roc: llvm alloca unitentional mutation bug

I also posted this on the chat, but I think actually making an issue sounds like a good idea 🙂

I tried to make this example simpler by using lists instead of strings, but could not reproduce it with a lists version. So it seems to maybe also be related to the specifics of how strings are handled? Unfortunately I have have no idea about roc’s internals to make a good guess what might be happening here, since I just started actually writing roc code.

prefixes = \str, soFar ->
    if Str.isEmpty str then
        soFar

    else
        next = str
            |> Str.graphemes
            |> List.dropFirst 1
            |> Str.joinWith ""

        prefixes next (List.append soFar str)

# this is the (erroneous) output roc produces
expect prefixes "abc" [] == ["abc", "c", ""]

# this is what I whould have expected
expect prefixes "abc" [] == ["abc", "bc", "c"] # fails

If it helps, I could repro this with todays nighly build and an old one I still had from 11/28. It also did not make a difference whether I used basic-cli 0.6.2 or 0.7.0.