Explicitly specify type parameter on FromResidual for Option and ControlFlow.

[zachs18]

Remove type parameter default R = <Self as Try>::Residual from FromResidual Specify default type parameter on FromResidual impls in the stdlib to work around #99940 / #87350 as mentioned in #84277 (comment).

This does not completely fix the issue, but works around it for Option and ControlFlow specifically (Result does not have the issue since it already did not use the default parameter of FromResidual).

(Does this need an ACP or similar?) This probably needs at least an FCP since it changes the API described in the RFC. Not sure if T-lang, T-libs-api, T-libs, or some combination (The tracking issue is tagged T-lang, T-libs-api). This probably doesn't need T-lang input, since it is not changing the API of FromResidual from the RFC? Maybe needs T-libs-api FCP?

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[joboet]

r? libs-api

[WaffleLapkin]

From the issue it seems that the default here should work and @lcnr is working on a compiler fix. Do we really need this temporary fix?

[lcnr]

We expect to fix the underlying issue here in the medium term, so afaict the impact of this change is to prevent some weird errors when implementing FromResidual without specifying the defaulted param? Because removing the default itself doesn't change anything, it just forces people to manually specify the argument for the param (in which case they likely use a manually normalized version)

[zachs18]

We expect to fix the underlying issue here in the medium term

Okay, if that is expected to happen before try_trait_v2 stabilizes, then this change is probably unnecessary, yeah.

the impact of this change is to prevent some weird errors when implementing FromResidual without specifying the defaulted param

Kind of, but the errors are nonlocal to the impl using defaulted param, as shown in #99940. The errors occur when implementing FromResidual<Local> for Upstream at all for a type where the upstream crate implemented FromResidual for Upstream (e.g. impl<T> FromResidual</* local */ Flip<T>> for ::core::option::Option<T>). Since the errors only occur downstream crates, it's not easy to work around by manually specifying the parameter in the upstream crate.

Because removing the default itself doesn't change anything, it just forces people to manually specify the argument for the param (in which case they likely use a manually normalized version)

It doesn't change anything within a crate, but across crates, it makes FromResidual<Local> for Upstream possible.

---

Alternately, this PR could be changed to leave the R = <Self as Try>::Residual, but still manually specify the type on all impls in the stdlib, which should resolve specifically not being able to implement FromResidual<Local> for Option/ControlFlow, without changing the API of FromResidual itself. Actually that seems like a much better idea, I'm going to change this PR to do that.

[zachs18]

@rustbot ready

This PR now does not modify FromResidual itself, instead only explicitly specifying the generic for the Option and ControlFlow's FromResidual impls in core, to work around #99940 and allow downstream impl FromResidual<Local> for Option<_>. Also adds a regression test library/core/tests/ops/from_residual.rs to ensure Option, ControlFlow, and Result can have downstream impl FromResidual<Local> impls.

This does not change the API except with respect to the coherence issue. Once the compiler fix happens, the explicit generics should be able to be removed without changing the API.

[scottmcm]

Updating just the impl seems entirely fine to me, especially since any third-party impls are necessarily unstable.

@bors r+

[bors]

📌 Commit 1b6df71 has been approved by scottmcm

It is now in the queue for this repository.