Difference between slice[] and slice

[WhatMeWorry]

Just got through debugging a line of code which uses dynamic 
array.  It boiled to to my use of [].  How should I "D think" 
about slice[]?  The run time error seems to say the the length of 
[] is zero.   I was assuming that [] meant "the entirety" of the 
array.

In short, is there anytime that one would want to use "slice[] = 
something" syntax?I

//waste[] = waste[0..$-1]; // object.Error@(0): Array lengths 
don't match for copy: 0 != 1

     waste = waste[0..$-1]; // works