Have a link to your code?

> HashMap<T: Hash+Eq, u64>

I can take a wild guess that T is a generic type defined on either the method, struct or impl in which this HashMap is declared. It's on the method/struct/impl. You don't necessarily need the type bound on the struct, but you do on the impl declaration:

struct MyStruct<T: Hash+Eq> { map: HashMap<T, u64> }

impl<T: Hash+Eq> MyStruct<T> { pub fn insert_something(&mut self, my_key: T, my_int: u64) { self.map.insert(my_key, my_int); } }

try it here: https://is.gd/RwUpQx