Add cargo features to build only UEFI or BIOS bootloader

[phil-opp]

Some projects only need one build standard. So we should allow to build only the UEFI or the BIOS bootloader to make it possible to reduce compile times. To achieve this, we could add new uefi and bios cargo features, which are both enabled by default

Proposed by @AviiNL on gitter.

[AlexJMohr]

Anyone working on this? I'd like to make an attempt with some guidance.

Obviously Cargo.toml needs something like this:

[features]
default = ["bios", "uefi"]
bios = []
uefi = []

And I was thinking I'd add add this to src/lib.rs

#[cfg(feature = "bios")]
mod bios; // src/bios/mod.rs
#[cfg(feature = "bios")]
pub use bios::BiosBoot;
#[cfg(feature = "uefi")]
mod uefi; // src/uefi/mod.rs
#[cfg(feature = "uefi")]
pub use uefi::UefiBoot;

Move gpt.rs to uefi/gpt.rs, move mbr.rs to bios/mbr.rs

[phil-opp]

Thanks, your approach looks good! The second step is then to update the build.rs, where we implement a hacky workaround for building the dependencies (until artifact dependencies are stable in cargo). I think it should be enough to add a #[cfg(feature = "uefi")] { ... } block around the build_uefi_bootloader call and a #[cfg(feature = "bios")] { ... } block around the build_bios_* calls in the main function. I'm happy to help you with the details, just ping me if you have any questions.

[jasoncouture]

See jasoncouture#1 ( #313 ) and let me know what you think @AlexJMohr