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The method to use ndarray::stack to construct a matrix from an iterator that contains column vectors is:

1. Import the ndarry crate and use the stack() method.
2. Define the axis on which you want to stack the column vectors. For example, if you want to stack them vertically to form a matrix, the axis should be 0.
3. Call the stack() method, passing in the axis and the iterator of column vectors.
4. The method will return an ndarray that contains the stacked matrix.

Here's an example code snippet that constructs a 3x2 matrix by stacking three 3x1 column vectors vertically:

use ndarray::stack;

let col_vecs = vec![array![1, 2, 3], array![4, 5, 6], array![7, 8, 9]];
let stacked_matrix = stack(Axis(0), &col_vecs).unwrap();
assert_eq!(stacked_matrix, array![[1, 4, 7], [2, 5, 8], [3, 6, 9]]);

In this example, the Axis(0) argument specifies that the column vectors should be stacked vertically. The &col_vecs argument is a reference to the iterator of column vectors that we want to stack. The resulting stacked matrix is a 3x2 array.