rust/compiler/rustc_infer/src/traits/mod.rs

//! 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, 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::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::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(
        cause: ObligationCause<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
        predicate: O,
    ) -> Obligation<'tcx, O> {
        Obligation { cause, param_env, recursion_depth: 0, predicate }
    }

    pub fn with_depth(
        cause: ObligationCause<'tcx>,
        recursion_depth: usize,
        param_env: ty::ParamEnv<'tcx>,
        predicate: O,
    ) -> Obligation<'tcx, O> {
        Obligation { cause, param_env, recursion_depth, predicate }
    }

    pub fn misc(
        span: Span,
        body_id: hir::HirId,
        param_env: ty::ParamEnv<'tcx>,
        trait_ref: O,
    ) -> Obligation<'tcx, O> {
        Obligation::new(ObligationCause::misc(span, body_id), param_env, trait_ref)
    }

    pub fn with<P>(&self, value: P) -> Obligation<'tcx, P> {
        Obligation {
            cause: self.cause.clone(),
            param_env: self.param_env,
            recursion_depth: self.recursion_depth,
            predicate: 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())
    }
}