I finally took a quick look at the current code (and the discussion so far)
and I have a few remarks:

* even within source directories it's useful to be able to exclude certain
subdirs from project-level operations. How is this going to be handled?
* it's not 100% clear to me what are major mode writes supposed to provide
as "project implementations"
* In 24.4 a `vc-root-dir` was added. Seems it overlaps a bit with
`project.el`
* how can Projectile (or a similar package) leverage `project.el`?

On 8 July 2015 at 03:25, Dmitry Gutov <dgutov@yandex.ru> wrote:

> On 06/10/2015 01:31 AM, Dmitry Gutov wrote:
>
>  If project-source-directories is a generic method, maybe we can say that
>> its implementation should almost always (unless it really knows what
>> it's doing) call the next applicable implementation.
>>
>> That implementation could be set up to dispatch based on the value of
>> major-mode.
>>
>
> I've pushed that to the branch 'scratch/project', please take a look.
>
> The split between "project directories" and "project source directories"
> is not 100% necessary, but I think it can be useful, provided the semantic
> difference between the two is clear.
>
> What I want to do with project-source-directories here, is to allow a
> major mode to provide a default list (or, actually, a function that would
> compute it, because `load-path' can change at runtime).
>
> This way, unless a project implementation explicitly overrides it, Elisp
> authors will have the whole load-path used for searching.
>
> Maybe using a specialized implementation using &context is not the best
> thing to do there. Alternatively, we can introduce a new variable (like
> `project-source-directories-function'), and refer to it in the default
> `project-source-directories' implementation, as well as its docstring.
>
>