False positives selecting system allocator

[birkenfeld]

The current most recent (I hope) unstable way to select the system allocator is:

#![feature(alloc_system, global_allocator, allocator_api)]
extern crate alloc_system;
#[global_allocator]
static A: alloc_system::System = alloc_system::System;

It produces the following warnings that are nonsensical in the given context:

warning: `0 as *mut _` detected. Consider using `ptr::null_mut()`
  --> src/main.rs:28:1
   |
28 | static A: alloc_system::System = alloc_system::System;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: #[warn(zero_ptr)] on by default
   = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.180/index.html#zero_ptr

warning: The function/method `alloc::heap::Alloc::usable_size` doesn't need a mutable reference
  --> src/main.rs:28:1
   |
28 | static A: alloc_system::System = alloc_system::System;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: #[warn(unnecessary_mut_passed)] on by default
   = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.180/index.html#unnecessary_mut_passed

If this select-the-allocator API is to be stabilized eventually, these should be fixed.

[oli-obk]

Probably just needs a macro check.

[phansch]

Triage: Doesn't seem to trigger with custom allocators anymore, but both lints still need an external macro check.

If someone wants to get started with contributing to Clippy, I'm happy to mentor. It would be similar to this PR.

[samueltardieu]

Reproducer is not usable anymore. Should we close the issue?