rust/src/librustc/traits/query/outlives_bounds.rs

// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use infer::InferCtxt;
use syntax::ast;
use syntax::source_map::Span;
use smallvec::SmallVec;
use traits::{FulfillmentContext, ObligationCause, TraitEngine, TraitEngineExt};
use traits::query::NoSolution;
use ty::{self, Ty, TyCtxt};

use ich::StableHashingContext;
use rustc_data_structures::stable_hasher::{HashStable, StableHasher,
                                           StableHasherResult};
use std::mem;

/// Outlives bounds are relationships between generic parameters,
/// whether they both be regions (`'a: 'b`) or whether types are
/// involved (`T: 'a`).  These relationships can be extracted from the
/// full set of predicates we understand or also from types (in which
/// case they are called implied bounds). They are fed to the
/// `OutlivesEnv` which in turn is supplied to the region checker and
/// other parts of the inference system.
#[derive(Clone, Debug)]
pub enum OutlivesBound<'tcx> {
    RegionSubRegion(ty::Region<'tcx>, ty::Region<'tcx>),
    RegionSubParam(ty::Region<'tcx>, ty::ParamTy),
    RegionSubProjection(ty::Region<'tcx>, ty::ProjectionTy<'tcx>),
}

EnumLiftImpl! {
    impl<'a, 'tcx> Lift<'tcx> for self::OutlivesBound<'a> {
        type Lifted = self::OutlivesBound<'tcx>;
        (self::OutlivesBound::RegionSubRegion)(a, b),
        (self::OutlivesBound::RegionSubParam)(a, b),
        (self::OutlivesBound::RegionSubProjection)(a, b),
    }
}

EnumTypeFoldableImpl! {
    impl<'tcx> TypeFoldable<'tcx> for self::OutlivesBound<'tcx> {
        (self::OutlivesBound::RegionSubRegion)(a, b),
        (self::OutlivesBound::RegionSubParam)(a, b),
        (self::OutlivesBound::RegionSubProjection)(a, b),
    }
}

impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for OutlivesBound<'tcx> {
    fn hash_stable<W: StableHasherResult>(&self,
                                          hcx: &mut StableHashingContext<'a>,
                                          hasher: &mut StableHasher<W>) {
        mem::discriminant(self).hash_stable(hcx, hasher);
        match *self {
            OutlivesBound::RegionSubRegion(ref a, ref b) => {
                a.hash_stable(hcx, hasher);
                b.hash_stable(hcx, hasher);
            }
            OutlivesBound::RegionSubParam(ref a, ref b) => {
                a.hash_stable(hcx, hasher);
                b.hash_stable(hcx, hasher);
            }
            OutlivesBound::RegionSubProjection(ref a, ref b) => {
                a.hash_stable(hcx, hasher);
                b.hash_stable(hcx, hasher);
            }
        }
    }
}

impl<'cx, 'gcx, 'tcx> InferCtxt<'cx, 'gcx, 'tcx> {
    /// Implied bounds are region relationships that we deduce
    /// automatically.  The idea is that (e.g.) a caller must check that a
    /// function's argument types are well-formed immediately before
    /// calling that fn, and hence the *callee* can assume that its
    /// argument types are well-formed. This may imply certain relationships
    /// between generic parameters. For example:
    ///
    ///     fn foo<'a,T>(x: &'a T)
    ///
    /// can only be called with a `'a` and `T` such that `&'a T` is WF.
    /// For `&'a T` to be WF, `T: 'a` must hold. So we can assume `T: 'a`.
    ///
    /// # Parameters
    ///
    /// - `param_env`, the where-clauses in scope
    /// - `body_id`, the body-id to use when normalizing assoc types.
    ///   Note that this may cause outlives obligations to be injected
    ///   into the inference context with this body-id.
    /// - `ty`, the type that we are supposed to assume is WF.
    /// - `span`, a span to use when normalizing, hopefully not important,
    ///   might be useful if a `bug!` occurs.
    pub fn implied_outlives_bounds(
        &self,
        param_env: ty::ParamEnv<'tcx>,
        body_id: ast::NodeId,
        ty: Ty<'tcx>,
        span: Span,
    ) -> Vec<OutlivesBound<'tcx>> {
        debug!("implied_outlives_bounds(ty = {:?})", ty);

        let mut orig_values = SmallVec::new();
        let key = self.canonicalize_query(&param_env.and(ty), &mut orig_values);
        let result = match self.tcx.global_tcx().implied_outlives_bounds(key) {
            Ok(r) => r,
            Err(NoSolution) => {
                self.tcx.sess.delay_span_bug(
                    span,
                    "implied_outlives_bounds failed to solve all obligations"
                );
                return vec![];
            }
        };
        assert!(result.value.is_proven());

        let result = self.instantiate_query_result_and_region_obligations(
            &ObligationCause::misc(span, body_id), param_env, &orig_values, &result);
        debug!("implied_outlives_bounds for {:?}: {:#?}", ty, result);
        let result = match result {
            Ok(v) => v,
            Err(_) => {
                self.tcx.sess.delay_span_bug(
                    span,
                    "implied_outlives_bounds failed to instantiate"
                );
                return vec![];
            }
        };

        // Instantiation may have produced new inference variables and constraints on those
        // variables. Process these constraints.
        let mut fulfill_cx = FulfillmentContext::new();
        fulfill_cx.register_predicate_obligations(self, result.obligations);
        if fulfill_cx.select_all_or_error(self).is_err() {
            self.tcx.sess.delay_span_bug(
                span,
                "implied_outlives_bounds failed to solve obligations from instantiation"
            );
        }

        result.value
    }
}

pub fn explicit_outlives_bounds<'tcx>(
    param_env: ty::ParamEnv<'tcx>,
) -> impl Iterator<Item = OutlivesBound<'tcx>> + 'tcx {
    debug!("explicit_outlives_bounds()");
    param_env
        .caller_bounds
        .into_iter()
        .filter_map(move |predicate| match predicate {
            ty::Predicate::Projection(..) |
            ty::Predicate::Trait(..) |
            ty::Predicate::Subtype(..) |
            ty::Predicate::WellFormed(..) |
            ty::Predicate::ObjectSafe(..) |
            ty::Predicate::ClosureKind(..) |
            ty::Predicate::TypeOutlives(..) |
            ty::Predicate::ConstEvaluatable(..) => None,
            ty::Predicate::RegionOutlives(ref data) => data.no_late_bound_regions().map(
                |ty::OutlivesPredicate(r_a, r_b)| OutlivesBound::RegionSubRegion(r_b, r_a),
            ),
        })
}