A few of the built-in functions are very special, in that they can evaluate to constant expressions. len and cap are two such functions. But they aren’t always evaluated to constant expressions, sometimes they’re more normal-ish runtime functions. Length and capacity … The expression len(s) is constant if s is a string constant. The expressions len(s) and cap(s) are constants if the type of s is an array or pointer to an array and the expression s does not contain channel receives or (n...
