Matrix constructors should be constant expressions whenever possible.

[wdanilo]

Hi! Currently the following code is rejected, but it should be accepted:

const DEFAULT_POSITION : Vector3<f32> = Vector3::new(1.0,0.0,0.0);

[sebcrozet]

Hi! I'll close this issue because it is a duplicate of #521. This was not possible a year ago because of limitations of const-expression in Rust. Though it has improved a lot since then so we should try again now.