feat(hal-x86_64): make alloc a feature flag (#372)

Currently, some code in `hal-x86_64` depends on `liballoc`, while other
code does not. This makes it difficult to use parts of the HAL crate in
early boot environments without an allocator.

This commit adds an "alloc" feature flag to `hal-x86_64`, which enables
code that depends on `liballoc`.

This commit was factored out of #371.

Author: hawkw

hal-x86_64/Cargo.toml

@@ -6,7 +6,9 @@ edition = "2018"
 license = "MIT"
 
 [features]
+default = ["alloc"]
 log = ["tracing/log"]
+alloc = []
 
 [dependencies]
 acpi = "4.1.1"

hal-x86_64/src/cpu.rs

@@ -3,6 +3,7 @@ use mycelium_util::bits;
 
 pub mod entropy;
 pub mod intrinsics;
+#[cfg(feature = "alloc")]
 pub mod local;
 mod msr;
 pub use self::msr::Msr;

hal-x86_64/src/lib.rs

@@ -3,18 +3,17 @@
 // Allow const operands in asm.
 #![feature(asm_const)]
 #![feature(abi_x86_interrupt)]
-#![feature(doc_cfg)]
+#![feature(doc_cfg, doc_auto_cfg)]
+#![feature(extern_types)]
 // A bunch of const fn features.
 #![feature(const_mut_refs)]
 // Oftentimes it's necessary to write to a value at a particular location in
 // memory, and these types don't implement Copy to ensure they aren't
 // inadvertantly copied.
 #![allow(clippy::trivially_copy_pass_by_ref)]
-// Macros generated by `tracing` often generate large amounts of code, which
-// causes this lint to complain about relatively simple methods.
-#![allow(clippy::cognitive_complexity)]
 
 pub(crate) use hal_core::{PAddr, VAddr};
+#[cfg(feature = "alloc")]
 extern crate alloc;
 
 pub mod control_regs;