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### What it does
Displays a warning when a union is declared with the default representation (without a `#[repr(C)]` attribute).
### Why is this bad?
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.
### 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
};
}
```
|
