Expand make variables in write_file content and expand_template substitutions

[SanjayVas]

If a user wants make variable substitution, they need to either write their own versions of these rules or else fall back to genrule.

I assume a boolean attribute would need to be added to toggle this behavior as it wouldn't be backwards-compatible.

[tetromino]

This is a reasonable request, but note that we'd need an ergonomic way to specify how to map from variable names to substitution keys (e.g. are you using $var or {{var}} or $(var) or ...)

[aiuto]

Strong vote for ${var}.

• The leading $ is widely used in scripting languages as a variable reference. It is universally familiar.
• $var is insufficient because you you can't parse where var ends.
• $(var) is very Makefile centric. While the Bazel engineers know that from history, it is not actually a widely used style anywhere else.
• {{var}} is also not commonly used.
• %var% is widely known in the windows world, but no where else.

As an alternative, {var} is sort of OK.

[SanjayVas]

This is a reasonable request, but note that we'd need an ergonomic way to specify how to map from variable names to substitution keys (e.g. are you using $var or {{var}} or $(var) or ...)

To clarify, this is about substitution of Make Variables. There's no need to specify any kind of mapping here, as that's already defined. The rule implementation just needs to call ctx.expand_make_variables (ignore the deprecation warning on that function, as a suitable replacement does not actually exist).

[tetromino]

a suitable replacement does not actually exist).

I think the appropriate solution to this issue would be such a suitable replacement. With sufficient flexibility to support ${var}, $(var), or any other reasonable syntax. I don't see any particular reason why it's impossible; just some ugly string munging. (We can add some better APIs to a future version of Bazel to make the string munging less ugly, a real regex module exposed to Starlark for example would help massively, but skylib unfortunately needs to work with the current stable version of Bazel too.)