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;
}