Why can't we assign a value when we return a struct instead of a pointer to a struct?

In C language, when we return a structure instead of a pointer to the structure, the reason why we cannot directly assign the value is that the entire structure will be copied when returning the structure content instead of returning a pointer to the structure. Since a structure may contain a large amount of data, copying the entire structure may be expensive. Therefore, in order to improve efficiency, the C language stipulates that when a structure is returned, we can only use it by assigning it to a variable of structure type. This can avoid unnecessary copy operations and improve code execution efficiency.

Question content

Suppose we have the following structure

type profile struct {
    id   int
    name string
}

func test(p profile) profile {
    return p
}

func main() {
    var profile profile
    test(profile).id = 20 // cannot assign to test(profile).id (value of type int)
}

But if we change the profile of the test function return type to *profile, the main function will work.

func test(p Profile) *Profile {
    return &p
}

func main() {
    var profile Profile
    test(profile).id = 20 // Works
}

Why is this?

Workaround

Assigning fields to Profile in this way has no noticeable effect. You are assigning a temporary struct value (of a field) and then immediately discarding it. Note that test's return value is a copy of the profile in main; it is copied into the test's parameters, and then copied again when returning from the test.

When returning a pointer, at least in principle, the structure pointed to will still be accessible after the assignment (although not in this particular case, since the pointer points to a copy of the argument being tested).

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