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89 lines
2.2 KiB
Rust
89 lines
2.2 KiB
Rust
// These are simplifications of the tower traits by the same name:
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pub trait Service<Request> {
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type Response;
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}
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pub trait Layer<C> {
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type Service;
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}
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// Any type will do here:
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pub struct Req;
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pub struct Res;
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// This is encoding a trait alias.
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pub trait ParticularService:
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Service<Req, Response = Res> {
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}
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impl<T> ParticularService for T
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where
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T: Service<Req, Response = Res>,
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{
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}
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// This is also a trait alias.
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// The weird = <Self as ...> bound is there so that users of the trait do not
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// need to repeat the bounds. See https://github.com/rust-lang/rust/issues/20671
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// for context, and in particular the workaround in:
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// https://github.com/rust-lang/rust/issues/20671#issuecomment-529752828
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pub trait ParticularServiceLayer<C>:
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Layer<C, Service = <Self as ParticularServiceLayer<C>>::Service>
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{
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type Service: ParticularService;
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}
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impl<T, C> ParticularServiceLayer<C> for T
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where
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T: Layer<C>,
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T::Service: ParticularService,
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{
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type Service = T::Service;
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}
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// These are types that implement the traits that the trait aliases refer to.
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// They should also implement the alias traits due to the blanket impls.
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struct ALayer<C>(C);
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impl<C> Layer<C> for ALayer<C> {
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type Service = AService;
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}
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struct AService;
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impl Service<Req> for AService {
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// However, AService does _not_ meet the blanket implementation,
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// since its Response type is bool, not Res as it should be.
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type Response = bool;
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}
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// This is a wrapper type around ALayer that uses the trait alias
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// as a way to communicate the requirements of the provided types.
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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>
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where
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ALayer<C>: ParticularServiceLayer<C>,
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{
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fn check(&self) {}
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}
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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() {
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// This gives a very poor error message that does nothing to point the user
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// at the underlying cause of why the types involved do not meet the bounds.
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Client(()).check(); //~ ERROR E0599
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// This gives a good(ish) error message that points the user at _why_ the
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// bound isn't met, and thus how they might fix it.
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check(()); //~ ERROR E0271
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}
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