why does this String init work without len?

buf = InlineArray[UInt8, 640](0)
...
s = String(buf.unsafe_ptr())

buf = InlineArray[UInt8, 640](0)
...
s = String(buf.unsafe_ptr())

fn __init__(inout self, ptr: UnsafePointer[UInt8], len: Int): 

fn __init__(inout self, ptr: UnsafePointer[UInt8], len: Int):