User defined convenience constructors

[Raildex]

I find myself writing factory functions a lot that would be better suited to be actual constructors.
I would like to have a language-level feature that allows writing convenience constructors.
Similar to how extension methods allow custom functionality on instances (using the public interface of a type), convenience constructors are used to construct instances with extended constructors.

Take Assimp.Matrix3x3 for example. It has exactly three (actually, just two) constructors:

Matrix3x3(Matrix4x4)
Matrix3x3(Single,Single,Single,Single,Single,Single,Single,Single,Single)
Matrix3x3()

But most of the time, you have 3 Vector3s that represent the rows or columns of the Matrix. A constructor for the client code is missing for this use case.

Now lets say we can write something like this:

static class MyMatrixExtensions {
  public convenience Matrix3x3(in Vector3 a,in Vector3 b,in Vector3 c): Matrix3x3(a.X,a.Y,a.Z,b.X,b.Y,b.Z,c.X,c.Y,c.Z); // chaining syntax like regular constructors, a convenience constructor must call an actual constructor

  public convenience Matrix3x3 (Vector3[] columns) : Matrix3x3() { // uses default constructor of the struct
    M11 = columns[0].X; // you can use the public properties in a convenience constructor
    M21 = columns[0].Y;
    M31 = columns[0].Z;
    M12 = columns[1].X;
    M22 = columns[1].Y;
    M32 = columns[1].Z;
    M13 = columns[2].X;
    M23 = columns[2].Y;
    M33 = columns[2].Z;
  }
}

//usage

Vector3 normal = ...
Vector3 tangent = ...
Vector3 bitangent = ...
Matrix3x3 tbn = new Matrix3x3(t,b,n); // looks neat and tidy

It could be implemented as syntactic sugar for a compiler generated static factory method.

[FaustVX]

With the Role/extension proposal,
May be you could add constructor to classes

public implicit extension E for Matrix3x3
{
    public Matrix3x3(in Vector3 a, in Vector3 b, in Vector3 c)
    : this(a.X, a.Y, a.Z, b.X, b.Y, b.Z, c.X, c.Y, c.Z)
    { }
}