rust/src/librustc/traits/coherence.rs

//! See rustc guide chapters on [trait-resolution] and [trait-specialization] for more info on how
//! this works.
//!
//! [trait-resolution]: https://rust-lang.github.io/rustc-guide/traits/resolution.html
//! [trait-specialization]: https://rust-lang.github.io/rustc-guide/traits/specialization.html

use hir::def_id::{DefId, LOCAL_CRATE};
use syntax_pos::DUMMY_SP;
use traits::{self, Normalized, SelectionContext, Obligation, ObligationCause};
use traits::IntercrateMode;
use traits::select::IntercrateAmbiguityCause;
use ty::{self, Ty, TyCtxt};
use ty::relate::TraitObjectMode;
use ty::fold::TypeFoldable;
use ty::subst::Subst;

use infer::{InferOk};

/// Whether we do the orphan check relative to this crate or
/// to some remote crate.
#[derive(Copy, Clone, Debug)]
enum InCrate {
    Local,
    Remote
}

#[derive(Debug, Copy, Clone)]
pub enum Conflict {
    Upstream,
    Downstream { used_to_be_broken: bool }
}

pub struct OverlapResult<'tcx> {
    pub impl_header: ty::ImplHeader<'tcx>,
    pub intercrate_ambiguity_causes: Vec<IntercrateAmbiguityCause>,
}

/// If there are types that satisfy both impls, invokes `on_overlap`
/// with a suitably-freshened `ImplHeader` with those types
/// substituted. Otherwise, invokes `no_overlap`.
pub fn overlapping_impls<'gcx, F1, F2, R>(
    tcx: TyCtxt<'_, 'gcx, 'gcx>,
    impl1_def_id: DefId,
    impl2_def_id: DefId,
    intercrate_mode: IntercrateMode,
    trait_object_mode: TraitObjectMode,
    on_overlap: F1,
    no_overlap: F2,
) -> R
where
    F1: FnOnce(OverlapResult<'_>) -> R,
    F2: FnOnce() -> R,
{
    debug!("overlapping_impls(\
           impl1_def_id={:?}, \
           impl2_def_id={:?},
           intercrate_mode={:?},
           trait_object_mode={:?})",
           impl1_def_id,
           impl2_def_id,
           intercrate_mode,
           trait_object_mode);

    let overlaps = tcx.infer_ctxt().with_trait_object_mode(trait_object_mode).enter(|infcx| {
        let selcx = &mut SelectionContext::intercrate(&infcx, intercrate_mode);
        overlap(selcx, impl1_def_id, impl2_def_id).is_some()
    });

    if !overlaps {
        return no_overlap();
    }

    // In the case where we detect an error, run the check again, but
    // this time tracking intercrate ambuiguity causes for better
    // diagnostics. (These take time and can lead to false errors.)
    tcx.infer_ctxt().with_trait_object_mode(trait_object_mode).enter(|infcx| {
        let selcx = &mut SelectionContext::intercrate(&infcx, intercrate_mode);
        selcx.enable_tracking_intercrate_ambiguity_causes();
        on_overlap(overlap(selcx, impl1_def_id, impl2_def_id).unwrap())
    })
}

fn with_fresh_ty_vars<'cx, 'gcx, 'tcx>(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
                                       param_env: ty::ParamEnv<'tcx>,
                                       impl_def_id: DefId)
                                       -> ty::ImplHeader<'tcx>
{
    let tcx = selcx.tcx();
    let impl_substs = selcx.infcx().fresh_substs_for_item(DUMMY_SP, impl_def_id);

    let header = ty::ImplHeader {
        impl_def_id,
        self_ty: tcx.type_of(impl_def_id).subst(tcx, impl_substs),
        trait_ref: tcx.impl_trait_ref(impl_def_id).subst(tcx, impl_substs),
        predicates: tcx.predicates_of(impl_def_id).instantiate(tcx, impl_substs).predicates,
    };

    let Normalized { value: mut header, obligations } =
        traits::normalize(selcx, param_env, ObligationCause::dummy(), &header);

    header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
    header
}

/// Can both impl `a` and impl `b` be satisfied by a common type (including
/// `where` clauses)? If so, returns an `ImplHeader` that unifies the two impls.
fn overlap<'cx, 'gcx, 'tcx>(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
                            a_def_id: DefId,
                            b_def_id: DefId)
                            -> Option<OverlapResult<'tcx>>
{
    debug!("overlap(a_def_id={:?}, b_def_id={:?})", a_def_id, b_def_id);

    // For the purposes of this check, we don't bring any placeholder
    // types into scope; instead, we replace the generic types with
    // fresh type variables, and hence we do our evaluations in an
    // empty environment.
    let param_env = ty::ParamEnv::empty();

    let a_impl_header = with_fresh_ty_vars(selcx, param_env, a_def_id);
    let b_impl_header = with_fresh_ty_vars(selcx, param_env, b_def_id);

    debug!("overlap: a_impl_header={:?}", a_impl_header);
    debug!("overlap: b_impl_header={:?}", b_impl_header);

    // Do `a` and `b` unify? If not, no overlap.
    let obligations = match selcx.infcx().at(&ObligationCause::dummy(), param_env)
                                         .eq_impl_headers(&a_impl_header, &b_impl_header)
    {
        Ok(InferOk { obligations, value: () }) => obligations,
        Err(_) => return None
    };

    debug!("overlap: unification check succeeded");

    // Are any of the obligations unsatisfiable? If so, no overlap.
    let infcx = selcx.infcx();
    let opt_failing_obligation =
        a_impl_header.predicates
                     .iter()
                     .chain(&b_impl_header.predicates)
                     .map(|p| infcx.resolve_type_vars_if_possible(p))
                     .map(|p| Obligation { cause: ObligationCause::dummy(),
                                           param_env,
                                           recursion_depth: 0,
                                           predicate: p })
                     .chain(obligations)
                     .find(|o| !selcx.predicate_may_hold_fatal(o));
    // FIXME: the call to `selcx.predicate_may_hold_fatal` above should be ported
    // to the canonical trait query form, `infcx.predicate_may_hold`, once
    // the new system supports intercrate mode (which coherence needs).

    if let Some(failing_obligation) = opt_failing_obligation {
        debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
        return None
    }

    let impl_header = selcx.infcx().resolve_type_vars_if_possible(&a_impl_header);
    let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
    debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
    Some(OverlapResult { impl_header, intercrate_ambiguity_causes })
}

pub fn trait_ref_is_knowable<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
                                             trait_ref: ty::TraitRef<'tcx>)
                                             -> Option<Conflict>
{
    debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
    if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
        // A downstream or cousin crate is allowed to implement some
        // substitution of this trait-ref.

        // A trait can be implementable for a trait ref by both the current
        // crate and crates downstream of it. Older versions of rustc
        // were not aware of this, causing incoherence (issue #43355).
        let used_to_be_broken =
            orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok();
        if used_to_be_broken {
            debug!("trait_ref_is_knowable({:?}) - USED TO BE BROKEN", trait_ref);
        }
        return Some(Conflict::Downstream { used_to_be_broken });
    }

    if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
        // This is a local or fundamental trait, so future-compatibility
        // is no concern. We know that downstream/cousin crates are not
        // allowed to implement a substitution of this trait ref, which
        // means impls could only come from dependencies of this crate,
        // which we already know about.
        return None;
    }

    // This is a remote non-fundamental trait, so if another crate
    // can be the "final owner" of a substitution of this trait-ref,
    // they are allowed to implement it future-compatibly.
    //
    // However, if we are a final owner, then nobody else can be,
    // and if we are an intermediate owner, then we don't care
    // about future-compatibility, which means that we're OK if
    // we are an owner.
    if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
        debug!("trait_ref_is_knowable: orphan check passed");
        return None;
    } else {
        debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
        return Some(Conflict::Upstream);
    }
}

pub fn trait_ref_is_local_or_fundamental<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
                                                         trait_ref: ty::TraitRef<'tcx>)
                                                         -> bool {
    trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, "fundamental")
}

pub enum OrphanCheckErr<'tcx> {
    NoLocalInputType,
    UncoveredTy(Ty<'tcx>),
}

/// Checks the coherence orphan rules. `impl_def_id` should be the
/// def-id of a trait impl. To pass, either the trait must be local, or else
/// two conditions must be satisfied:
///
/// 1. All type parameters in `Self` must be "covered" by some local type constructor.
/// 2. Some local type must appear in `Self`.
pub fn orphan_check<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
                                    impl_def_id: DefId)
                                    -> Result<(), OrphanCheckErr<'tcx>>
{
    debug!("orphan_check({:?})", impl_def_id);

    // We only except this routine to be invoked on implementations
    // of a trait, not inherent implementations.
    let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
    debug!("orphan_check: trait_ref={:?}", trait_ref);

    // If the *trait* is local to the crate, ok.
    if trait_ref.def_id.is_local() {
        debug!("trait {:?} is local to current crate",
               trait_ref.def_id);
        return Ok(());
    }

    orphan_check_trait_ref(tcx, trait_ref, InCrate::Local)
}

/// Check whether a trait-ref is potentially implementable by a crate.
///
/// The current rule is that a trait-ref orphan checks in a crate C:
///
/// 1. Order the parameters in the trait-ref in subst order - Self first,
///    others linearly (e.g., `<U as Foo<V, W>>` is U < V < W).
/// 2. Of these type parameters, there is at least one type parameter
///    in which, walking the type as a tree, you can reach a type local
///    to C where all types in-between are fundamental types. Call the
///    first such parameter the "local key parameter".
///     - e.g., `Box<LocalType>` is OK, because you can visit LocalType
///       going through `Box`, which is fundamental.
///     - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
///       the same reason.
///     - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
///       not local), `Vec<LocalType>` is bad, because `Vec<->` is between
///       the local type and the type parameter.
/// 3. Every type parameter before the local key parameter is fully known in C.
///     - e.g., `impl<T> T: Trait<LocalType>` is bad, because `T` might be
///       an unknown type.
///     - but `impl<T> LocalType: Trait<T>` is OK, because `LocalType`
///       occurs before `T`.
/// 4. Every type in the local key parameter not known in C, going
///    through the parameter's type tree, must appear only as a subtree of
///    a type local to C, with only fundamental types between the type
///    local to C and the local key parameter.
///     - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
///     is bad, because the only local type with `T` as a subtree is
///     `LocalType<T>`, and `Vec<->` is between it and the type parameter.
///     - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
///     the second occurrence of `T` is not a subtree of *any* local type.
///     - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
///     `LocalType<Vec<T>>`, which is local and has no types between it and
///     the type parameter.
///
/// The orphan rules actually serve several different purposes:
///
/// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where
///    every type local to one crate is unknown in the other) can't implement
///    the same trait-ref. This follows because it can be seen that no such
///    type can orphan-check in 2 such crates.
///
///    To check that a local impl follows the orphan rules, we check it in
///    InCrate::Local mode, using type parameters for the "generic" types.
///
/// 2. They ground negative reasoning for coherence. If a user wants to
///    write both a conditional blanket impl and a specific impl, we need to
///    make sure they do not overlap. For example, if we write
///    ```
///    impl<T> IntoIterator for Vec<T>
///    impl<T: Iterator> IntoIterator for T
///    ```
///    We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
///    We can observe that this holds in the current crate, but we need to make
///    sure this will also hold in all unknown crates (both "independent" crates,
///    which we need for link-safety, and also child crates, because we don't want
///    child crates to get error for impl conflicts in a *dependency*).
///
///    For that, we only allow negative reasoning if, for every assignment to the
///    inference variables, every unknown crate would get an orphan error if they
///    try to implement this trait-ref. To check for this, we use InCrate::Remote
///    mode. That is sound because we already know all the impls from known crates.
///
/// 3. For non-#[fundamental] traits, they guarantee that parent crates can
///    add "non-blanket" impls without breaking negative reasoning in dependent
///    crates. This is the "rebalancing coherence" (RFC 1023) restriction.
///
///    For that, we only a allow crate to perform negative reasoning on
///    non-local-non-#[fundamental] only if there's a local key parameter as per (2).
///
///    Because we never perform negative reasoning generically (coherence does
///    not involve type parameters), this can be interpreted as doing the full
///    orphan check (using InCrate::Local mode), substituting non-local known
///    types for all inference variables.
///
///    This allows for crates to future-compatibly add impls as long as they
///    can't apply to types with a key parameter in a child crate - applying
///    the rules, this basically means that every type parameter in the impl
///    must appear behind a non-fundamental type (because this is not a
///    type-system requirement, crate owners might also go for "semantic
///    future-compatibility" involving things such as sealed traits, but
///    the above requirement is sufficient, and is necessary in "open world"
///    cases).
///
/// Note that this function is never called for types that have both type
/// parameters and inference variables.
fn orphan_check_trait_ref<'tcx>(tcx: TyCtxt<'_, '_, '_>,
                                trait_ref: ty::TraitRef<'tcx>,
                                in_crate: InCrate)
                                -> Result<(), OrphanCheckErr<'tcx>>
{
    debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})",
           trait_ref, in_crate);

    if trait_ref.needs_infer() && trait_ref.needs_subst() {
        bug!("can't orphan check a trait ref with both params and inference variables {:?}",
             trait_ref);
    }

    // First, create an ordered iterator over all the type parameters to the trait, with the self
    // type appearing first.
    // Find the first input type that either references a type parameter OR
    // some local type.
    for input_ty in trait_ref.input_types() {
        if ty_is_local(tcx, input_ty, in_crate) {
            debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);

            // First local input type. Check that there are no
            // uncovered type parameters.
            let uncovered_tys = uncovered_tys(tcx, input_ty, in_crate);
            for uncovered_ty in uncovered_tys {
                if let Some(param) = uncovered_ty.walk()
                    .find(|t| is_possibly_remote_type(t, in_crate))
                {
                    debug!("orphan_check_trait_ref: uncovered type `{:?}`", param);
                    return Err(OrphanCheckErr::UncoveredTy(param));
                }
            }

            // OK, found local type, all prior types upheld invariant.
            return Ok(());
        }

        // Otherwise, enforce invariant that there are no type
        // parameters reachable.
        if let Some(param) = input_ty.walk()
            .find(|t| is_possibly_remote_type(t, in_crate))
        {
            debug!("orphan_check_trait_ref: uncovered type `{:?}`", param);
            return Err(OrphanCheckErr::UncoveredTy(param));
        }
    }

    // If we exit above loop, never found a local type.
    debug!("orphan_check_trait_ref: no local type");
    return Err(OrphanCheckErr::NoLocalInputType);
}

fn uncovered_tys<'tcx>(tcx: TyCtxt<'_, '_, '_>, ty: Ty<'tcx>, in_crate: InCrate)
                       -> Vec<Ty<'tcx>> {
    if ty_is_local_constructor(ty, in_crate) {
        vec![]
    } else if fundamental_ty(ty) {
        ty.walk_shallow()
          .flat_map(|t| uncovered_tys(tcx, t, in_crate))
          .collect()
    } else {
        vec![ty]
    }
}

fn is_possibly_remote_type(ty: Ty<'_>, _in_crate: InCrate) -> bool {
    match ty.sty {
        ty::Projection(..) | ty::Param(..) => true,
        _ => false,
    }
}

fn ty_is_local(tcx: TyCtxt<'_, '_, '_>, ty: Ty<'_>, in_crate: InCrate) -> bool {
    ty_is_local_constructor(ty, in_crate) ||
        fundamental_ty(ty) && ty.walk_shallow().any(|t| ty_is_local(tcx, t, in_crate))
}

fn fundamental_ty(ty: Ty<'_>) -> bool {
    match ty.sty {
        ty::Ref(..) => true,
        ty::Adt(def, _) => def.is_fundamental(),
        _ => false
    }
}

fn def_id_is_local(def_id: DefId, in_crate: InCrate) -> bool {
    match in_crate {
        // The type is local to *this* crate - it will not be
        // local in any other crate.
        InCrate::Remote => false,
        InCrate::Local => def_id.is_local()
    }
}

fn ty_is_local_constructor(ty: Ty<'_>, in_crate: InCrate) -> bool {
    debug!("ty_is_local_constructor({:?})", ty);

    match ty.sty {
        ty::Bool |
        ty::Char |
        ty::Int(..) |
        ty::Uint(..) |
        ty::Float(..) |
        ty::Str |
        ty::FnDef(..) |
        ty::FnPtr(_) |
        ty::Array(..) |
        ty::Slice(..) |
        ty::RawPtr(..) |
        ty::Ref(..) |
        ty::Never |
        ty::Tuple(..) |
        ty::Param(..) |
        ty::Projection(..) => {
            false
        }

        ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => match in_crate {
            InCrate::Local => false,
            // The inference variable might be unified with a local
            // type in that remote crate.
            InCrate::Remote => true,
        },

        ty::Adt(def, _) => def_id_is_local(def.did, in_crate),
        ty::Foreign(did) => def_id_is_local(did, in_crate),

        ty::Dynamic(ref tt, ..) => def_id_is_local(tt.principal().def_id(), in_crate),

        ty::Error => true,

        ty::UnnormalizedProjection(..) |
        ty::Closure(..) |
        ty::Generator(..) |
        ty::GeneratorWitness(..) |
        ty::Opaque(..) => {
            bug!("ty_is_local invoked on unexpected type: {:?}", ty)
        }
    }
}