[cfe-dev] Tooling RFC: Find build root from source root with a symlink?

[Roman Lebedev via cfe-dev]

On Thu, Oct 11, 2018 at 1:42 PM Sam McCall via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
>
> Many projects/build systems have the concept of a "build tree" that can be separate from the source tree. (e.g. CMake supports this, and LLVM recommends this approach).
>
> Tools sometimes need to find the build tree.
> I'd like to suggest a convention for the Tooling library to support:
>     If $SRC/buildroot exists, it's the default build directory for the source tree $SRC.
>     Otherwise, the build directory is $SRC itself.
>     $SRC/buildroot may be a symlink.
>     Configuration files like compile_commands.json are searched for in the build directory.
> (Bikeshedding the "buildroot" string is welcome. "build" will conflict too often, sadly).
Is "buildroot" supposed to be a hardcoded constant?
Personally, i have never ever named any of my build dirs that. They
were always named "build".
Also, what if there are multiple build trees - build-clang-release,
build-gcc-release, ... ?

> Implications:
>  - for /foo/bar/baz.cc, we'd search for compile_commands.json in {/foo/bar/buildroot/,/foo/bar/, /foo/buildroot/, etc}. This is backwards-compatible.
>  - where users currently symlink compile_commands.json itself, they could create the buildroot symlink instead. This would work in the same way, and enable new cases.
>  - if we implement e.g. a Ninja-backed compilation DB that doesn't need compile_commands.json, the same symlink convention would work.
>  - it provides a simple model for multi-configuration-aware tools: a configuration is defined by a build dir, there's a default configuration, tools can let the user override the build dir. e.g. clangd can write its index files into the build dir instead of the source dir, which is multi-configuration-friendly (tinyurl.com/clangd-automatic-index)
>  - non-clang tools that consume compile_commands.json will need to be updated over time.
>
> What do you think?
> Cheers, Sam
Roman.

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