macros.md

Macros and ijars

Using ijar poses a challenge in Scala because Scala macros are resolved during compilation. If a Scala macro references methods stripped away by ijar, things break. However, forgoing ijar use altogether is sub-optimal---using ijar dramatically decreases unnecessary recompilation between builds.

lucidsoftware/rules_scala strives to optimally integrate ijar and Scala macros as follows:

1. scala_library, scala_binary, and scala_test have a Boolean macro attribute which must be set to true if the rule contains any Scala macros
2. During the classpath resolution phase, we set macro_classpath to contain the transitive runtime classpaths of all deps for which macro = true
3. We let JavaInfo always use ijar
   • If macro was false, we get useful ijars
   • If macro was true, we get problematic ijars; however, this won't be an issue:
4. During compilation, we have macro_classpath precede the compile classpath of the merged JavaInfo, forcing the compiler to use the full classes

This lets us make use of ijars as much as possible, without breaking things due to Scala macro usage.