I was thinking and the problem is, what happens with multiple Builds? They need to combine their commands into a single JSON array, so just appending to a file doesn't work.

Option 1: be explicit. Create a CompilationDatabase, pass it to each Build, emit with a method on CompilationDatabase. Maybe on Drop as well to deal with panicking commands?
Option 2: Create a static CompilationDatabase, each Build (optionally) adds itself to it, on Drop removes itself. When all are removed, the database is written.
Option 3: before executing every new command read the current compilation database from the file, parse it, add the new command entry and write back.

All options have something to dislike, I am not sure which is better. Or maybe there is an option 4.

@alexeyr, what do you think about following option:

let mut database = CompilationDatabase::new();

cc::Build::new().file("blobstore.c").compile_and_store("blobstore", &mut database);
// … more builds

database.store(/* maybe with a path */);

If we find a nice API I'm happy to contribute that API.

Came here to request this very same feature!

In the meantime I think I may be able to capture the compiler commands by wrapping the compiler with a shell script...

In the meantime I think I may be able to capture the compiler commands by wrapping the compiler with a shell script...

That was ultimately our solution in shadow, which is in the process of incrementally migrating from C to Rust. We already had a setup script to wrap our build process, so we changed it to use the bear wrapper to generate compile_commands.json.

@sporksmith Could you point to how you setup the bear wrapper for cc::Build?

@sporksmith Could you point to how you setup the bear wrapper for cc::Build?

https://github.com/shadow/shadow/blob/main/setup#L249

Our build system has accumulated a number of layers. We use a custom python script setup, which invokes cmake, which generates makefiles, which have both pure C targets and Rust targets, some of which have their own build scripts that compile C code.

Instead of running just make to build everything from the top-level generated makefiles, we run bear --append -- make, which adds an LD_PRELOAD to the environment before invoking make, which in turn intercepts and records compiler commands.

For a project that doesn't have the cmake layer, I think you could run bear --append -- cargo build.

bear --append -- cargo build works perfectly here - thanks @sporksmith!