Add `boulder recipe eval`

[lumi-me-not]

I feel it could be useful to be able to do boulder recipe eval '%cargo_install' and similar invocations to be able to find out the value of an action or definition. This would then evaluate the given expression and output whatever it outputs.

If this is desired, I can write this, as well. Just checking if others would also think it's worth it.

[ermo]

So it would basically resolve the given macro (or macros) per the profile used? If so, this seems very useful.

[lumi-me-not]

Yeah, I experimented with something like this while messing with the new evaluator. My naive implementation was insufficient, though, as it did not load all the builtins. (they are set in job/phase.rs)

[ermo]

I wonder if we actually want such a standalone eval subcommand to work with both recipes and macros, and then have it output the evaluated recipe / macro to stdout?

Example:

# Evaluate a recipe by resolving all macros etc. and print the resolved recipe to stdout
# It should be possible to specify a profile via -p|--profile. Defaults to -p default-${arch}.kdl
# recipe eval should default to `./recipe.kdl` (when enabled) and then `./stone.yaml` (in that order)
boulder recipe eval [somerecipe.kdl]
# Evaluate an individual macro (action or definition)
boulder recipe macro eval '%(somedefinition)' 
boulder recipe macro eval '%someaction'

[lumi-me-not]

That sounds useful, as well. I would suggest boulder recipe eval 'expr' and boulder recipe eval -f 'recipe.kdl', then, as commands. But I don't have strong opinions in either direction.

[ermo]

Let's go with -f|--file then. Just be aware that we may already have input and output handling. It may be wise to obey those flags.