Q:

Does conversion between alias types in Go create copies?

Example:
type MyString string 
var s = "very long string"
var ms = MyString(s)
var s2 = string(s)

Are ms or s2 a full copy of s (as it would be done with []byte(s))? Or they are just a string struct copies (which keeps the real value in a pointer)?
What if we are passing this to a function? E.g.:
func foo(s MyString){
  ...
}
foo(ms(s))  // do we copy s here?

A:

Spec: Conversions:

Specific rules apply to (non-constant) conversions between numeric types or to and from a string type. These conversions may change the representation of x and incur a run-time cost. All other conversions only change the type but not the representation of x.

So converting to and from the underlying type of your custom type does not make a copy if it.
When you pass a value to a function or method, a copy is made and passed. If you pass a string to a function, only the structure describing the string will be copied and passed, since strings are immutable.
Same is true if you pass a slice (slices are also descriptors). Passing a slice will make a copy of the slice descriptor but it will refer to the same underlying array.

