Calls to struct methods and immutable

[Kenji Hara]

On Thursday, 15 November 2012 at 23:40:16 UTC, Joseph Rushton 
Wakeling wrote:
> On 11/15/2012 07:48 PM, Ali Çehreli wrote:
>> Do you have a reference type in your struct?
>
> Assuming I do, what can I do to ensure the struct instance is 
> immutable?  Is cast(immutable) now the only option?

This code works with dmd git head (and might work with dmd 2.060).

struct Foo
{
     int a, b;
     string[string] aa;
}
immutable(Foo) makeFoo() pure
{
     Foo foo;
     foo.aa["a"] = "hello";
     foo.aa["b"] = "world";
     return foo;
     // compiler allows this implicit casting from Foo to 
immutable(Foo),
     // because compiler can guarantee the instance 'foo' doesn't 
have
     // mutable indirections to any global data so makeFoo is a 
pure function.
}
void main()
{
     immutable ifoo = makeFoo();
}

Kenji Hara