Fix doctests

[00xc]

Doctests are a Rust feature where code tests can be inlined into the documentation itself. That way, documentation examples are also ran as integration tests. For example:

/// Sums two integers.
///
/// ```rust
/// let a = 1;
/// let b = 2;
/// assert_eq!(my_sum(a, b), 3);
/// ```
fn my_sum(a: u32, b: u32) -> u32 {
    ...
}

These tests can be ran via cargo test --target=x86_64-unknown-linux-gnu --doc, which at the moment we do NOT do via our Makefile test target. Using --tests instead of --doc will also work.

Currently, we have some doctests for ImmutAfterInitCell and ImmutAfterInitRef, which are currently failing. Moreover, more doctests will be added while implementing the tasks for #74. Thus, we must:

• Fix ImmutAfterInit* doctests.
• Run doctests via make test

Check the doctest documentation here:
https://doc.rust-lang.org/rustdoc/write-documentation/documentation-tests.html

[00xc]

So I see @nicstange already noticed this when pushing 8cdea8e and reached the same conclusion as I have: doctests are broken because cfg(doctest) is not set properly during compilation. This means that we cannot disable our custom global allocator during doctests.

For now I'm able to bypass this by doing:

diff --git a/Makefile b/Makefile
index 127e6b6..0cefcc2 100644
--- a/Makefile
+++ b/Makefile
@@ -18,6 +18,7 @@ all: svsm.bin
 
 test:
        cargo test --target=x86_64-unknown-linux-gnu
+       RUSTFLAGS="--cfg doctest" cargo test --target=x86_64-unknown-linux-gnu --doc
 
 utils/gen_meta: utils/gen_meta.c
        cc -O3 -Wall -o $@ $<
diff --git a/src/mm/alloc.rs b/src/mm/alloc.rs
index 6b3cf63..6e0d4ae 100644
--- a/src/mm/alloc.rs
+++ b/src/mm/alloc.rs
@@ -1282,7 +1282,7 @@ unsafe impl GlobalAlloc for SvsmAllocator {
     }
 }
 
-#[cfg_attr(not(test), global_allocator)]
+#[cfg_attr(not(any(test, doctest)), global_allocator)]
 pub static mut ALLOCATOR: SvsmAllocator = SvsmAllocator::new();
 
 pub fn root_mem_init(pstart: PhysAddr, vstart: VirtAddr, page_count: usize) {

I'm tempted to simply push this as I can think of no other alternative.