E0284 - Error codes index
Description: Book listing all Rust error codes

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# Error code E0284

This error occurs when the compiler is unable to unambiguously infer the return type of a function or method which is generic on return type, such as the `collect` method for `Iterator`s.

For example:

    fn main() {
        let n: u32 = 1;
        let mut d: u64 = 2;
        d = d + n.into();
    }

Here we have an addition of `d` and `n.into()`. Hence, `n.into()` can return any type `T` where `u64: Add<T>`. On the other hand, the `into` method can return any type where `u32: Into<T>`.

The author of this code probably wants `into()` to return a `u64`, but the compiler can’t be sure that there isn’t another type `T` where both `u32: Into<T>` and `u64: Add<T>`.

To resolve this error, use a concrete type for the intermediate expression:

    fn main() {
        let n: u32 = 1;
        let mut d: u64 = 2;
        let m: u64 = n.into();
        d = d + m;
    }