Description
What it does
Detect assignments to Copy values that are immediately discarded.
rustc is pretty good at detecting when variables are assigned but never read, and Clippy has a temporary_assignments lint to catch assignments to temporaries, but neither of them catch the following case:
struct Point { x: u64, y: u64 }
fn new_point() -> Point {
Point {
x: 10,
y: 30,
}
}
fn main() {
new_point().x = 11;
}Whether or not new_point has side-effects should be irrelevant: assigning to its .x field and then immediately throwing it away is definitely pointless (no pun intended 😏). Things are more complicated if Point has a drop implementation, hence the lint should probably be restricted to types that implement Copy.
Categories
- Kind: correctness
Drawbacks
Potentially difficult to implement
Example
This example is more similar to the one I encountered in the real world: a Copy type with a mutable getter and a copying getter, where the copying getter is called by mistake:
#[derive(Debug, Clone, Copy)]
struct Point { x: u64, y: u64 }
#[derive(Debug)]
struct Container {
point: Point,
}
impl Container {
fn get_point(&self) -> Point {
self.point
}
#[allow(unused)]
fn get_point_mut(&mut self) -> &mut Point {
&mut self.point
}
}
fn main() {
let container = Container {
point: Point { x: 10, y: 30 },
};
container.get_point().x = 0; // oops! I meant `get_point_mut`!
println!("{:?}", container);
}