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 Iterators.

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;
}