[cfe-dev] clangd, completion in header files

[Milian Wolff via cfe-dev]

On Montag, 26. März 2018 10:22:05 CEST Sam McCall via cfe-dev wrote:
> Thanks for trying things out, and sorry for the bad header-file experience.
> 
> On Mon, Mar 26, 2018 at 4:05 AM Jan Včelák via cfe-dev <
> 
> cfe-dev at lists.llvm.org> wrote:
> > Hello list,
> > 
> > thank you for all the work on clangd. I tried it for the first time
> > with VSCode and I'm really impressed how useful it is just out of the
> > box.
> > 
> > I however encountered a problem with code completion in header files
> > and I would like to know if it's a bug in clangd or a problem in my
> > setup.
> 
> Yeah, this is a known missing feature - guessing the right compile flags
> for headers.
> If you have a compilation database (e.g. compile_commands.json) that
> provides compile commands for headers, then clangd works as expected.
> However most build systems don't provide this information. (Bazel, for one,
> does).
> 
> When there's no compile command provided, we fall back to 'clang
> $filename'. Clang treats .h files as C.
> But it's also missing a bunch of other information: include paths, defines
> etc that are likely required to get useful results.
> So I don't think just diverging from clang here will actually help many
> projects (feel free to try this out by editing compile_commands.json - if
> this works for you it'd be good to know).
> 
> What can we do then? A few ideas:
>  - we can preprocess all the files from compile_commands.json on startup (
> https://reviews.llvm.org/D41911 is the start of this). But we can't
> guarantee we get to the file you care about in time, so behavior will be
> erratic.
>  - we can pick a compile command arbitrarily and take its flags, which will
> get include paths and defines right if they're uniform across the project
>  - we can just refuse to provide any diagnostics/completions where we don't
> have a known-good set of flags.

Another trick that we apply in KDevelop:

- Try to find the *.cpp file for the header based on some file naming 
heuristics and use that instead.

This works well in the majority of cases, but fails for system headers, 
header-only utilities, and headers you just started to write where the 
accompanying *.cpp file is still empty.

Bye

-- 
Milian Wolff
mail at milianw.de
http://milianw.de
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