Vector2 operator improvements

I recently asked a question about structs, and optimizing some overloaded operators.

The original question is here

Now, I have taken those improvements to heart (or some/most of them), and I return with the following functions (nonmember functions, as I wish for them to be C compliant if possible).

inline Vector2& operator+=(Vector2 &a, const Vector2 &b)
 {
     a.x += b.x;
     a.y += b.y;
     return a;
 }
 
 inline Vector2 operator+(Vector2 a, const Vector2 &b) 
 {
     a += b;
     return a;
 }
 
 inline Vector2& operator*=(Vector2 &a, const float &n)
 {
     a.x *= n;
     a.y *= n;
     return a;
 }
 
 inline Vector2 operator*(Vector2 a, const float &n) 
 {
     a *= n;
     return a;
 }
 
 inline float operator*(const Vector2 &a, const Vector2 &b)
 {
     return (a.x * b.x) + (a.y * b.y);
 }
 
 inline Vector2 rotate(const Vector2 &a, const float &angle)
 {
     Vector2 out = a;
     out *= cos(angle);
     out.x -= sin(angle) * a.y;
     out.y += sin(angle) * a.x;
     return out;
 }
 

(Please note, I omitted subtraction, and another multiplication operator, as they were equivalent to other operators listed here).

I am currently unable to notice any other potential improvements.
Have I missed anything, that will (potentially) make these functions, as they currently stand, inefficient?

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