Clippy lint · Restriction · checked with Clippy 0.1.99

clippy::default_union_representation: this union has the default representation

What it meansDisplays a warning when a union is declared with the default representation (without a #[repr(C)] attribute). It is in the restriction group and off by default.

What you see

warning: this union has the default representation
 --> main.rs:1:1
  |
1 | / union Foo {
2 | |     a: i32,
3 | |     b: u32,
4 | | }
  | |_^
  |
  = help: consider annotating `Foo` with `#[repr(C)]` to explicitly specify memory layout
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#default_union_representation
  = note: requested on the command line with `-W clippy::default-union-representation`

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?

Unions in Rust have unspecified layout by default, despite many people thinking that they lay out each field at the start of the union (like C does). That is, there are no guarantees about the offset of the fields for unions with multiple non-ZST fields without an explicitly specified layout. These cases may lead to undefined behavior in unsafe blocks.

Lints that forbid language features for specific codebases (no unwrap, no indexing...). Off by default; pick them one by one, never the whole group.

How to fix it

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

Flagged

union Foo {
    a: i32,
    b: u32,
}

fn main() {
    let _x: u32 = unsafe {
        Foo { a: 0_i32 }.b // Undefined behavior: `b` is allowed to be padding
    };
}

Write this instead

#[repr(C)]
union Foo {
    a: i32,
    b: u32,
}

fn main() {
    let _x: u32 = unsafe {
        Foo { a: 0_i32 }.b // Now defined behavior, this is just an i32 -> u32 transmute
    };
}

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]
default_union_representation = "warn"

In code

#![warn(clippy::default_union_representation)]

One run

cargo clippy -- -W clippy::default-union-representation

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.

Restrictiongroup

allowdefault level

nofixed by cargo clippy --fix

1.60.0added in Rust

Clippy's own documentation

What it does

Displays a warning when a union is declared with the default representation (without a #[repr(C)] attribute).

Why restrict this?

Example

union Foo {
    a: i32,
    b: u32,
}

fn main() {
    let _x: u32 = unsafe {
        Foo { a: 0_i32 }.b // Undefined behavior: `b` is allowed to be padding
    };
}

Use instead:

#[repr(C)]
union Foo {
    a: i32,
    b: u32,
}

fn main() {
    let _x: u32 = unsafe {
        Foo { a: 0_i32 }.b // Now defined behavior, this is just an i32 -> u32 transmute
    };
}

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

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