Fix build error on latest nightly

[toku-sa-n]

error[E0658]: function pointers cannot appear in constant functions
   --> /home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/x86_64-0.12.1/src/structures/idt.rs:378:31
    |
378 |                 divide_error: Entry::missing(),
    |                               ^^^^^^^^^^^^^^^^
    |
    = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
    = help: add `#![feature(const_fn_fn_ptr_basics)]` to the crate attributes to enable

bisect: https://gist.github.com/50b4d25cbcd7f712a2253316ca76f1d9

I wonder why we have to add this crate attribute...

[toku-sa-n]

Maybe the same situation as #182?

[josephlr]

Maybe the same situation as #182?

Essentially yes. However, this isn't a major breaking change, so my commit (ce4afba) can make sure we override the crate correctly for indirect deps.

[josephlr]

I wonder why we have to add this crate attribute...

The const_fn feature (is finally) being split up into multiple sub-features rust-lang/rust#76618, so as they get added, we will have to update our attributes.

[josephlr]

Maybe the same situation as #182?

Essentially yes. However, this isn't a major breaking change, so my commit (ce4afba) can make sure we override the crate correctly for indirect deps.

Crap, this didn't work, because Cargo.toml overrides don't work for bootimage runner which builds bootloader without the overrides.

Looks like this has to be force pulled (and we will have to push a new release).

[phil-opp]

The const_fn feature (is finally) being split up into multiple sub-features rust-lang/rust#76618, so as they get added, we will have to update our attributes.

It seems like there is only one remaining const_fn feature that we're using that requires the general feature(const_fn): Trait bounds other than Sized (error message is "trait bounds other than Sized on const fn parameters are unstable"). I'm not aware of any proposed sub-feature for this though.

[josephlr]

I'm not aware of any proposed sub-feature for this though.

One is probably going to be split out soonish. The interesting thing is that we only need the Trait bounds, but we don't need to actually call const fn trait methods. It looks like we could even remove some of our generic use-cases right now.

[josephlr]

I'm not aware of any proposed sub-feature for this though.

const_trait_bound_opt_out (see rust-lang/rust#67794) might end up being what we need, as we don't (currently) relay on any const fn trait methods.

[phil-opp]

Interesting, thanks for the info!