src/librustc/traits/coherence.rs - third_party/rust - Git at Google

 //! 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 crate::infer::{CombinedSnapshot, InferOk};
 use crate::hir::def_id::{DefId, LOCAL_CRATE};
 use crate::traits::{self, Normalized, SelectionContext, Obligation, ObligationCause};
 use crate::traits::IntercrateMode;
 use crate::traits::select::IntercrateAmbiguityCause;
 use crate::ty::{self, Ty, TyCtxt};
 use crate::ty::fold::TypeFoldable;
 use crate::ty::subst::Subst;
 use syntax::symbol::sym;
 use syntax_pos::DUMMY_SP;

 /// 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>,

     /// `true` if the overlap might've been permitted before the shift
     /// to universes.
     pub involves_placeholder: bool,
 }

 pub fn add_placeholder_note(err: &mut errors::DiagnosticBuilder<'_>) {
     err.note(&format!(
         "this behavior recently changed as a result of a bug fix; \
          see rust-lang/rust#56105 for details"
     ));
 }

 /// 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<F1, F2, R>(
     tcx: TyCtxt<'_>,
     impl1_def_id: DefId,
     impl2_def_id: DefId,
     intercrate_mode: IntercrateMode,
     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={:?})",
            impl1_def_id,
            impl2_def_id,
            intercrate_mode);

     let overlaps = tcx.infer_ctxt().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().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, 'tcx>(
     selcx: &mut SelectionContext<'cx, '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, 'tcx>(
     selcx: &mut SelectionContext<'cx, '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);

     selcx.infcx().probe(|snapshot| overlap_within_probe(selcx, a_def_id, b_def_id, snapshot))
 }

 fn overlap_within_probe(
     selcx: &mut SelectionContext<'cx, 'tcx>,
     a_def_id: DefId,
     b_def_id: DefId,
     snapshot: &CombinedSnapshot<'_, 'tcx>,
 ) -> Option<OverlapResult<'tcx>> {
     // 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_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_vars_if_possible(&a_impl_header);
     let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
     debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);

     let involves_placeholder = match selcx.infcx().region_constraints_added_in_snapshot(snapshot) {
         Some(true) => true,
         _ => false,
     };

     Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder })
 }

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

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

 /// Checks the coherence orphan rules. `impl_def_id` should be the
 /// `DefId` 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(
     tcx: TyCtxt<'_>,
     impl_def_id: DefId,
 ) -> Result<(), OrphanCheckErr<'_>> {
     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)
 }

 /// Checks 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);
     }

     if tcx.features().re_rebalance_coherence {
         // Given impl<P1..=Pn> Trait<T1..=Tn> for T0, an impl is valid only
         // if at least one of the following is true:
         //
         // - Trait is a local trait
         // (already checked in orphan_check prior to calling this function)
         // - All of
         //     - At least one of the types T0..=Tn must be a local type.
         //      Let Ti be the first such type.
         //     - No uncovered type parameters P1..=Pn may appear in T0..Ti (excluding Ti)
         //
         for input_ty in trait_ref.input_types() {
             debug!("orphan_check_trait_ref: check ty `{:?}`", input_ty);
             if ty_is_local(tcx, input_ty, in_crate) {
                 debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
                 return Ok(());
             } else if let ty::Param(_) = input_ty.sty {
                 debug!("orphan_check_trait_ref: uncovered ty: `{:?}`", input_ty);
                 return Err(OrphanCheckErr::UncoveredTy(input_ty))
             }
         }
         // If we exit above loop, never found a local type.
         debug!("orphan_check_trait_ref: no local type");
         Err(OrphanCheckErr::NoLocalInputType)
     } else {
         // 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");
         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, ..) => {
             if let Some(principal) = tt.principal() {
                 def_id_is_local(principal.def_id(), in_crate)
             } else {
                 false
             }
         }

         ty::Error => true,

         ty::UnnormalizedProjection(..) |
         ty::Closure(..) |
         ty::Generator(..) |
         ty::GeneratorWitness(..) |
         ty::Opaque(..) => {
             bug!("ty_is_local invoked on unexpected type: {:?}", ty)
         }
     }
 }
	//! 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 crate::infer::{CombinedSnapshot, InferOk};
	use crate::hir::def_id::{DefId, LOCAL_CRATE};
	use crate::traits::{self, Normalized, SelectionContext, Obligation, ObligationCause};
	use crate::traits::IntercrateMode;
	use crate::traits::select::IntercrateAmbiguityCause;
	use crate::ty::{self, Ty, TyCtxt};
	use crate::ty::fold::TypeFoldable;
	use crate::ty::subst::Subst;
	use syntax::symbol::sym;
	use syntax_pos::DUMMY_SP;

	/// 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>,

	/// `true` if the overlap might've been permitted before the shift
	/// to universes.
	pub involves_placeholder: bool,
	}

	pub fn add_placeholder_note(err: &mut errors::DiagnosticBuilder<'_>) {
	err.note(&format!(
	"this behavior recently changed as a result of a bug fix; \
	see rust-lang/rust#56105 for details"
	));
	}

	/// 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<F1, F2, R>(
	tcx: TyCtxt<'_>,
	impl1_def_id: DefId,
	impl2_def_id: DefId,
	intercrate_mode: IntercrateMode,
	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={:?})",
	impl1_def_id,
	impl2_def_id,
	intercrate_mode);

	let overlaps = tcx.infer_ctxt().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().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, 'tcx>(
	selcx: &mut SelectionContext<'cx, '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, 'tcx>(
	selcx: &mut SelectionContext<'cx, '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);

	selcx.infcx().probe(\|snapshot\| overlap_within_probe(selcx, a_def_id, b_def_id, snapshot))
	}

	fn overlap_within_probe(
	selcx: &mut SelectionContext<'cx, 'tcx>,
	a_def_id: DefId,
	b_def_id: DefId,
	snapshot: &CombinedSnapshot<'_, 'tcx>,
	) -> Option<OverlapResult<'tcx>> {
	// 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_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_vars_if_possible(&a_impl_header);
	let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
	debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);

	let involves_placeholder = match selcx.infcx().region_constraints_added_in_snapshot(snapshot) {
	Some(true) => true,
	_ => false,
	};

	Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder })
	}

	pub fn trait_ref_is_knowable<'tcx>(
	tcx: TyCtxt<'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<'tcx>(
	tcx: TyCtxt<'tcx>,
	trait_ref: ty::TraitRef<'tcx>,
	) -> bool {
	trait_ref.def_id.krate == LOCAL_CRATE \|\| tcx.has_attr(trait_ref.def_id, sym::fundamental)
	}

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

	/// Checks the coherence orphan rules. `impl_def_id` should be the
	/// `DefId` 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(
	tcx: TyCtxt<'_>,
	impl_def_id: DefId,
	) -> Result<(), OrphanCheckErr<'_>> {
	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)
	}

	/// Checks 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);
	}

	if tcx.features().re_rebalance_coherence {
	// Given impl<P1..=Pn> Trait<T1..=Tn> for T0, an impl is valid only
	// if at least one of the following is true:
	//
	// - Trait is a local trait
	// (already checked in orphan_check prior to calling this function)
	// - All of
	// - At least one of the types T0..=Tn must be a local type.
	// Let Ti be the first such type.
	// - No uncovered type parameters P1..=Pn may appear in T0..Ti (excluding Ti)
	//
	for input_ty in trait_ref.input_types() {
	debug!("orphan_check_trait_ref: check ty `{:?}`", input_ty);
	if ty_is_local(tcx, input_ty, in_crate) {
	debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
	return Ok(());
	} else if let ty::Param(_) = input_ty.sty {
	debug!("orphan_check_trait_ref: uncovered ty: `{:?}`", input_ty);
	return Err(OrphanCheckErr::UncoveredTy(input_ty))
	}
	}
	// If we exit above loop, never found a local type.
	debug!("orphan_check_trait_ref: no local type");
	Err(OrphanCheckErr::NoLocalInputType)
	} else {
	// 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");
	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, ..) => {
	if let Some(principal) = tt.principal() {
	def_id_is_local(principal.def_id(), in_crate)
	} else {
	false
	}
	}

	ty::Error => true,

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