test: fix doctests and run in Makefile target

Our custom bare-metal allocator (`SvsmAllocator`) does not work during
tests, as they run as regular userspace binaries. Thus, we must not
set it as the global Rust allocator. To do this, we need a
compile-time directive that lets us know we are building doctests and
not the regular binary (`cfg(doctests)`), but sadly it is not
properly set during test compilation(see rust-lang/rust#45599,
rust-lang/rust#67295 and coconut-svsm#93).

In order to bypass this, set the compile time flag ourselves via the
RUSTFLAGS environment variable so that tests work. Also add the new
invocation to the `test` target in the Makefile.

Fixes: coconut-svsm#93
Fixes: 8cdea8e ("Cargo manifest: disable doctests")
Signed-off-by: Carlos López <carlos.lopez@suse.com>

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 $@ $<

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) {