It should already be the case in 0.12 that you can put .sbt files in the base directory of a subproject and the settings there will be included in that project’s scope.
Code is reused between .sbt files by creating a normal .scala file in project/. The code in project/ will be available for use in the .sbt files. The definitions in one .sbt are not visible to other .sbt files, at least in 0.13. This is mainly an implementation restriction and it is undetermined whether this will be lifted in future versions.
The default root project will aggregate all subprojects, including those coming from projects defined in subProject/build.sbt.
The current difficulty is making it explicit.
For example, the following build.sbt in the root directory would define a subproject in sub/.
This is a full definition, defining the ID, base directory, etc… for the project.
<root>/build.sbt
lazy val sub = project
However, it cannot reference anything defined in <sub>/build.sbt. (The existence of sub/build.sbt is not known until after <root>/build.sbt is compiled and evaluated.)
So, to explicitly define what sub aggregates, you’d need something like:
sub/build.sbt
lazy val sub = project.in(file(".")).aggregates(subSub)
//or: lazy val sub = project in file(".") aggregate subSub
lazy val subSub = project
However, this duplicates the definition of sub.
A possible solution going forward is to make the root definition just a reference, like:
<root>/build.sbt
lazy val sub = LocalProject("sub")