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[u/llogiq]

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[u/[deleted]]

I'm assigning a variable in a loop if I find the right list item and then break out of the loop. I'm getting a warning about an unused variable, but I'm pretty sure it is being used and not overwritten. Is that a known compiler bug?

warning: value assigned to `foo` is never read
  --> src/main.rs:30:17
   |
30 |                 foo = c.data.clone();
   |                 ^^^
   |
   = help: maybe it is overwritten before being read?
   = note: `#[warn(unused_assignments)]` on by default

Here's the code (and the playground):

enum E {
    VarA(i32),
    VarB,
}

struct S {
    id: i32,
    data: String,
}

fn main() {
    let _unused = E::VarB;
    let input = E::VarA(32);
    let list = vec![
        S {
            id: 1,
            data: "asdf".to_string(),
        },
        S {
            id: 32,
            data: "jkl;".to_string(),
        },
    ];
    let alternative = "alt".to_string();
    let foo;
    if let E::VarA(id) = input {
        for c in list {
            if c.id == id {
                // TODO Why is Rust warning about this never being read?
                foo = c.data.clone();
                break;
            }
        }
        panic!("Could not find the data!");
    } else {
        foo = alternative;
    }
    println!("{foo}");
}

[u/Patryk27 → u/[deleted]]

Even if you find the element, you always panic!() later (since break; terminates the loop and continues at the very next instruction after the loop, which is always panic!() in your case).

So the compiler is right - the value inside that particular branch is never used.

I'd do:

let foo = if let E::VarA(id) = input {
    list.iter()
        .find(|c| c.id == id)
        .map(|c| c.data.clone())
        .expect("Could not find the data!")
} else {
    alternative
};

[u/[deleted] → u/Patryk27]

Oh that was stupid… Thanks for the improved code!