Thanks for discussing this with me and helping me work through it. To answer your question:

Let's annotate the variable lifetimes. `'` indicates last use, `_n` indicates that a variable holds the same value. I'll only be annotating the values in question:

foo = (list1_0 list2_0') -> {
mutate_last list1_0'
}

big_list_0 = [ ... ]
foo big_list_0 big_list_0
print big_list_0'

Let's focus on this line. Because these both aren't the last reference to `big_list`, they're passed as a CoW reference. I'll use `&` to denote this:

foo &big_list_0 &big_list_0

In `foo`, `list1_0 = &big_list_0` and `list2_0' = &big_list_0`. `list2_0` is never used, so this reference is immediately dropped (not the value itself, rather the reference to that value). Let's now look at the body of foo:

mutate_last list1_0'

Because list1_0 is a final reference, it's passed the value of the CoW reference, which is, well, the CoW reference. mutate list mutates this, and because the reference is copy-on-write, a copy of `&big_list_0` is returned. We'll call this `big_list_1`.

So the CoW reference of a value is a CoW<Value>. a CoW reference of a CoW<Value> stays the same, it's still a CoW<Value>.

> So how do you tell inside foo whether the call is the last use of the object referenced by list1? Heroic interprocedural pointer analysis? A uniqueness type system? Something else?

Something else. Vaporization is not all compile-time magic, it depends on the runtime as well. You can basically sum it up with two operations:

reference: Value or CoW<Value> -> CoW<Value>
mutate: Value or CoW<Value> + mutation -> Value

So when `mutate_list` mutates the list, depending on whether `list1` is a value or a CoW reference, either a copy will be made or it'll be mutated in place. I'm not claiming that Vaporization is the best approach to compile-time memory management, rather, it's more of a set of compiletime+runtime strategies that allow the interpretation of programs without garbage collection (in the traditional sense)

So, the question becomes: how do we indicate whether something is a value or a CoW reference? Something along the lines of a tagged pointer will do. Passerine already uses NaN-tagging, so this fits right in.

What about compiling to low-level targets, like LLVM IR, which don't really have a 'runtime' and rather specify the manual layout of structs and references?

I'm not quite sure, but here's one possibility. All values that can escape a local scope exist in some region of memory allocated for local variables. When a function is called, we can pass a bitfield indicating which of the locals variables on the stack are CoW references, and which are not. When the runtime goes to mutate a reference, it quickly checks the bitfield (which can be stored in a register or in the call frame) whether it needs to make a copy before mutating. In practice, the check is just a quick bit-shift and `&`.

Note that we can go a bit further: the bit-fields of all calls are known at compile time, so instead of copying and passing the bitfield itself, we can pass the index of the bitfield (which is a constant) to the function to use. In fact, I imagine you could somewhat 'monomorphize' this, so to speak, and generate different functions for different bitfields... but I'd have to think some more about this.

Anyway, that's about it. I hope I've helped clear some things up. If you notice anything else slightly off, or if you'd like to discuss this further, please let me know :)