The implementation described here is inspired by Lua. Even if you already know what a closure is, it might still be a worthwile read, since it probably implements them very differently from what you are thinking! Instead of always heap-allocating variables that are used by closures, it starts by stack-allocating them and only moves them to the heap if the closure outlives its parent function. In order for this to wor…
Another approach is to do the opposite - always store all local variables in the heap, and rely on scalar replacement of aggregates to put them back onto the stack where possible. This is what I do in my Ruby interpreter. That’s definitely not compatible with a single-pass though!
A third option that can still be single pass is the Smalltalk way, where the stack frames themselves are first-class objects: the closures then keep a reference to the stack frame and the GC won't collect any live stack frames, which will contain closed-over variables.