Custom build dir

[rgrinberg]

Fix #291

[ghost]

A few thoughts:

• if the build directory is set to an external path, then Path.is_local p will no longer be true for paths
  inside the build directory. I expect that this will break quite a few things
• renaming the build directory completely invalidate the cache, given that the real build directory is present in all rules
• _build is hard-coded in the the documentation as well, so the documentation needs to be updated as well

Initially, I had thought that we only needed to know about the actual build directory at the edges, i.e. when converting to/from strings. And in between we would only manipulate symbolic paths, in the same way that local paths are not absolute but relative to the chosen root. Basically we could represent Path.t as follow:

type t =
  | External of Absolute.t
  | In_source_tree of Relative.t
  | In_build_dir of Relative.t

[rgrinberg]

if the build directory is set to an external path, then Path.is_local p will no longer be true for paths inside the build directory. I expect that this will break quite a few things

So I suppose we should change the meaning of local to be anything under the workspace or in the build dir. Regardless if the build dir is in the workspace.

renaming the build directory completely invalidate the cache, given that the real build directory is present in all rules

That seems to be expected. At least until we have the global build cache.

Initially, I had thought that we only needed to know about the actual build directory at the edges, i.e. when converting to/from strings. And in between we would only manipulate symbolic paths, in the same way that local paths are not absolute but relative to the chosen root. Basically we could represent Path.t as follow:

Okay, I'll try making it work with this type

[rgrinberg]

Closed in favor of #735