Mill 0.12 and Chisel source locators

[jackkoenig]

Chisel¹ uses source locators to help our users map from generated Verilog back to their original Scala source code. These source locators are relative to the project root, which Chisel determines at compilation time via the user.dir Java system property (basically "pwd" of the Scala compilation process)--with the option for user override via system property chisel.project.root².

This has worked well for both Mill and SBT projects which (traditionally) run compilation from the root of the project directory. Mill 0.12 changes this to run the compilation process in the root directory (/) which causes issues with our approach.

It's not clear what the best solution is, some options:

1. Tell Chisel users to set chisel.project.root when compiling their projects.
2. Set chisel.project.root in the Mill bootstrap script.
3. Make it possible to disable PWD sandboxing via some override.
4. Enhance .mill-jvm-opts to support environment variable expansion.

(1) and (2) require no changes on Mill's part but have some big disadvantages for users of Chisel.

(1) makes the typical use of Chisel pretty annoying:

mill -Dchisel.project.root=$PWD __.compile

(2) effectively requires Chisel to maintain and distribute its own mill bootstrap script. This would be especially be problematic for users who decide to install mill themselves since they would not get this very Chisel-specific behavior.

(3) @lihaoyi described as "a last-resort kind of thing" and I am inclined to agree--I don't think we want to disable the sandbox, we just want to tweak the environment slightly in a way that can be committed to a file on disk.

(4) Sounds the simplest to me. I just want to put -Dchisel.project.root=$PWD in a file on disk that we include in the Chisel project template--then users don't really have to mess with it most of the time. I think there are variations on this idea that could work--it'd be fine to do this in some snippet in mill.build as well, but I suspect doing it in .mill-jvm-opts is simpler.

Just trying to get the conversation started, and as requested by @lihaoyi, put somewhere that isn't ephemeral like Discord chat. Any thoughts or ideas?

---

This Discussion is spun out of a conversation on Discord³

Footnotes

1. https://github.com/chipsalliance/chisel ↩

2. https://github.com/chipsalliance/chisel/blob/a7a68e522ccc71d08a0a0aaea0a1460d266f2d7f/core/src/main/scala/chisel3/internal/SourceInfoFileResolver.scala#L17 ↩

3. https://discord.com/channels/632150470000902164/1295786923410984981/1299045339789262919 ↩

[lihaoyi]

@jackkoenig we'll probably go with $ENV interpolation in the .mill-jvm-opts file. I think that's the least invasive approach. PR #3841

[jackkoenig]

Thank you!

[lihaoyi]

@jackkoenig can you try this unstable version 0.12.0-11-7e7a54 which contains the implementation for environment variable interpolation, if it works we can cut a stable release later

[jackkoenig]

I tested it and it works!

[lihaoyi]

@jackkoenig great! I cut 0.12.0-19-f66304 which changes the $PWD syntax to a less ambiguous ${PWD}, try it out and that should be the final syntax going forward

[jackkoenig]

@lihaoyi apologies for the delay, I see ${PWD} syntax (which is better) was released in 0.12.1--I can confirm it works for us. Thanks!