(
tcx: TyCtxt<'tcx>,
span: Span,
body_id: hir::HirId,
param_env: ty::ParamEnv<'tcx>,
trait_ref: impl ToPredicate<'tcx, O>,
) -> Obligation<'tcx, O> {
Obligation::new(tcx, ObligationCause::misc(span, body_id), param_env, trait_ref)
}
pub fn with<P>(
&self,
tcx: TyCtxt<'tcx>,
value: impl ToPredicate<'tcx, P>,
) -> Obligation<'tcx, P> {
Obligation::with_depth(tcx, self.cause.clone(), self.recursion_depth, self.param_env, value)
}
}
impl<'tcx> FulfillmentError<'tcx> {
pub fn new(
obligation: PredicateObligation<'tcx>,
code: FulfillmentErrorCode<'tcx>,
root_obligation: PredicateObligation<'tcx>,
) -> FulfillmentError<'tcx> {
FulfillmentError { obligation, code, root_obligation }
}
}
impl<'tcx> TraitObligation<'tcx> {
pub fn polarity(&self) -> ty::ImplPolarity {
self.predicate.skip_binder().polarity
}
pub fn self_ty(&self) -> ty::Binder<'tcx, Ty<'tcx>> {
self.predicate.map_bound(|p| p.self_ty())
}
}
|
