Support library development.

[Falven]

Hello,
When developing a library for Arduino as the documentation states, you are to create the following folder structure:

...Documents\Arduino\libraries\my_project
    - .vscode
        arduino.json
    - examples
        -my_example
            my_example.ino
    - src
        my_header_only_library.hpp
    keywords.txt
    library.properties
    README.adoc

Right now, as far as I know, it only understands sketches. If you try to build without a sketch, it will complain to you.
This makes it impossible to develop a library with proper IDE support.

Fine.
So we add "sketch": "examples\\my_example\\my_example.ino" to our arduino.json and reference our library from sketch.

#include <my_header_only_library.hpp>

Because it's considered a System Library, and our project resides in the System Library location, Arduino knows where to find my_header_only_library.hpp. The problem is, because we are including it as a System Library, Intellisense will not work as Intellisense is only generated for non-system libraries (which is a good thing!)

What if instead, we refer to it via a relative path as a header local to this sketch?

#include "../../src/my_header_only_library.hpp"

Arduino does not respect this syntax; it will only respect local paths that are within our sketch's root directory.
However, if we move our library within the sketch's root directory, then we are no longer a library as we are not following the required structure for a library.

So as far as I can see, the only way to develop a Library using VS Code with Intellisense, static analysis, formatting, etc. support is to create a top-level, temporary, sketch that imports your library and you have to delete it once you're done. Because, otherwise, the plugin will not generate a compile_commands.json that references the library source files.

To make matters worse, this scenario does not support header-only libraries as the generated compile_commands.json does not contain a file declaration for header-only libraries which makes it impossible to develop templated libraries, etc.