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Suggest -> impl Trait and -> Box<dyn Trait> on fn that doesn't return
During development, a function could have a return type set that is a bare trait object by accident. We already suggest using either a boxed trait object or `impl Trait` if the return paths will allow it. We now do so too when there are *no* return paths or they all resolve to `!`. We still don't handle cases where the trait object is *not* the entirety of the return type gracefully.
This commit is contained in:
Esteban Küber
parent
8ce3f840ae
commit
d3c96f03b5
@ -826,12 +826,12 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
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.iter()
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.filter_map(|expr| tables.node_type_opt(expr.hir_id))
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.map(|ty| self.resolve_vars_if_possible(&ty));
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let (last_ty, all_returns_have_same_type) = ret_types.clone().fold(
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(None, true),
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|(last_ty, mut same): (std::option::Option<Ty<'_>>, bool), ty| {
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let (last_ty, all_returns_have_same_type, count) = ret_types.clone().fold(
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(None, true, 0),
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|(last_ty, mut same, count): (std::option::Option<Ty<'_>>, bool, usize), ty| {
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let ty = self.resolve_vars_if_possible(&ty);
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same &= last_ty.map_or(true, |last_ty| last_ty == ty) && ty.kind != ty::Error;
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(Some(ty), same)
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(Some(ty), same, count + 1)
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},
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);
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let all_returns_conform_to_trait =
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@ -846,7 +846,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
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let obl = Obligation::new(cause.clone(), param_env, pred);
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self.predicate_may_hold(&obl)
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})
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})
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}) || count == 0
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}
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_ => false,
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}
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@ -855,21 +855,19 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
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};
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let sm = self.tcx.sess.source_map();
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let (snippet, last_ty) =
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if let (true, hir::TyKind::TraitObject(..), Ok(snippet), true, Some(last_ty)) = (
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// Verify that we're dealing with a return `dyn Trait`
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ret_ty.span.overlaps(span),
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&ret_ty.kind,
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sm.span_to_snippet(ret_ty.span),
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// If any of the return types does not conform to the trait, then we can't
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// suggest `impl Trait` nor trait objects, it is a type mismatch error.
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all_returns_conform_to_trait,
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last_ty,
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) {
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(snippet, last_ty)
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} else {
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return false;
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};
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let snippet = if let (true, hir::TyKind::TraitObject(..), Ok(snippet), true) = (
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// Verify that we're dealing with a return `dyn Trait`
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ret_ty.span.overlaps(span),
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&ret_ty.kind,
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sm.span_to_snippet(ret_ty.span),
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// If any of the return types does not conform to the trait, then we can't
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// suggest `impl Trait` nor trait objects, it is a type mismatch error.
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all_returns_conform_to_trait,
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) {
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snippet
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} else {
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return false;
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};
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err.code(error_code!(E0746));
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err.set_primary_message("return type cannot have an unboxed trait object");
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err.children.clear();
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@ -881,13 +879,50 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
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#using-trait-objects-that-allow-for-values-of-different-types>";
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let has_dyn = snippet.split_whitespace().next().map_or(false, |s| s == "dyn");
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let trait_obj = if has_dyn { &snippet[4..] } else { &snippet[..] };
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if all_returns_have_same_type {
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if count == 0 {
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// No return paths. Suggest `-> T`, `-> impl Trait`, and if `Trait` is object safe,
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// `-> Box<dyn Trait>`.
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err.note(
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"currently nothing is being returned, depending on the final implementation \
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you could change the return type in different ways",
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);
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err.span_suggestion(
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ret_ty.span,
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"you could use some type `T` that is `T: Sized` as the return type if all return \
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paths will have the same type",
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"T".to_string(),
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Applicability::MaybeIncorrect,
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);
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err.span_suggestion(
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ret_ty.span,
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&format!(
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"you could use `impl {}` as the return type if all return paths will have the \
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same type but you want to expose only the trait in the signature",
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trait_obj,
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),
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format!("impl {}", trait_obj),
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Applicability::MaybeIncorrect,
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);
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err.note(impl_trait_msg);
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if is_object_safe {
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err.span_suggestion(
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ret_ty.span,
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&format!(
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"you could use a boxed trait object if all return paths `impl` trait `{}`",
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trait_obj,
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),
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format!("Box<dyn {}>", trait_obj),
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Applicability::MaybeIncorrect,
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);
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err.note(trait_obj_msg);
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}
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} else if let (Some(last_ty), true) = (last_ty, all_returns_have_same_type) {
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// Suggest `-> impl Trait`.
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err.span_suggestion(
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ret_ty.span,
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&format!(
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"return `impl {1}` instead, as all return paths are of type `{}`, \
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which implements `{1}`",
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"use `impl {1}` as the return type, as all return paths are of type `{}`, \
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which implements `{1}`",
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last_ty, trait_obj,
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),
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format!("impl {}", trait_obj),
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@ -925,8 +960,8 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
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}
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err.note(trait_obj_msg);
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err.note(&format!(
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"if all the returned values were of the same type you could use \
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`impl {}` as the return type",
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"if all the returned values were of the same type you could use `impl {}` as the \
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return type",
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trait_obj,
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));
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err.note(impl_trait_msg);
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@ -5,7 +5,7 @@ LL | fn foo() -> dyn Trait { Struct }
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| ^^^^^^^^^ doesn't have a size known at compile-time
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|
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= note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
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help: return `impl Trait` instead, as all return paths are of type `Struct`, which implements `Trait`
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help: use `impl Trait` as the return type, as all return paths are of type `Struct`, which implements `Trait`
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LL | fn foo() -> impl Trait { Struct }
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| ^^^^^^^^^^
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@ -17,7 +17,7 @@ LL | fn bar() -> dyn Trait {
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| ^^^^^^^^^ doesn't have a size known at compile-time
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= note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
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help: return `impl Trait` instead, as all return paths are of type `{integer}`, which implements `Trait`
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help: use `impl Trait` as the return type, as all return paths are of type `{integer}`, which implements `Trait`
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|
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LL | fn bar() -> impl Trait {
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| ^^^^^^^^^^
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@ -14,7 +14,7 @@ fn bap() -> Trait { Struct }
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//~^ ERROR E0746
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fn ban() -> dyn Trait { Struct }
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//~^ ERROR E0746
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fn bak() -> dyn Trait { unimplemented!() } //~ ERROR E0277
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fn bak() -> dyn Trait { unimplemented!() } //~ ERROR E0746
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// Suggest using `Box<dyn Trait>`
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fn bal() -> dyn Trait { //~ ERROR E0746
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if true {
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@ -49,7 +49,7 @@ LL | fn bap() -> Trait { Struct }
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| ^^^^^ doesn't have a size known at compile-time
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= note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
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help: return `impl Trait` instead, as all return paths are of type `Struct`, which implements `Trait`
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help: use `impl Trait` as the return type, as all return paths are of type `Struct`, which implements `Trait`
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LL | fn bap() -> impl Trait { Struct }
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| ^^^^^^^^^^
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@ -61,20 +61,32 @@ LL | fn ban() -> dyn Trait { Struct }
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| ^^^^^^^^^ doesn't have a size known at compile-time
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= note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
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help: return `impl Trait` instead, as all return paths are of type `Struct`, which implements `Trait`
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help: use `impl Trait` as the return type, as all return paths are of type `Struct`, which implements `Trait`
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LL | fn ban() -> impl Trait { Struct }
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| ^^^^^^^^^^
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error[E0277]: the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
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error[E0746]: return type cannot have an unboxed trait object
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:17:13
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LL | fn bak() -> dyn Trait { unimplemented!() }
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| ^^^^^^^^^ doesn't have a size known at compile-time
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= help: the trait `std::marker::Sized` is not implemented for `(dyn Trait + 'static)`
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= note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
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= note: the return type of a function must have a statically known size
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= note: currently nothing is being returned, depending on the final implementation you could change the return type in different ways
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= note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
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= note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
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help: you could use some type `T` that is `T: Sized` as the return type if all return paths will have the same type
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LL | fn bak() -> T { unimplemented!() }
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| ^
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help: you could use `impl Trait` as the return type if all return paths will have the same type but you want to expose only the trait in the signature
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LL | fn bak() -> impl Trait { unimplemented!() }
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| ^^^^^^^^^^
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help: you could use a boxed trait object if all return paths `impl` trait `Trait`
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LL | fn bak() -> Box<dyn Trait> { unimplemented!() }
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| ^^^^^^^^^^^^^^
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error[E0746]: return type cannot have an unboxed trait object
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:19:13
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@ -249,7 +261,7 @@ LL | fn bat() -> dyn Trait {
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| ^^^^^^^^^ doesn't have a size known at compile-time
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= note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
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help: return `impl Trait` instead, as all return paths are of type `{integer}`, which implements `Trait`
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help: use `impl Trait` as the return type, as all return paths are of type `{integer}`, which implements `Trait`
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LL | fn bat() -> impl Trait {
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| ^^^^^^^^^^
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@ -261,7 +273,7 @@ LL | fn bay() -> dyn Trait {
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| ^^^^^^^^^ doesn't have a size known at compile-time
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= note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
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help: return `impl Trait` instead, as all return paths are of type `{integer}`, which implements `Trait`
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help: use `impl Trait` as the return type, as all return paths are of type `{integer}`, which implements `Trait`
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LL | fn bay() -> impl Trait {
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| ^^^^^^^^^^
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@ -2,7 +2,7 @@ pub trait AbstractRenderer {}
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fn _create_render(_: &()) ->
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dyn AbstractRenderer
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//~^ ERROR the size for values of type
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//~^ ERROR return type cannot have an unboxed trait object
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{
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match 0 {
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_ => unimplemented!()
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@ -1,13 +1,25 @@
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error[E0277]: the size for values of type `(dyn AbstractRenderer + 'static)` cannot be known at compilation time
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error[E0746]: return type cannot have an unboxed trait object
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--> $DIR/issue-18107.rs:4:5
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LL | dyn AbstractRenderer
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| ^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
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= help: the trait `std::marker::Sized` is not implemented for `(dyn AbstractRenderer + 'static)`
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= note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
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= note: the return type of a function must have a statically known size
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= note: currently nothing is being returned, depending on the final implementation you could change the return type in different ways
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= note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
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= note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
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help: you could use some type `T` that is `T: Sized` as the return type if all return paths will have the same type
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LL | T
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help: you could use `impl AbstractRenderer` as the return type if all return paths will have the same type but you want to expose only the trait in the signature
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LL | impl AbstractRenderer
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help: you could use a boxed trait object if all return paths `impl` trait `AbstractRenderer`
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LL | Box<dyn AbstractRenderer>
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error: aborting due to previous error
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For more information about this error, try `rustc --explain E0277`.
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For more information about this error, try `rustc --explain E0746`.
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