Help, what is the code mean?

[lilijreey via Digitalmars-d-learn]

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     }
```