Assume we have a project with the following structure:
root/
.hhconfig
├── directory1
├── directory2
├── directory3
.........................
├── directory10
Is there a way to have a single .hhconfig file, and exclude onlydirectory8 from the typechecking? I think it would be really painful to put separate .hhconfig files, inside every directory or declare as UNSAFE all the files on directory8 in order to be excluded from the typechecking.
This is not supported. A Hack project is designed to be checked as a single project, with full analysis going across all of the different parts of it. If it doesn't typecheck as a whole, then the behavior of HHVM on it is undefined.
You should really, really carefully consider why you're trying to exclude part of the project from typechecking. You really shouldn't have a large body of type-incorrect code. You may want to consider leaving that code back in PHP -- it sounds unlikely to be valid Hack code, or to become such soon. Hiding these type errors is crippling the typechecker's ability to help you find problems in the other code in your project.
You may also be able to use a different mode, decl mode which will exclude all the code in a file from having function bodies typechecked (but which will still make the definitions available to other files). But again, this is just shoving a problem under the rug. Ideally you'd fix all of the type errors instead!
Also, definitely don't put separate .hhconfig files in each directory -- they'll be checked as separate subprojects and none of the analysis will look across the borders of the subdirectories!