Make From<UnexpectedUniFFICallbackError> optional #1790
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mhammond
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Oct 12, 2023
| // Autoref-based specialization for converting UnexpectedUniFFICallbackError into error types. | ||
| // | ||
| // For more details, see: | ||
| // https://github.com/dtolnay/case-studies/blob/master/autoref-specialization/README.md |
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far out, that's crazy :) Thanks for the link!
| @@ -216,14 +216,33 @@ pub(crate) fn tagged_impl_header( | |||
| } | |||
| } | |||
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| pub(crate) fn derive_ffi_traits(ty: &Ident, udl_mode: bool) -> TokenStream { | |||
| pub(crate) fn derive_all_ffi_traits(ty: &Ident, udl_mode: bool) -> TokenStream { | |||
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These names kinda bother me a little - derive_all_ffi_traits seems to be deriving for a single ident where as derive_ffi_traits takes an array of identifiers. It would be great if you could maybe come up with alternatives to make this a little clearer?
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They both take a single type identifier, but derive_ffi_traits also takes a list of trait names to derive. What about moving that param to the end, so it's more clear what's the same and what's different?
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oh right, doh! Swapping those 2 args sounds great, thanks!
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This is in preparation of mozilla#1578 -- allowing foreign languages to implement traits. One issue there is that if we want to allow foreign languages to implement traits, then any Result return types need to implement `LiftReturn`, and that impl currently has a bound on `E: From<UnexpectedUniFFICallbackError`. IOW, it's only implemented for errors that can be converted from UnexpectedUniFFICallbackError. This makes sense when the trait is actually used by the foreign languages, however not all traits are intended for that use. For that matter, some libraries may be fine panicking in the the face of unexpected callback errors. By using the auto-ref specialization hack, we're able to convert the error if there's a `From<UnexpectedUniFFICallbackError>` impl, but use a panicking version if not. The trait bound is no longer required to implement `LiftReturn`.
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This is in preparation of #1578 -- allowing foreign languages to implement traits.
One issue there is that if we want to allow foreign languages to implement traits, then any Result return types need to implement
LiftReturn, and that impl currently has a bound onE: From<UnexpectedUniFFICallbackError. IOW, it's only implemented for errors that can be converted from UnexpectedUniFFICallbackError.This makes sense when the trait is actually used by the foreign languages, however some traits are only intended to be implemented by Rust code. For that matter, some libraries may be fine panicking in the the face of unexpected callback errors.
By using the auto-ref specialization hack, we're able to convert the error if there's a
From<UnexpectedUniFFICallbackError>impl, but use a panicking version if not. The trait bound is no longer required to implementLiftReturn.