Reported by joseph07 in #haskell. stack setup failed due to not having enough space on the device, despite there being 38GB. According to pdxleif, archlinux defaults to using tmpfs, which stores it in ram, and so is limited in size.

Here's where a system temp directory is used as a place to unpack the GHC tarball:

It'd also likely be worthwhile to take a look at other usages of the system temp directory. On one hand, it seems odd to have a system configuration that has such limited space in tmp. On the other hand, I see few downsides to instead storing such temp directories somewhere in ~/.stack (and the big upside of resolving this issue)