Clippy lint · Nursery · checked with Clippy 0.1.99

clippy::volatile_composites: type `main::MyDevice` is not volatile-compatible

What it meansThis lint warns when volatile load/store operations (write_volatile/read_volatile) are applied to composite types. It is in the nursery group and off by default.

What you see

warning: type `main::MyDevice` is not volatile-compatible
  --> main.rs:10:9
   |
10 |         device.write_volatile(MyDevice { addr, count });
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#volatile_composites
   = note: `-W clippy::volatile-composites` implied by `-W clippy::nursery`

Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.

Does it matter?

Volatile operations are typically used with memory mapped IO devices, where the precise number and ordering of load and store instructions is important because they can have side effects. This is well defined for primitive types like u32, but less well defined for structures and other composite types. In practice it’s implementation defined, and the behavior can be rustc-version dependent. As a result, code should only apply write_volatile/read_volatile to primitive types to be fully well-defined.

New lints still being finished. Off by default and may have false positives.

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

struct MyDevice {
    addr: usize,
    count: usize
}

fn start_device(device: *mut MyDevice, addr: usize, count: usize) {
    unsafe {
        device.write_volatile(MyDevice { addr, count });
    }
}

Write this instead

struct MyDevice {
    addr: usize,
    count: usize
}

fn start_device(device: *mut MyDevice, addr: usize, count: usize) {
    unsafe {
        (&raw mut (*device).addr).write_volatile(addr);
        (&raw mut (*device).count).write_volatile(count);
    }
}

Turn it off, or on

It is off by default. To use it, set it to warn or deny:

Whole crate (Cargo.toml)

[lints.clippy]
volatile_composites = "warn"

In code

#![warn(clippy::volatile_composites)]

One run

cargo clippy -- -W clippy::volatile-composites

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.

Nurserygroup

allowdefault level

nofixed by cargo clippy --fix

1.92.0added in Rust

Clippy's own documentation

What it does

This lint warns when volatile load/store operations (write_volatile/read_volatile) are applied to composite types.

Why is this bad?

Volatile operations are typically used with memory mapped IO devices, where the precise number and ordering of load and store instructions is important because they can have side effects. This is well defined for primitive types like u32, but less well defined for structures and other composite types. In practice it’s implementation defined, and the behavior can be rustc-version dependent.

As a result, code should only apply write_volatile/read_volatile to primitive types to be fully well-defined.

Example

struct MyDevice {
    addr: usize,
    count: usize
}

fn start_device(device: *mut MyDevice, addr: usize, count: usize) {
    unsafe {
        device.write_volatile(MyDevice { addr, count });
    }
}

Instead, operate on each primtive field individually:

struct MyDevice {
    addr: usize,
    count: usize
}

fn start_device(device: *mut MyDevice, addr: usize, count: usize) {
    unsafe {
        (&raw mut (*device).addr).write_volatile(addr);
        (&raw mut (*device).count).write_volatile(count);
    }
}

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

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