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Check suitability of the provided default
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‎src/librustc_typeck/check/wfcheck.rs

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@@ -425,6 +425,78 @@ fn check_trait(tcx: TyCtxt<'_>, item: &hir::Item<'_>) {
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for_item(tcx, item).with_fcx(|fcx, _| {
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check_where_clauses(tcx, fcx, item.span, trait_def_id, None);
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// Type-check associated type defaults (if there are any):
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// Assuming the defaults are used, check that all predicates (bounds on
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// the assoc type and where clauses on the trait) hold.
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let substs = InternalSubsts::identity_for_item(tcx, trait_def_id);
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// For all assoc. types with defaults, build a map from
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// `<Self as Trait<...>>::Assoc` to the default type.
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let map = tcx.associated_items(trait_def_id)
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.filter_map(|item| {
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if item.kind == ty::AssocKind::Type && item.defaultness.has_value() {
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// `<Self as Trait<...>>::Assoc`
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let proj = ty::ProjectionTy {
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substs,
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item_def_id: item.def_id,
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};
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let default_ty = tcx.type_of(item.def_id);
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debug!("assoc. type default mapping: {} -> {}", proj, default_ty);
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Some((proj, default_ty))
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} else {
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None
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}
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})
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.collect::<FxHashMap<_, _>>();
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struct DefaultNormalizer<'tcx> {
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tcx: TyCtxt<'tcx>,
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map: FxHashMap<ty::ProjectionTy<'tcx>, Ty<'tcx>>,
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}
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impl<'tcx> ty::fold::TypeFolder<'tcx> for DefaultNormalizer<'tcx> {
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fn tcx<'a>(&'a self) -> TyCtxt<'tcx> {
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self.tcx
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}
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fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
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match t.sty {
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ty::Projection(proj_ty) => {
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if let Some(default) = self.map.get(&proj_ty) {
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default
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} else {
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t.super_fold_with(self)
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}
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}
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_ => t.super_fold_with(self),
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}
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}
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}
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// Now take all predicates defined on the trait, replace any mention of
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// the assoc. types with their default, and prove them.
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// We only consider predicates that directly mention the assoc. type.
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let mut norm = DefaultNormalizer { tcx, map };
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let predicates = fcx.tcx.predicates_of(trait_def_id);
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for &(orig_pred, span) in predicates.predicates.iter() {
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let pred = orig_pred.fold_with(&mut norm);
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if pred != orig_pred {
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// Mentions one of the defaulted assoc. types
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debug!("default suitability check: proving predicate: {} -> {}", orig_pred, pred);
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let pred = fcx.normalize_associated_types_in(span, &pred);
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let cause = traits::ObligationCause::new(
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span,
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fcx.body_id,
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traits::ItemObligation(trait_def_id),
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);
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let obligation = traits::Obligation::new(cause, fcx.param_env, pred);
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fcx.register_predicate(obligation);
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}
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}
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vec![]
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});
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}

‎src/test/ui/associated-types/defaults-suitability.rs

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@@ -0,0 +1,93 @@
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//! Checks that associated type defaults are properly validated.
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//!
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//! This means:
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//! * Default types are wfchecked
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//! * Default types are checked against where clauses on the assoc. type
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//! (eg. `type Assoc: Clone = NotClone`), and also against where clauses on
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//! the trait itself when possible
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// compile-fail
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#![feature(associated_type_defaults)]
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struct NotClone;
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// Assoc. type bounds must hold for the default type
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trait Tr {
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type Ty: Clone = NotClone;
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//~^ ERROR the trait bound `NotClone: std::clone::Clone` is not satisfied
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}
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// Where-clauses defined on the trait must also be considered
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trait Tr2 where Self::Ty: Clone {
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//~^ ERROR the trait bound `NotClone: std::clone::Clone` is not satisfied
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type Ty = NotClone;
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}
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// Independent of where-clauses (there are none here), default types must always be wf
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trait Tr3 {
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type Ty = Vec<[u8]>;
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//~^ ERROR the size for values of type `[u8]` cannot be known at compilation time
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}
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// Involved type parameters must fulfill all bounds required by defaults that mention them
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trait Foo<T> {
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type Bar: Clone = Vec<T>;
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//~^ ERROR the trait bound `T: std::clone::Clone` is not satisfied
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}
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trait Bar: Sized {
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// `(): Foo<Self>` might hold for some possible impls but not all.
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type Assoc: Foo<Self> = ();
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//~^ ERROR the trait bound `(): Foo<Self>` is not satisfied
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}
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trait IsU8<T> {}
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impl<T> IsU8<u8> for T {}
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// Test that mentioning the assoc. type inside where clauses works
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trait C where
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Vec<Self::Assoc>: Clone,
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Self::Assoc: IsU8<Self::Assoc>,
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bool: IsU8<Self::Assoc>,
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{
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type Assoc = u8;
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}
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// Test that we get all expected errors if that default is unsuitable
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trait D where
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Vec<Self::Assoc>: Clone,
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//~^ ERROR the trait bound `NotClone: std::clone::Clone` is not satisfied
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Self::Assoc: IsU8<Self::Assoc>,
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//~^ ERROR the trait bound `NotClone: IsU8<NotClone>` is not satisfied
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bool: IsU8<Self::Assoc>,
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//~^ ERROR the trait bound `bool: IsU8<NotClone>` is not satisfied
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{
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type Assoc = NotClone;
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}
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trait Foo2<T> where
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<Self as Foo2<T>>::Bar: Clone,
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//~^ ERROR the trait bound `<Self as Foo2<T>>::Baz: std::clone::Clone` is not satisfied
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{
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type Bar = Vec<Self::Baz>;
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type Baz = T;
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}
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trait Foo3<T: Clone> where
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<Self as Foo3<T>>::Bar: Clone,
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//~^ ERROR the trait bound `<Self as Foo3<T>>::Baz: std::clone::Clone` is not satisfied
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{
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type Bar = Vec<Self::Baz>;
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type Baz = T;
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}
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trait Foo4<T> where
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<Self as Foo4<T>>::Bar: Clone,
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{
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type Bar = Vec<Self::Baz>;
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type Baz: Clone = T;
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//~^ ERROR the trait bound `T: std::clone::Clone` is not satisfied
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}
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fn main() {}

‎src/test/ui/associated-types/defaults-suitability.stderr

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error[E0277]: the trait bound `NotClone: std::clone::Clone` is not satisfied
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--> $DIR/defaults-suitability.rs:17:14
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|
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LL | type Ty: Clone = NotClone;
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| ^^^^^ the trait `std::clone::Clone` is not implemented for `NotClone`
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|
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note: required by `Tr`
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--> $DIR/defaults-suitability.rs:16:1
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|
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LL | trait Tr {
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| ^^^^^^^^
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error[E0277]: the trait bound `NotClone: std::clone::Clone` is not satisfied
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--> $DIR/defaults-suitability.rs:22:27
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|
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LL | trait Tr2 where Self::Ty: Clone {
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| ^^^^^ the trait `std::clone::Clone` is not implemented for `NotClone`
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|
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note: required by `Tr2`
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--> $DIR/defaults-suitability.rs:22:1
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|
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LL | trait Tr2 where Self::Ty: Clone {
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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error[E0277]: the trait bound `T: std::clone::Clone` is not satisfied
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--> $DIR/defaults-suitability.rs:35:15
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|
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LL | type Bar: Clone = Vec<T>;
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| ^^^^^ the trait `std::clone::Clone` is not implemented for `T`
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|
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= help: consider adding a `where T: std::clone::Clone` bound
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= note: required because of the requirements on the impl of `std::clone::Clone` for `std::vec::Vec<T>`
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note: required by `Foo`
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--> $DIR/defaults-suitability.rs:34:1
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|
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LL | trait Foo<T> {
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| ^^^^^^^^^^^^
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error[E0277]: the trait bound `(): Foo<Self>` is not satisfied
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--> $DIR/defaults-suitability.rs:41:17
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|
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LL | type Assoc: Foo<Self> = ();
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| ^^^^^^^^^ the trait `Foo<Self>` is not implemented for `()`
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|
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note: required by `Bar`
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--> $DIR/defaults-suitability.rs:39:1
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|
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LL | trait Bar: Sized {
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| ^^^^^^^^^^^^^^^^
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error[E0277]: the trait bound `NotClone: IsU8<NotClone>` is not satisfied
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--> $DIR/defaults-suitability.rs:61:18
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|
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LL | Self::Assoc: IsU8<Self::Assoc>,
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| ^^^^^^^^^^^^^^^^^ the trait `IsU8<NotClone>` is not implemented for `NotClone`
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|
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note: required by `D`
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--> $DIR/defaults-suitability.rs:58:1
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|
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LL | / trait D where
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LL | | Vec<Self::Assoc>: Clone,
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LL | |
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LL | | Self::Assoc: IsU8<Self::Assoc>,
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... |
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LL | | type Assoc = NotClone;
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LL | | }
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| |_^
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error[E0277]: the trait bound `bool: IsU8<NotClone>` is not satisfied
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--> $DIR/defaults-suitability.rs:63:11
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|
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LL | bool: IsU8<Self::Assoc>,
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| ^^^^^^^^^^^^^^^^^ the trait `IsU8<NotClone>` is not implemented for `bool`
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|
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note: required by `D`
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--> $DIR/defaults-suitability.rs:58:1
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|
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LL | / trait D where
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LL | | Vec<Self::Assoc>: Clone,
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LL | |
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LL | | Self::Assoc: IsU8<Self::Assoc>,
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... |
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LL | | type Assoc = NotClone;
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LL | | }
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| |_^
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error[E0277]: the trait bound `NotClone: std::clone::Clone` is not satisfied
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--> $DIR/defaults-suitability.rs:59:23
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|
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LL | Vec<Self::Assoc>: Clone,
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| ^^^^^ the trait `std::clone::Clone` is not implemented for `NotClone`
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|
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= note: required because of the requirements on the impl of `std::clone::Clone` for `std::vec::Vec<NotClone>`
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note: required by `D`
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--> $DIR/defaults-suitability.rs:58:1
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|
97+
LL | / trait D where
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LL | | Vec<Self::Assoc>: Clone,
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LL | |
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LL | | Self::Assoc: IsU8<Self::Assoc>,
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... |
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LL | | type Assoc = NotClone;
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LL | | }
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| |_^
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error[E0277]: the trait bound `<Self as Foo2<T>>::Baz: std::clone::Clone` is not satisfied
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--> $DIR/defaults-suitability.rs:70:29
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|
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LL | <Self as Foo2<T>>::Bar: Clone,
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| ^^^^^ the trait `std::clone::Clone` is not implemented for `<Self as Foo2<T>>::Baz`
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|
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= help: consider adding a `where <Self as Foo2<T>>::Baz: std::clone::Clone` bound
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= note: required because of the requirements on the impl of `std::clone::Clone` for `std::vec::Vec<<Self as Foo2<T>>::Baz>`
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note: required by `Foo2`
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--> $DIR/defaults-suitability.rs:69:1
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|
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LL | / trait Foo2<T> where
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LL | | <Self as Foo2<T>>::Bar: Clone,
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LL | |
120+
LL | | {
121+
LL | | type Bar = Vec<Self::Baz>;
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LL | | type Baz = T;
123+
LL | | }
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| |_^
125+
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error[E0277]: the trait bound `<Self as Foo3<T>>::Baz: std::clone::Clone` is not satisfied
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--> $DIR/defaults-suitability.rs:78:29
128+
|
129+
LL | <Self as Foo3<T>>::Bar: Clone,
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| ^^^^^ the trait `std::clone::Clone` is not implemented for `<Self as Foo3<T>>::Baz`
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|
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= help: consider adding a `where <Self as Foo3<T>>::Baz: std::clone::Clone` bound
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= note: required because of the requirements on the impl of `std::clone::Clone` for `std::vec::Vec<<Self as Foo3<T>>::Baz>`
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note: required by `Foo3`
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--> $DIR/defaults-suitability.rs:77:1
136+
|
137+
LL | / trait Foo3<T: Clone> where
138+
LL | | <Self as Foo3<T>>::Bar: Clone,
139+
LL | |
140+
LL | | {
141+
LL | | type Bar = Vec<Self::Baz>;
142+
LL | | type Baz = T;
143+
LL | | }
144+
| |_^
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146+
error[E0277]: the trait bound `T: std::clone::Clone` is not satisfied
147+
--> $DIR/defaults-suitability.rs:89:15
148+
|
149+
LL | type Baz: Clone = T;
150+
| ^^^^^ the trait `std::clone::Clone` is not implemented for `T`
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|
152+
= help: consider adding a `where T: std::clone::Clone` bound
153+
note: required by `Foo4`
154+
--> $DIR/defaults-suitability.rs:85:1
155+
|
156+
LL | / trait Foo4<T> where
157+
LL | | <Self as Foo4<T>>::Bar: Clone,
158+
LL | | {
159+
LL | | type Bar = Vec<Self::Baz>;
160+
LL | | type Baz: Clone = T;
161+
LL | |
162+
LL | | }
163+
| |_^
164+
165+
error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
166+
--> $DIR/defaults-suitability.rs:29:5
167+
|
168+
LL | type Ty = Vec<[u8]>;
169+
| ^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
170+
|
171+
= help: the trait `std::marker::Sized` is not implemented for `[u8]`
172+
= note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
173+
= note: required by `std::vec::Vec`
174+
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error: aborting due to 11 previous errors
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For more information about this error, try `rustc --explain E0277`.

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