Let Option derive #[must_use]

[dhardy]

Let Option and Box derive #[must_use] from their generic parameter T.

Rendered

[burdges]

Is there some reason #[must_use] should not be a marker trait?

[Lokathor]

I would say that actually must_use is more a property of a particular function, not really of types, and sticking it on types is a shorthanded way to put it on a lot of functions "for free" cognitively. I'd say it's best if we, as an ecosystem, stick to having must_use on functions/methods rather than types.

[dlight]

I would say that actually must_use is more a property of a particular function, not really of types, and sticking it on types is a shorthanded way to put it on a lot of functions "for free" cognitively. I'd say it's best if we, as an ecosystem, stick to having must_use on functions/methods rather than types.

So #[must_use] could be a marker trait on the function's type where it is applied (each function has a unique type) and on each type it is applied. Then you add a blanked impl on functions whenever their output type impls the trait.

(it could be two traits, one for types you apply #[must_use] and another for functions, so that returning the function doesn't trigger it)

[dhardy]

I would say that actually must_use is more a property of a particular function, not really of types

Yes, but #[must_use] on types already has stable support (since 1.0 I think).

So #[must_use] could be a marker trait

Perhaps, but that would be quite a significant revision. Definitely out-of-scope for this RFC.