rust-lang/rust-clippy

False positives selecting system allocator

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#2373 opened on Jan 18, 2018

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C-bugI-false-positiveT-macrosgood first issue

Description

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.

Contributor guide

Tech stack
rust
Domain
tooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Rust programming basicsUnderstanding of global allocator attributesFamiliarity with clippy lint registration
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50
Research direction
The issue reports false positive warnings for `zero ptr` and `unnecessary mut passed` when using the system allocator. To fix, locate the source of these lints in clippy, likely in files like `clippy lints/src/zero ptr.rs` and `clippy lints/src/unnecessary mut passed.rs`. Understand why they trigger on `alloc system::System` static initialization, and add an exemption or suppress the lint for legal uses of `0 as *mut ` in this context. Check if there are existing tests for these lints and add a test case for the allocator pattern. The PR should also verify that no other false positives arise from the fix.