`bevy_input` is now optional, but there's no way to enable it

[rparrett]

Bevy version

main, since #17955

What you did

Attempted to migrate a crate dealing with input and using default-features = false to Bevy 0.16-dev

What went wrong

error[E0433]: failed to resolve: could not find `input` in `bevy`
  --> src/lib.rs:28:5
   |
28 |     input::keyboard::{Key, KeyboardInput},
   |     ^^^^^ could not find `input` in `bevy`

And there doesn't appear to be any way of rectifying this. It seems that we need a new bevy_input feature, but it's also possible that bevy_input becoming optional was a mistake. I haven't been following the no_std effort closely, so I'm not sure.

A bad workaround is to add the std feature, which includes bevy_input. (This may be a separate bug. Seems like with bevy_input optional, bevy_internal/std should be using bevy_input?/std).

[alice-i-cecile]

@bushrat011899 may know better, but my instincts say we're just missing a feature flag.

[bushrat011899]

Tl;Dr enable either std or libm and you'll get bevy_input.

The problem is bevy_math, bevy_inout, etc. need a floating point backend, so you must enable either libm or std. To allow no-default-features to still compile, the only option is to make those crates optional.

I'll give this another look though and see if there's a way to make this more intuitive!

[alice-i-cecile]

Cutting from the milestone: that's a good enough workaround for me to be comfortable shipping it. Rust please give us enum features 🥺

[rparrett]

Tl;Dr enable either std or libm and you'll get bevy_input.

Gotcha, thanks. So the play for maximum compatibility would be to add a matching std and libm feature to my crate? (and include std in my own default features?)

Side note: we may want to amend https://bevyengine.org/learn/quick-start/plugin-development/#small-crate-size

[bushrat011899]

Yep! In the release notes we'll call out a newly added example in examples/no_std/library (which you should check as well!) which shows how to make a maximally compatible Bevy library, and some suggestions for how to test it for no_std compatibility.

[alice-i-cecile]

Pre-RFC for a cargo feature in this space: https://internals.rust-lang.org/t/pre-rfc-mutually-excusive-global-features/19618