This is a short writeup about how we managed to use Emacs eglot mode
work with ESP-IDF. This should also work equivalently with any other
LSP client implementation in Emacs.
Procedure
Firstly, the clang toolchain for ESP-IDF should be installed by
idf_tools.py install esp-clang. From this point, if the
IDF_TOOLCHAIN environment variable is set to clang, then any
freshly configured project will be configured to use the clang
toolchain. When reconfiguring an existing project, it will be
necessary to clean the build/ directory first.
From that point on, entering the ESP-IDF environment will place its
own version of clangd on $PATH. idf.py build will generate a
file titled compile_commands.json in the build directory, which is
required by clangd to find important information in the
project. This file should be symlinked this file to the project root,
i.e ln -s build/compile_commands.json.
To make eglot or another LSP client call the correct version of
clangd and see the correct libraries, a buffer-local environment has
to be set before the mode starts. The best way we have found to do
this is to use direnv in combination with the envrc Emacs
plugin. Install and configure direnv on the system, and write a
.envrc file in the project root which sources the ESP-IDF
environment script. Additionally, enable envrc-global-mode in
Emacs. With this configuration, Emacs will automatically load the
ESP-IDF environment for the project, and eglot or other LSP clients
will also operate correctly in this environment.
Discussion
When visiting ESP-IDF source files outside a directory configured with
the ESP-IDF environment, eglot will misbehave because it is
operating with the default system environment, which is
undesirable. The solution we have been using for this is adding an
.envrc file in the ESP-IDF directory, and symlinking a
build_commands.json from any ESP-IDF project. This does not strike
us as ideal, but it works well enough. There may be a better way, and
possibly simpler to configure clangd by using a .dir-locals.el,
but this has not yet been investigated in any depth.