Help, what is the code mean?

[lilijreey]

Hi:
    In dlang core.thread.osthread has below code, the 654 line 
code i can understand why write () first, and {m_fn = fn;}()  do 
what?
```
    this( void function() fn, size_t sz = 0 ) @safe pure nothrow 
@nogc
    647     in
    648     {
    649         assert( fn );
    650     }
    651     do
    652     {
    653         this(sz);
    654         () @trusted { m_fn   = fn; }(); //What is the code 
mean?
    655         m_call = Call.FN;
    656         m_curr = &m_main;
    657     }
```