"Static Initialization of Structs"

There is inconsistency between the examples.
Replace X with S in all the examples.
        
In the 4th example, this line:
S s = { 1, i };   // q.a = 1, q.b = i, q.c = 0, q.d = 7

Should be replaced with:        
S q = { 1, i };   // q.a = 1, q.b = i, q.c = 0, q.d = 7



"Const and Invariant Structs"

In the code example the statement t = s; is not compilable:

const struct S { int a; int b = 2; }

void main()
{
    S s = S(3);    // initializes s.a to 3
    S t;           // initializes t.a to 0
    t = s;         // ok, t.a is now 3
    //~ t.a = 4;   // error, t.a is const
}

test3.d(29): Error: variable test3.main.t cannot modify const



"Struct Constructors"

There are no code examples here. A simple one could be added:

struct S {
    int x;
    int y;
    
    this (int x_val, int y_val) {
        x = x_val;
        y = y_val;
    }
}

void main() {
    S s = S(4, 5);
}