rust/src/librustc/middle/implicator.rs

// Copyright 2012 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.

// #![warn(deprecated_mode)]

use middle::infer::{InferCtxt, GenericKind};
use middle::subst::Substs;
use middle::traits;
use middle::ty::{self, RegionEscape, ToPolyTraitRef, ToPredicate, Ty};
use middle::ty_fold::{TypeFoldable, TypeFolder};

use syntax::ast;
use syntax::codemap::Span;

use util::common::ErrorReported;
use util::nodemap::FnvHashSet;

// Helper functions related to manipulating region types.

#[derive(Debug)]
pub enum Implication<'tcx> {
    RegionSubRegion(Option<Ty<'tcx>>, ty::Region, ty::Region),
    RegionSubGeneric(Option<Ty<'tcx>>, ty::Region, GenericKind<'tcx>),
    RegionSubClosure(Option<Ty<'tcx>>, ty::Region, ast::DefId, &'tcx Substs<'tcx>),
    Predicate(ast::DefId, ty::Predicate<'tcx>),
}

struct Implicator<'a, 'tcx: 'a> {
    infcx: &'a InferCtxt<'a,'tcx>,
    body_id: ast::NodeId,
    stack: Vec<(ty::Region, Option<Ty<'tcx>>)>,
    span: Span,
    out: Vec<Implication<'tcx>>,
    visited: FnvHashSet<Ty<'tcx>>,
}

/// This routine computes the well-formedness constraints that must hold for the type `ty` to
/// appear in a context with lifetime `outer_region`
pub fn implications<'a,'tcx>(
    infcx: &'a InferCtxt<'a,'tcx>,
    body_id: ast::NodeId,
    ty: Ty<'tcx>,
    outer_region: ty::Region,
    span: Span)
    -> Vec<Implication<'tcx>>
{
    debug!("implications(body_id={}, ty={:?}, outer_region={:?})",
           body_id,
           ty,
           outer_region);

    let mut stack = Vec::new();
    stack.push((outer_region, None));
    let mut wf = Implicator { infcx: infcx,
                              body_id: body_id,
                              span: span,
                              stack: stack,
                              out: Vec::new(),
                              visited: FnvHashSet() };
    wf.accumulate_from_ty(ty);
    debug!("implications: out={:?}", wf.out);
    wf.out
}

impl<'a, 'tcx> Implicator<'a, 'tcx> {
    fn tcx(&self) -> &'a ty::ctxt<'tcx> {
        self.infcx.tcx
    }

    fn accumulate_from_ty(&mut self, ty: Ty<'tcx>) {
        debug!("accumulate_from_ty(ty={:?})",
               ty);

        // When expanding out associated types, we can visit a cyclic
        // set of types. Issue #23003.
        if !self.visited.insert(ty) {
            return;
        }

        match ty.sty {
            ty::TyBool |
            ty::TyChar |
            ty::TyInt(..) |
            ty::TyUint(..) |
            ty::TyFloat(..) |
            ty::TyBareFn(..) |
            ty::TyError |
            ty::TyStr => {
                // No borrowed content reachable here.
            }

            ty::TyClosure(def_id, substs) => {
                let &(r_a, opt_ty) = self.stack.last().unwrap();
                self.out.push(Implication::RegionSubClosure(opt_ty, r_a, def_id, substs));
            }

            ty::TyTrait(ref t) => {
                let required_region_bounds =
                    object_region_bounds(self.tcx(), &t.principal, t.bounds.builtin_bounds);
                self.accumulate_from_object_ty(ty, t.bounds.region_bound, required_region_bounds)
            }

            ty::TyEnum(def_id, substs) |
            ty::TyStruct(def_id, substs) => {
                let item_scheme = self.tcx().lookup_item_type(def_id);
                self.accumulate_from_adt(ty, def_id, &item_scheme.generics, substs)
            }

            ty::TyArray(t, _) |
            ty::TySlice(t) |
            ty::TyRawPtr(ty::TypeWithMutability { ty: t, .. }) |
            ty::TyBox(t) => {
                self.accumulate_from_ty(t)
            }

            ty::TyRef(r_b, mt) => {
                self.accumulate_from_rptr(ty, *r_b, mt.ty);
            }

            ty::TyParam(p) => {
                self.push_param_constraint_from_top(p);
            }

            ty::TyProjection(ref data) => {
                // `<T as TraitRef<..>>::Name`

                self.push_projection_constraint_from_top(data);
            }

            ty::TyTuple(ref tuptys) => {
                for &tupty in tuptys {
                    self.accumulate_from_ty(tupty);
                }
            }

            ty::TyInfer(_) => {
                // This should not happen, BUT:
                //
                //   Currently we uncover region relationships on
                //   entering the fn check. We should do this after
                //   the fn check, then we can call this case a bug().
            }
        }
    }

    fn accumulate_from_rptr(&mut self,
                            ty: Ty<'tcx>,
                            r_b: ty::Region,
                            ty_b: Ty<'tcx>) {
        // We are walking down a type like this, and current
        // position is indicated by caret:
        //
        //     &'a &'b ty_b
        //         ^
        //
        // At this point, top of stack will be `'a`. We must
        // require that `'a <= 'b`.

        self.push_region_constraint_from_top(r_b);

        // Now we push `'b` onto the stack, because it must
        // constrain any borrowed content we find within `T`.

        self.stack.push((r_b, Some(ty)));
        self.accumulate_from_ty(ty_b);
        self.stack.pop().unwrap();
    }

    /// Pushes a constraint that `r_b` must outlive the top region on the stack.
    fn push_region_constraint_from_top(&mut self,
                                       r_b: ty::Region) {

        // Indicates that we have found borrowed content with a lifetime
        // of at least `r_b`. This adds a constraint that `r_b` must
        // outlive the region `r_a` on top of the stack.
        //
        // As an example, imagine walking a type like:
        //
        //     &'a &'b T
        //         ^
        //
        // when we hit the inner pointer (indicated by caret), `'a` will
        // be on top of stack and `'b` will be the lifetime of the content
        // we just found. So we add constraint that `'a <= 'b`.

        let &(r_a, opt_ty) = self.stack.last().unwrap();
        self.push_sub_region_constraint(opt_ty, r_a, r_b);
    }

    /// Pushes a constraint that `r_a <= r_b`, due to `opt_ty`
    fn push_sub_region_constraint(&mut self,
                                  opt_ty: Option<Ty<'tcx>>,
                                  r_a: ty::Region,
                                  r_b: ty::Region) {
        self.out.push(Implication::RegionSubRegion(opt_ty, r_a, r_b));
    }

    /// Pushes a constraint that `param_ty` must outlive the top region on the stack.
    fn push_param_constraint_from_top(&mut self,
                                      param_ty: ty::ParamTy) {
        let &(region, opt_ty) = self.stack.last().unwrap();
        self.push_param_constraint(region, opt_ty, param_ty);
    }

    /// Pushes a constraint that `projection_ty` must outlive the top region on the stack.
    fn push_projection_constraint_from_top(&mut self,
                                           projection_ty: &ty::ProjectionTy<'tcx>) {
        let &(region, opt_ty) = self.stack.last().unwrap();
        self.out.push(Implication::RegionSubGeneric(
            opt_ty, region, GenericKind::Projection(projection_ty.clone())));
    }

    /// Pushes a constraint that `region <= param_ty`, due to `opt_ty`
    fn push_param_constraint(&mut self,
                             region: ty::Region,
                             opt_ty: Option<Ty<'tcx>>,
                             param_ty: ty::ParamTy) {
        self.out.push(Implication::RegionSubGeneric(
            opt_ty, region, GenericKind::Param(param_ty)));
    }

    fn accumulate_from_adt(&mut self,
                           ty: Ty<'tcx>,
                           def_id: ast::DefId,
                           _generics: &ty::Generics<'tcx>,
                           substs: &Substs<'tcx>)
    {
        let predicates =
            self.tcx().lookup_predicates(def_id).instantiate(self.tcx(), substs);
        let predicates = match self.fully_normalize(&predicates) {
            Ok(predicates) => predicates,
            Err(ErrorReported) => { return; }
        };

        for predicate in predicates.predicates.as_slice() {
            match *predicate {
                ty::Predicate::Trait(ref data) => {
                    self.accumulate_from_assoc_types_transitive(data);
                }
                ty::Predicate::Equate(..) => { }
                ty::Predicate::Projection(..) => { }
                ty::Predicate::RegionOutlives(ref data) => {
                    match self.tcx().no_late_bound_regions(data) {
                        None => { }
                        Some(ty::OutlivesPredicate(r_a, r_b)) => {
                            self.push_sub_region_constraint(Some(ty), r_b, r_a);
                        }
                    }
                }
                ty::Predicate::TypeOutlives(ref data) => {
                    match self.tcx().no_late_bound_regions(data) {
                        None => { }
                        Some(ty::OutlivesPredicate(ty_a, r_b)) => {
                            self.stack.push((r_b, Some(ty)));
                            self.accumulate_from_ty(ty_a);
                            self.stack.pop().unwrap();
                        }
                    }
                }
            }
        }

        let obligations = predicates.predicates
                                    .into_iter()
                                    .map(|pred| Implication::Predicate(def_id, pred));
        self.out.extend(obligations);

        let variances = self.tcx().item_variances(def_id);

        for (&region, &variance) in substs.regions().iter().zip(&variances.regions) {
            match variance {
                ty::Contravariant | ty::Invariant => {
                    // If any data with this lifetime is reachable
                    // within, it must be at least contravariant.
                    self.push_region_constraint_from_top(region)
                }
                ty::Covariant | ty::Bivariant => { }
            }
        }

        for (&ty, &variance) in substs.types.iter().zip(&variances.types) {
            match variance {
                ty::Covariant | ty::Invariant => {
                    // If any data of this type is reachable within,
                    // it must be at least covariant.
                    self.accumulate_from_ty(ty);
                }
                ty::Contravariant | ty::Bivariant => { }
            }
        }
    }

    /// Given that there is a requirement that `Foo<X> : 'a`, where
    /// `Foo` is declared like `struct Foo<T> where T : SomeTrait`,
    /// this code finds all the associated types defined in
    /// `SomeTrait` (and supertraits) and adds a requirement that `<X
    /// as SomeTrait>::N : 'a` (where `N` is some associated type
    /// defined in `SomeTrait`). This rule only applies to
    /// trait-bounds that are not higher-ranked, because we cannot
    /// project out of a HRTB. This rule helps code using associated
    /// types to compile, see Issue #22246 for an example.
    fn accumulate_from_assoc_types_transitive(&mut self,
                                              data: &ty::PolyTraitPredicate<'tcx>)
    {
        debug!("accumulate_from_assoc_types_transitive({:?})",
               data);

        for poly_trait_ref in traits::supertraits(self.tcx(), data.to_poly_trait_ref()) {
            match self.tcx().no_late_bound_regions(&poly_trait_ref) {
                Some(trait_ref) => { self.accumulate_from_assoc_types(trait_ref); }
                None => { }
            }
        }
    }

    fn accumulate_from_assoc_types(&mut self,
                                   trait_ref: ty::TraitRef<'tcx>)
    {
        debug!("accumulate_from_assoc_types({:?})",
               trait_ref);

        let trait_def_id = trait_ref.def_id;
        let trait_def = self.tcx().lookup_trait_def(trait_def_id);
        let assoc_type_projections: Vec<_> =
            trait_def.associated_type_names
                     .iter()
                     .map(|&name| self.tcx().mk_projection(trait_ref.clone(), name))
                     .collect();
        debug!("accumulate_from_assoc_types: assoc_type_projections={:?}",
               assoc_type_projections);
        let tys = match self.fully_normalize(&assoc_type_projections) {
            Ok(tys) => { tys }
            Err(ErrorReported) => { return; }
        };
        for ty in tys {
            self.accumulate_from_ty(ty);
        }
    }

    fn accumulate_from_object_ty(&mut self,
                                 ty: Ty<'tcx>,
                                 region_bound: ty::Region,
                                 required_region_bounds: Vec<ty::Region>)
    {
        // Imagine a type like this:
        //
        //     trait Foo { }
        //     trait Bar<'c> : 'c { }
        //
        //     &'b (Foo+'c+Bar<'d>)
        //         ^
        //
        // In this case, the following relationships must hold:
        //
        //     'b <= 'c
        //     'd <= 'c
        //
        // The first conditions is due to the normal region pointer
        // rules, which say that a reference cannot outlive its
        // referent.
        //
        // The final condition may be a bit surprising. In particular,
        // you may expect that it would have been `'c <= 'd`, since
        // usually lifetimes of outer things are conservative
        // approximations for inner things. However, it works somewhat
        // differently with trait objects: here the idea is that if the
        // user specifies a region bound (`'c`, in this case) it is the
        // "master bound" that *implies* that bounds from other traits are
        // all met. (Remember that *all bounds* in a type like
        // `Foo+Bar+Zed` must be met, not just one, hence if we write
        // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
        // 'y.)
        //
        // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
        // am looking forward to the future here.

        // The content of this object type must outlive
        // `bounds.region_bound`:
        let r_c = region_bound;
        self.push_region_constraint_from_top(r_c);

        // And then, in turn, to be well-formed, the
        // `region_bound` that user specified must imply the
        // region bounds required from all of the trait types:
        for &r_d in &required_region_bounds {
            // Each of these is an instance of the `'c <= 'b`
            // constraint above
            self.out.push(Implication::RegionSubRegion(Some(ty), r_d, r_c));
        }
    }

    fn fully_normalize<T>(&self, value: &T) -> Result<T,ErrorReported>
        where T : TypeFoldable<'tcx> + ty::HasTypeFlags
    {
        let value =
            traits::fully_normalize(self.infcx,
                                    traits::ObligationCause::misc(self.span, self.body_id),
                                    value);
        match value {
            Ok(value) => Ok(value),
            Err(errors) => {
                // I don't like reporting these errors here, but I
                // don't know where else to report them just now. And
                // I don't really expect errors to arise here
                // frequently. I guess the best option would be to
                // propagate them out.
                traits::report_fulfillment_errors(self.infcx, &errors);
                Err(ErrorReported)
            }
        }
    }
}

/// Given an object type like `SomeTrait+Send`, computes the lifetime
/// bounds that must hold on the elided self type. These are derived
/// from the declarations of `SomeTrait`, `Send`, and friends -- if
/// they declare `trait SomeTrait : 'static`, for example, then
/// `'static` would appear in the list. The hard work is done by
/// `ty::required_region_bounds`, see that for more information.
pub fn object_region_bounds<'tcx>(
    tcx: &ty::ctxt<'tcx>,
    principal: &ty::PolyTraitRef<'tcx>,
    others: ty::BuiltinBounds)
    -> Vec<ty::Region>
{
    // Since we don't actually *know* the self type for an object,
    // this "open(err)" serves as a kind of dummy standin -- basically
    // a skolemized type.
    let open_ty = tcx.mk_infer(ty::FreshTy(0));

    // Note that we preserve the overall binding levels here.
    assert!(!open_ty.has_escaping_regions());
    let substs = tcx.mk_substs(principal.0.substs.with_self_ty(open_ty));
    let trait_refs = vec!(ty::Binder(ty::TraitRef::new(principal.0.def_id, substs)));

    let mut predicates = others.to_predicates(tcx, open_ty);
    predicates.extend(trait_refs.iter().map(|t| t.to_predicate()));

    tcx.required_region_bounds(open_ty, predicates)
}