Only include dependencies when binary feature is enabled

[phil-opp]

This change vastly improves compile times when only lib.rs is compiled because no dependencies need to be compiled in this case. See rust-lang/cargo#1982 (comment) for more information about the approach.

This is a breaking change since a build of the executable without the binary feature will result in an error.

Depends on rust-osdev/bootimage#43, so we will need to increase the minimally supported bootimage version. To avoid public breakage, we should probably wait a bit after releasing the new bootimage version to give people time to upgrade.

[phil-opp]

To try this change today:

• Install bootimage 0.7.6 or later.

• Update the bootloader dependency in your Cargo.toml:

  [dependencies]
  bootloader = { git = "https://github.com/rust-osdev/bootloader.git", branch="binary-feature", features = ["map_physical_memory"]}

Now building the bootloader library should no longer require compiling the bootloader dependencies:

>  cargo xbuild
   […sysroot build…]

   Compiling llvm-tools v0.1.1
   Compiling bootloader v0.6.4 (https://github.com/rust-osdev/bootloader.git?branch=binary-feature#aa2bbc14)
   Compiling blog_os v0.1.0 (/home/philipp/Documents/blog_os/code)
    Finished dev [unoptimized + debuginfo] target(s) in 1.36s

[64]

LGTM

[64]

Actually there is one more thing - do we need the build script when the binary feature is disabled? We are still pulling in llvm-tools twice

[phil-opp]

Good point! I disabled the build script and the llvm-tools dependency too now

[phil-opp]

I think it's fine to merge this now. We will need to release a new semver-breaking version anyway, so no breakage should occur unless you manually increase the bootloader version in your Cargo.toml.

[phil-opp]

Published as version 0.7.0