Trying template types

gistfile1.txt

use std::num::{Zero};
use std::vec::Vec;
use std::mem::size_of;

pub struct Image<T> {
    width: uint,
    height: uint,
    // ...
    buffer: Vec<T>,
}

impl<T: Zero, Clone> Image<T> {

    fn stride_for_type(w: uint, ch: uint) -> uint {
        let sz = size_of::<T>();
        //...
        3*w*sz
    }

    fn new(w: uint, h: uint) -> Image<T> {
        let s = Image::<T>::stride_for_type(w, 3);
        Image { 
            width: w,
            height: h,
            buffer: Vec::from_elem(s*h, Zero::zero()),
        }
    }
}

The error message:

$ rustc --crate-type lib templt1.rs 
templt1.rs:22:17: 22:44 error: the impl referenced by this path needs 2 type parameters, but 1 type parameter were supplied
templt1.rs:22         let s = Image::<T>::stride_for_type(w, 3);
                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~
templt1.rs:22:17: 22:44 error: not enough type parameters provided: expected 2, found 1
templt1.rs:22         let s = Image::<T>::stride_for_type(w, 3);
                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~
templt1.rs:22:17: 22:44 error: cannot determine a type for this bounded type parameter: unconstrained type
templt1.rs:22         let s = Image::<T>::stride_for_type(w, 3);
                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~