Rollup merge of rust-lang#123976 - ChrisDenton:no-libc-in-std-doc-tests, r=Mark-Simulacrum

Use fake libc in core test

The war on libc continues.

Some platforms may not need to link to the libc crate (and it's possible some may not even have a libc), therefore we shouldn't require it for tests. This creates dummy `malloc` and `free` implementations for use in the pointer docs, but, keeps the public documentation looking the same as before.

Author: jieyouxu

core/src/primitive_docs.rs

@@ -537,7 +537,11 @@ impl () {}
 /// ## 4. Get it from C.
 ///
 /// ```
-/// # #![feature(rustc_private)]
+/// # mod libc {
+/// # pub unsafe fn malloc(_size: usize) -> *mut core::ffi::c_void { core::ptr::NonNull::dangling().as_ptr() }
+/// # pub unsafe fn free(_ptr: *mut core::ffi::c_void) {}
+/// # }
+/// # #[cfg(any())]
 /// #[allow(unused_extern_crates)]
 /// extern crate libc;
 ///
@@ -548,7 +552,7 @@ impl () {}
 ///     if my_num.is_null() {
 ///         panic!("failed to allocate memory");
 ///     }
-///     libc::free(my_num as *mut libc::c_void);
+///     libc::free(my_num as *mut core::ffi::c_void);
 /// }
 /// ```
 ///