Summary
Writing to a union is considered safe. However, when a structure contains a field that is a union, multiple_unsafe_ops_per_block considers writing to that union to be unsafe.
Lint Name
multiple_unsafe_ops_per_block
Reproducer
I tried this code:
#![warn(clippy::multiple_unsafe_ops_per_block)]
union Example {
f1: u32,
f2: bool,
}
struct HoldsExample {
e: Example,
}
fn set_union_value(target: &mut HoldsExample, value: *const u32) {
unsafe {
target.e.f1 = *value;
};
}
fn main() {
let mut example = HoldsExample { e: Example { f2: true } };
set_union_value(&mut example, &5);
println!("Value: {}", unsafe { example.e.f1 } );
}I saw this happen:
Checking playground v0.0.1 (/playground)
warning: this `unsafe` block contains 2 unsafe operations, expected only one
--> src/main.rs:12:5
|
12 | / unsafe {
13 | | target.e.f1 = *value;
14 | | };
| |_____^
|
note: raw pointer dereference occurs here
--> src/main.rs:13:23
|
13 | target.e.f1 = *value;
| ^^^^^^
note: union field access occurs here
--> src/main.rs:13:9
|
13 | target.e.f1 = *value;
| ^^^^^^^^^^^
= help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.97.0/index.html#multiple_unsafe_ops_per_block
note: the lint level is defined here
--> src/main.rs:1:9
|
1 | #![warn(clippy::multiple_unsafe_ops_per_block)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
warning: `playground` (bin "playground") generated 1 warning
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.62s
I expected to see this happen:
Lint not triggered, the note: union field access occurs here note is for something that is entirely safe, can be verified by switching set_union_value() to be:
fn set_union_value(target: &mut HoldsExample, value: *const u32) {
let dereferenced = unsafe { *value };
target.e.f1 = dereferenced;
}Version
Clippy 0.1.97 (2026-07-14 8bab26f4f6)
Playground 1.97.1
Additional Labels
No response
Summary
Writing to a union is considered safe. However, when a structure contains a field that is a union,
multiple_unsafe_ops_per_blockconsiders writing to that union to be unsafe.Lint Name
multiple_unsafe_ops_per_block
Reproducer
I tried this code:
I saw this happen:
I expected to see this happen:
Lint not triggered, the
note: union field access occurs herenote is for something that is entirely safe, can be verified by switching set_union_value() to be:Version
Additional Labels
No response