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chalk lowering rule: ProjectionEq-Normalize #52153

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Nov 14, 2018
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44 changes: 43 additions & 1 deletion src/librustc_traits/lowering/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -496,9 +496,51 @@ pub fn program_clauses_for_associated_type_def<'a, 'tcx>(
};
let from_env_clause = Clause::ForAll(ty::Binder::bind(from_env_clause));

// Rule ProjectionEq-Normalize
//
// ProjectionEq can succeed by normalizing:
// ```
// forall<Self, P1..Pn, Pn+1..Pm, U> {
// ProjectionEq(<Self as Trait<P1..Pn>>::AssocType<Pn+1..Pm> = U) :-
// Normalize(<Self as Trait<P1..Pn>>::AssocType<Pn+1..Pm> -> U)
// }
// ```

let offset = tcx.generics_of(trait_id).params
.iter()
.map(|p| p.index)
.max()
.unwrap_or(0);
// Add a new type param after the existing ones (`U` in the comment above).
let ty_var = ty::Bound(
ty::BoundTy::new(ty::INNERMOST, ty::BoundVar::from_u32(offset + 1))
);

// `ProjectionEq(<Self as Trait<P1..Pn>>::AssocType<Pn+1..Pm> = U)`
let projection = ty::ProjectionPredicate {
projection_ty,
ty: tcx.mk_ty(ty_var),
};

// `Normalize(<A0 as Trait<A1..An>>::AssocType<Pn+1..Pm> -> U)`
let hypothesis = tcx.mk_goal(
DomainGoal::Normalize(projection).into_goal()
);

// ProjectionEq(<Self as Trait<P1..Pn>>::AssocType<Pn+1..Pm> = U) :-
// Normalize(<Self as Trait<P1..Pn>>::AssocType<Pn+1..Pm> -> U)
let normalize_clause = ProgramClause {
goal: DomainGoal::Holds(WhereClause::ProjectionEq(projection)),
hypotheses: tcx.mk_goals(iter::once(hypothesis)),
category: ProgramClauseCategory::Other,
};
let normalize_clause = Clause::ForAll(ty::Binder::bind(normalize_clause));

let clauses = iter::once(projection_eq_clause)
.chain(iter::once(wf_clause))
.chain(iter::once(from_env_clause));
.chain(iter::once(from_env_clause))
.chain(iter::once(normalize_clause));

tcx.mk_clauses(clauses)
}

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1 change: 1 addition & 0 deletions src/test/ui/chalkify/lower_trait.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -15,6 +15,7 @@ error: program clause dump
LL | #[rustc_dump_program_clauses] //~ ERROR program clause dump
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: forall<Self, S, T, ^3> { ProjectionEq(<Self as Foo<S, T>>::Assoc == ^3) :- Normalize(<Self as Foo<S, T>>::Assoc -> ^3). }
= note: forall<Self, S, T> { FromEnv(Self: Foo<S, T>) :- FromEnv(Unnormalized(<Self as Foo<S, T>>::Assoc)). }
= note: forall<Self, S, T> { ProjectionEq(<Self as Foo<S, T>>::Assoc == Unnormalized(<Self as Foo<S, T>>::Assoc)). }
= note: forall<Self, S, T> { WellFormed(Unnormalized(<Self as Foo<S, T>>::Assoc)) :- Implemented(Self: Foo<S, T>). }
Expand Down