E0161 - Error codes index
Description: Book listing all Rust error codes

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# Error code E0161

A value was moved whose size was not known at compile time.

Erroneous code example:

    trait Bar {
        fn f(self);
    }

    impl Bar for i32 {
        fn f(self) {}
    }

    fn main() {
        let b: Box<dyn Bar> = Box::new(0i32);
        b.f();
        // error: cannot move a value of type dyn Bar: the size of dyn Bar cannot
        //        be statically determined
    }

In Rust, you can only move a value when its size is known at compile time.

To work around this restriction, consider “hiding” the value behind a reference: either `&x` or `&mut x`. Since a reference has a fixed size, this lets you move it around as usual. Example:

    trait Bar {
        fn f(&self);
    }

    impl Bar for i32 {
        fn f(&self) {}
    }

    fn main() {
        let b: Box<dyn Bar> = Box::new(0i32);
        b.f();
        // ok!
    }