src/docs/clangd.md

# Clangd

## Introduction

[clangd](https://clangd.llvm.org/) is a clang-based [language server](https://langserver.org/).
It brings IDE features (e.g. diagnostics, code completion, code navigations) to
your editor.

## Quick Start

* [Get clangd](#getting-clangd)
* Make sure generated ninja files are up-to-date
* Optional: build chrome normally to get generated headers
* Generate compilation database (note: it's not regenerated automatically):
```
tools/clang/scripts/generate_compdb.py -p out/Default > compile_commands.json
```

*** note
Note: If you're using a different build directory, you'll need to replace `out/Default`
in this and other commands with your build directory.
***

* Indexing is enabled by default (since clangd 9), note that this might consume
  lots of CPU and RAM. There's also a
  [remote-index service](https://github.com/clangd/chrome-remote-index/blob/main/docs/index.md)
  to have an instant project-wide index without consuming local resources
  (requires clangd 12+ built with remote index support).
* Use clangd in your favourite editor

## Getting clangd

For the best results, you should use a clangd that exactly matches the version
of Clang used by Chromium. This avoids problems like mismatched versions of
compiler diagnostics.

The easiest way to do this is to set the `checkout_clangd` var in `.gclient`:

```
solutions = [
  {
    "url": "https://chromium.googlesource.com/chromium/src.git",
    "managed": False,
    "name": "src",
    "custom_deps": {},
    "custom_vars": {
      "checkout_clangd": True,
    },
  },
]
```

After this, `gclient` will keep the binary at
`third_party/llvm-build/Release+Asserts/bin/clangd` in sync with the version of
Clang used by Chromium.

Alternatively, you may use the `build_clang_tools_extra.py` script to build
clangd from source:

```
tools/clang/scripts/build_clang_tools_extra.py --fetch out/Default clangd
```

The resulting binary will be at
`out/Default/tools/clang/third_party/llvm/build/bin/clangd`.

Once you have an appropriate clangd binary, you must configure your editor to
use it, either by placing it first on your `PATH`, or through editor-specific
configuration.

*** note
Note: The clangd provided by Chromium does not support optional features like
remote indexing (see https://crbug.com/1358258), such that `clangd --version`
will not mention `grpc`, and you will see “Unknown Index key External” warnings
in the clangd log.

If you want those features, you'll need to use a different build of clangd,
such as the [clangd/clangd releases on
GitHub](https://github.com/clangd/clangd/releases).
***

## Setting Up

1. Make sure generated ninja files are up-to-date.

```
gn gen out/Default
```

2. Generate the compilation database, clangd needs it to know how to build a
source file.

```
tools/clang/scripts/generate_compdb.py -p out/Default > compile_commands.json
```

Note: the compilation database is not regenerated automatically. You need to
regenerate it manually whenever build rules change, e.g., when you have new files
checked in or when you sync to head.

If using Windows PowerShell, use the following command instead to set the
output's encoding to UTF-8 (otherwise Clangd will hit "YAML:1:4: error: Got
empty plain scalar" while parsing it).

```
tools/clang/scripts/generate_compdb.py -p out/Default | out-file -encoding utf8 compile_commands.json
```

3. Optional: build chrome normally. This ensures generated headers exist and are
up-to-date. clangd will still work without this step, but it may give errors or
inaccurate results for files which depend on generated headers.

```
ninja -C out/Default chrome
```

4. Optional: configure clangd to use remote-index service for an instant
   project-wide index and reduced local CPU and RAM usage. See
   [instructions](https://github.com/clangd/chrome-remote-index/blob/main/docs/index.md).

5. Use clangd in your favourite editor, see detailed [instructions](
https://clangd.llvm.org/installation.html#editor-plugins).

    * Optional: You may want to add `--header-insertion=never` to the clangd
      flags, so that your editor doesn't automatically add incorrect #include
      lines. The feature doesn't correctly handle some common Chromium headers
      like `base/functional/callback_forward.h`.

## Background Indexing

clangd builds an incremental index of your project (all files listed in the
compilation database). The index improves code navigation features
(go-to-definition, find-references) and code completion.

* clangd only uses idle cores to build the index, you can limit the total amount
  of cores by passing the *-j=\<number\>* flag;
* the index is saved to the `.cache/clangd/index` in the project root; index
  shards for common headers e.g. STL will be stored in
  *$HOME/.cache/clangd/index*;
* background indexing can be disabled by the `--background-index=false` flag;
  Note that, disabling background-index will limit clangd’s knowledge about your
  codebase to files you are currently editing.

Note: the first index time may take hours (for reference, it took 2~3 hours on
a 48-core, 64GB machine). A full index of Chromium (including v8, blink) takes
~550 MB disk space and ~2.7 GB memory in clangd.

Note: [Remote-index service](https://github.com/clangd/chrome-remote-index/blob/main/docs/index.md)
replaces background-index with some downsides like being ~a day old (Clangd will
still know about your changes in the current editing session) and not covering
all configurations (not available for mac&windows specific code or non-main
branches).

## Questions

If you have any questions, reach out to
[clangd/clangd](https://github.com/clangd/clangd) or clangd-dev@lists.llvm.org.