Remove the From derive macro from prelude

[Kobzol]

The new #[derive(From)] functionality (implemented in #144922) caused name resolution ambiguity issues (#145524). The reproducer looks e.g. like this:

mod foo { pub use derive_more::From; } use foo::*; #[derive(From)] // ERROR: `From` is ambiguous struct S(u32);

It's pretty unfortunate that it works like this, but I guess that there's not much to be done here, and we'll have to wait for the next edition to put the From macro into the prelude. That will probably require #139493 to land.

I created a new module in core (and re-exported it in std) called from, where I re-exported the From macro. I think that since this is a new module, it should not have the same backwards incompatibility issue.

Happy to hear suggestions about the naming - maybe it would make sense as core::macros::from::From? But we already had a precedent in the core::assert_matches module, so I just followed suit.

Fixes: #145524

r? @petrochenkov

[rustbot]

Changes to the code generated for builtin derived traits.

cc @nnethercote

[Voultapher]

If #139493 lands, this could just be core::From and in a future version From.

[petrochenkov]

@bors r+

[bors]

📌 Commit a6a760e has been approved by petrochenkov

It is now in the queue for this repository.

[RalfJung]

Wouldn't convert be the most sensible location, right next to the corresponding trait?

[Kobzol]

It can't be std::convert::From, because then existing code that already imported this path would suddenly start triggering an unstable derive_from feature.

[Zalathar]

Bors hasn't noticed that this was merged.

@bors r- retry