A few points:

• I don't really like overloading the type of options (i.e. either bool string). Why not just add a separate size option, like boot.devShmSize?

• We really should continue to use the upstream unit to prevent getting out of sync with systemd. The upstream unit also has several options omitted here (e.g. Description, Conflicts, After, etc.). It would be better to use systemd's override mechanisms, e.g.

systemd.mounts = [ { where = "tmp"; options = [...]; } ];

or even better

systemd.mounts."/tmp".options = [...];

but that will require changing systemd.mounts into an attrset, like systemd.services.

@edolstra:

1. I wanted to keep the interface unchanged for people who already have set this option, otherwise i'd make two options: boot.tmpOnTmpfs.enable and boot.tmpOnTmpfs.size.
2. can you give me any pointers how to tackle this? this would be a major refactoring, if i understand correctly.

Should converting systemd.mounts to an attrset be part of the effort? I cannot think of a way to do this in a backwards compatible way.

For the author, reviewers, and committers: this PR was scanned and appears to add a use of the deprecated types.string, which emits a warning as of #66346. Before merging, please change this to another type, possibly:

• types.str for a single string where merging does not make sense, or cannot work
• types.lines for multi-line configuration or scripts where merging is possible
• types.listOf types.str for a mergeable list of strings

Doesn't seem to be much interest in this, closing for now, feel free to reopen