[clangd-dev] VS Code Clangd and CMake integration

[Sam McCall via clangd-dev]

On Wed, Nov 28, 2018, 23:00 Anton Södergren <anton.sodergren at gmail.com
wrote:

> I'm not familiar with CMake Tools, I guess it helps with editing
>> CMakeLists, CMake configs, and running configure/generate?
>
> Except for the file editing part (that's handled by another extension),
> you're pretty much right. It also allows for building and launching
> debuggers. But it's the configure/generate part that's the most interesting
> in our case.
>
> Clangd works like other clang-based tools. Starting at the source file in
>> question, it walks up directories until it finds compile_commands.json
>> <https://clang.llvm.org/docs/JSONCompilationDatabase.html>.
>>
> So it turned out that it indeed seemed to generate a compile_commands.json
> file in the build folder (at least yesterday, today it doesn't seem to work
> well at all). I tried copying that file over to my project root folder and
> Clangd found it and started working correctly indeed!
>
> The current best-practice is to manually create a symlink
>> src/compile_commands.json -> build/compile_commands.json. I think
>> symlinking src/.build to build/ is a slightly more principled approach, but
>> something still has to create a symlink. (Maybe we could get CMake or CMake
>> Tools to do it?)
>>
> While this is a possible approach, I think is sounds a little unwieldy
> (though that might just be personal preference). When I looked around a bit
> more, I saw that the Clang tooling docs
> <https://clang.llvm.org/docs/HowToSetupToolingForLLVM.html> mentioned
> that it's possible to pass the build path directly to Clang tools with a -p
> parameter, is this possible in Clangd too?
>
It is in fact possible as a command flag (it's spelled
-compile-commands-dir). It works in a pinch, but it's a subtly broken idea
and I don't think it's the right long-term direction.

1) a clangd instance is typically 1:1 with an editor session. But an editor
can open files from multiple projects, so what should the flag value be?
2) how does the editor know which dir to pass to clangd? If it's an editor
preference, how does this work for users who work in different projects at
different times?
3) each tool (clangd, clang-tidy etc) needs to get this configuration in
some appropriate way, it's not shared (whereas all clang-based tools share
discovery logic for compile_commands.json)

Requiring the symlink (and adding it to gitignore etc) is a little fiddly
and certainly unconventional, but it's the best thing we've found so far.

For example, other IDE's that utilize clang-tidy (e.g. CLion) seem to do so
> without any visible symlinks.
>
Yes, if you're an IDE that already knows how to build the code (has
opinions/integrations for the build system) then directly injecting the
configuration is feasible. (Clangd has LSP extensions for this). But in
VSCode, the build system is just another plugin that may or may not be
installed, so a decoupled design seems most likely to work in many
configurations.

To me, that sounds more robust and clean than having to pollute the source
> tree with symlinks. Of course, that would require some work probably on
> both of the plugins involved. Also, having a general and flexible approach
> for this would be nice, as I'm also interested in getting work on a build2
> extension going, which would require the same functionality.
>
Definitely open to new ideas here, but I'm not sure exactly what you're
proposing.

Clangd needs to be able to determine the compile command given a filename,
at any time, within the clangd process (i.e. without exchanging LSP
messages with the editor).

Maybe one idea is to stick to compile_commands.json but extend the way they
are discovered, e.g. let the Cmake Tools plugin notify Clangd that a
certain compile_commands file is relevant and should be considered, even if
it's not in an ancestor dir of the source file.

clangd has to reload the data when appropriate. Today, it caches forever
>> :-( Ilya is working on fixing it!
>>
> I did notice this problem when testing it out yesterday. But I trust that
> it will get fixed then!
>
It's... not trivial. (Other clang tools don't handle this as they tend to
be batch)
So we'll fix it, but it might take a little time :-)


> Cheers,
> Anton
>
> On Tue, 27 Nov 2018 at 20:25, Sam McCall <sammccall at google.com> wrote:
>
>> On Tue, Nov 27, 2018 at 8:09 PM Anton Södergren via clangd-dev <
>> clangd-dev at lists.llvm.org> wrote:
>>
>>> Hello! I'm not sure if this is the best place to ask, but it's better to
>>> get a discussion going rather than just being confused.
>>>
>> Right place! Welcome :-)
>>
>>
>>> I tried using Clangd in Visual Studio Code, but noticed that it doesn't
>>> integrate well with the CMake Tools extension, which makes it hard to use
>>> together with a CMake workflow. I would like a way for Clangd to
>>> automatically get the correct info when CMake runs in the editor.
>>> Specifically, I want seamless and automatic integration between these
>>> two extensions without the user having to do any additional config:
>>>
>>>    - vscode-clangd (
>>>    https://marketplace.visualstudio.com/items?itemName=llvm-vs-code-extensions.vscode-clangd
>>>    )
>>>    - CMake Tools (
>>>    https://marketplace.visualstudio.com/items?itemName=vector-of-bool.cmake-tools
>>>    )
>>>
>>> I'm not familiar with CMake Tools, I guess it helps with editing
>> CMakeLists, CMake configs, and running configure/generate?
>> (Sorry, I would dig into it, about to head out on vacation)
>>
>>
>>> I see three possible approaches for this:
>>>
>>>    1. Work with the author of CMake Tools to make the extension
>>>    generate the appropriate info for vscode-clangd
>>>    2. Make vscode-clangd retrieve the info from CMake Tools somehow
>>>    3. Create a new extension that acts as a bridge between the two
>>>
>>> Or alternatively, some combination of the different approaches.
>>> I know that the CMake Tools extension integrates well with Microsoft's
>>> C/C++ extension, so maybe it would be a good idea to do this in a similar
>>> way that they did it. If my understanding is correct, a separate bridge
>>> extension was previously used for this, but I think they now use a
>>> combination of something similar to approach 1 and 2 for the extensions to
>>> communicate with each other.
>>> I would be interested to hear what you all think would be the best way
>>> to go about this, and maybe I can get in touch with vector-of-bools (the
>>> author of the CMake Tools extension) about it too.
>>>
>>> Another question I had regarding this: how does Clangd find the correct
>>> include paths and other info in the first place? Does it use some kind of
>>> compilation commands file for this? Is it possible to tell Clangd where to
>>> look for such a file? I couldn't really find any documentation about this
>>> when I looked around.
>>>
>> So if my understanding of CMake Tools is correct, these questions are
>> very much related :-)
>>
>> Clangd works like other clang-based tools. Starting at the source file in
>> question, it walks up directories until it finds compile_commands.json
>> <https://clang.llvm.org/docs/JSONCompilationDatabase.html>. This lists
>> the flags required to compile each file in its scope.
>>
>> So I think there are three relevant parts here:
>>
>>    - this file has to be created somehow. CMake can do this, it's
>>    controlled by CMAKE_EXPORT_COMPILE_COMMANDS
>>    <https://cmake.org/cmake/help/v3.5/variable/CMAKE_EXPORT_COMPILE_COMMANDS.html>,
>>    which I believe is on by default now. It writes the files into the root of
>>    the build tree.
>>    - the file has to be discovered somehow (if the CMake build tree !=
>>    the source tree). The current best-practice is to manually create a symlink
>>    src/compile_commands.json -> build/compile_commands.json. I think
>>    symlinking src/.build to build/ is a slightly more principled approach, but
>>    something still has to create a symlink. (Maybe we could get CMake or CMake
>>    Tools to do it?). Or maybe there's some other clever way to find the build
>>    tree.
>>    - clangd has to reload the data when appropriate. Today, it caches
>>    forever :-( Ilya is working on fixing it!
>>
>> Gotta run, hope this helps!
>> Patches and ideas welcome :-)
>>
>
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