#[derive(FromPrimitive)] should use the num crate

[lilyball]

The compiler still supports #[derive(FromPrimitive)] but it uses std::num::FromPrimitive which doesn't exist anymore. The compiler should remove its built-in support for #[derive_FromPrimitive] and the num crate should implement it instead.

Alternatively, to avoid the need for #![feature(custom_derive)], the compiler's built-in support could be adjusted to use num::traits::FromPrimitive instead, with the expectation that clients will adopt the external num crate, but this approach is inflexible (the num crate can't change FromPrimitive without breaking compatibility).

[nwin]

This is really an issue. At the moment there is no convenient way of converting an integer value to an enum. enum_primitive also doesn’t work because of #24189.

[bombless]

I think people can invoke a macro to define an enum, in this way they can impl From easily.
edit: typo

[nwin]

I don’t want to write a new macro for every enum, this is as convenient as just implementing From manually.

[durka]

#[derive(FromPrimitive)] was removed in #31400 so this can be closed.