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Rollup merge of rust-lang#106788 - estebank:elaborate_pred_E0599, r=c…
…ompiler-errors Tweak E0599 and elaborate_predicates CC rust-lang#86377.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,88 @@ | ||
| // These are simplifications of the tower traits by the same name: | ||
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| pub trait Service<Request> { | ||
| type Response; | ||
| } | ||
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| pub trait Layer<C> { | ||
| type Service; | ||
| } | ||
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| // Any type will do here: | ||
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| pub struct Req; | ||
| pub struct Res; | ||
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| // This is encoding a trait alias. | ||
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| pub trait ParticularService: | ||
| Service<Req, Response = Res> { | ||
| } | ||
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| impl<T> ParticularService for T | ||
| where | ||
| T: Service<Req, Response = Res>, | ||
| { | ||
| } | ||
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| // This is also a trait alias. | ||
| // The weird = <Self as ...> bound is there so that users of the trait do not | ||
| // need to repeat the bounds. See https://github.com/rust-lang/rust/issues/20671 | ||
| // for context, and in particular the workaround in: | ||
| // https://github.com/rust-lang/rust/issues/20671#issuecomment-529752828 | ||
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| pub trait ParticularServiceLayer<C>: | ||
| Layer<C, Service = <Self as ParticularServiceLayer<C>>::Service> | ||
| { | ||
| type Service: ParticularService; | ||
| } | ||
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| impl<T, C> ParticularServiceLayer<C> for T | ||
| where | ||
| T: Layer<C>, | ||
| T::Service: ParticularService, | ||
| { | ||
| type Service = T::Service; | ||
| } | ||
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| // These are types that implement the traits that the trait aliases refer to. | ||
| // They should also implement the alias traits due to the blanket impls. | ||
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| struct ALayer<C>(C); | ||
| impl<C> Layer<C> for ALayer<C> { | ||
| type Service = AService; | ||
| } | ||
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| struct AService; | ||
| impl Service<Req> for AService { | ||
| // However, AService does _not_ meet the blanket implementation, | ||
| // since its Response type is bool, not Res as it should be. | ||
| type Response = bool; | ||
| } | ||
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| // This is a wrapper type around ALayer that uses the trait alias | ||
| // as a way to communicate the requirements of the provided types. | ||
| struct Client<C>(C); | ||
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| // The method and the free-standing function below both have the same bounds. | ||
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| impl<C> Client<C> | ||
| where | ||
| ALayer<C>: ParticularServiceLayer<C>, | ||
| { | ||
| fn check(&self) {} | ||
| } | ||
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| fn check<C>(_: C) where ALayer<C>: ParticularServiceLayer<C> {} | ||
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| // But, they give very different error messages. | ||
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| fn main() { | ||
| // This gives a very poor error message that does nothing to point the user | ||
| // at the underlying cause of why the types involved do not meet the bounds. | ||
| Client(()).check(); //~ ERROR E0599 | ||
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| // This gives a good(ish) error message that points the user at _why_ the | ||
| // bound isn't met, and thus how they might fix it. | ||
| check(()); //~ ERROR E0271 | ||
| } |
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