[Rust] add support for optionally compiling libc functions (#763)

* add SNMALLOC_RUST_LIBC_API option

CMakeLists.txt

@@ -32,6 +32,7 @@ option(SNMALLOC_ENABLE_FUZZING "Enable fuzzing instrumentation tests" OFF)
 option(SNMALLOC_USE_SELF_VENDORED_STL "Avoid using system STL" OFF)
 # Options that apply only if we're not building the header-only library
 cmake_dependent_option(SNMALLOC_RUST_SUPPORT "Build static library for rust" OFF "NOT SNMALLOC_HEADER_ONLY_LIBRARY" OFF)
+cmake_dependent_option(SNMALLOC_RUST_LIBC_API "Include libc API in the rust library" OFF "SNMALLOC_RUST_SUPPORT" OFF)
 cmake_dependent_option(SNMALLOC_STATIC_LIBRARY "Build static libraries" ON "NOT SNMALLOC_HEADER_ONLY_LIBRARY" OFF)
 cmake_dependent_option(SNMALLOC_CHECK_LOADS "Perform bounds checks on the source argument to memcpy with heap objects" OFF "NOT SNMALLOC_HEADER_ONLY_LIBRARY" OFF)
 cmake_dependent_option(SNMALLOC_OPTIMISE_FOR_CURRENT_MACHINE "Compile for current machine architecture" Off "NOT SNMALLOC_HEADER_ONLY_LIBRARY" OFF)
@@ -585,6 +586,10 @@ endif()
     add_shim(snmallocshim-rust STATIC src/snmalloc/override/rust.cc)
     add_shim(snmallocshim-checks-rust STATIC src/snmalloc/override/rust.cc)
     target_compile_definitions(snmallocshim-checks-rust PRIVATE SNMALLOC_CHECK_CLIENT)
+    if (SNMALLOC_RUST_LIBC_API)
+      target_compile_definitions(snmallocshim-rust PRIVATE SNMALLOC_RUST_LIBC_API)
+      target_compile_definitions(snmallocshim-checks-rust PRIVATE SNMALLOC_RUST_LIBC_API)
+    endif()
   endif()
 
   if (SNMALLOC_BUILD_TESTING)

src/snmalloc/override/rust.cc

@@ -1,5 +1,11 @@
 #define SNMALLOC_NAME_MANGLE(a) sn_##a
-#include "snmalloc/snmalloc.h"
+
+// The libc API provided by malloc.cc will always be mangled per above.
+#ifdef SNMALLOC_RUST_LIBC_API
+#  include "malloc.cc"
+#else
+#  include "snmalloc/snmalloc.h"
+#endif
 
 #include <string.h>