fbuild should resolve project-local boards/*.json like PlatformIO does #515

Problem

fbuild's board database is baked in at compile time and there is no runtime fallback to project-local boards/<id>.json, so a well-formed platformio.ini that builds cleanly under PlatformIO fails under fbuild with:

build error: config error: unknown board 'lpc845brk' (no built-in defaults)

Reproducible failure from a real CI run:
https://github.com/zackees/ArduinoCore-LPC8xx/actions/runs/27172681316/job/80214987717

Where the gap is in the code

crates/fbuild-config/src/board/db.rs:

static BOARDS_DIR: include_dir::Dir =
    include_dir::include_dir!(\"$CARGO_MANIFEST_DIR/assets/boards/json\");

The board DB is a compile-time include_dir! snapshot of crates/fbuild-config/assets/boards/json/. There is no code path that reads <project>/boards/*.json, ~/.platformio/boards/*.json, or the resolved platform package's boards/ directory. crates/fbuild-config/src/board/loaders.rs calls get_board_defaults(board_id); when that returns None, the build aborts with the message above.

How PlatformIO solves the same problem

PlatformIO's board resolution is dynamic and cascades:

1. <project>/boards/<id>.json (project-local override — this is what makes the failing case work in pio)
2. ~/.platformio/boards/<id>.json (user override)
3. <installed_platform>/boards/<id>.json (platform-provided)

A board JSON sitting next to platformio.ini is automatically picked up; no special config required.

Proposed feature

Add a runtime loader to fbuild-config that scans <project>/boards/*.json (the dir containing the resolved platformio.ini) and merges those entries into the lookup BEFORE falling through to the "unknown board" error. Optionally also resolve ~/.platformio/boards/ and the installed-platform boards/ for full PIO parity, but project-local alone closes the most common case.

Sketch:

// pseudo
pub(super) fn get_board_defaults(board_id: &str) -> Option<HashMap<String, String>> {
    if let Some(v) = get_board_db().get(board_id) {
        return Some(flatten(v));
    }
    if let Some(v) = load_project_local_board(board_id) {  // NEW
        return Some(flatten(v));
    }
    None
}

load_project_local_board would look in <project_dir>/boards/<board_id>.json using whatever already locates the active platformio.ini (e.g., the same path the build is being invoked against — fbuild build <dir> -e <env>).

Why this matters

• Generalizes. Every new board for any platform works the moment its JSON exists in the project. No per-board upstream PR + fbuild release cycle.
• PIO parity. The fbuild marketing line is "directly compatible with platformio.ini". Project-local board JSONs are a documented part of PIO's platformio.ini story; not honoring them is a divergence users will hit again and again.
• Unblocks LPC8xx today without waiting on a full Stage 2 LPC8xx orchestrator (Native support for NXP LPC8xx (LPC845-BRK, LPCXpresso804, LPCXpresso845-MAX) #513). LPC8xx is just the first reproducer — anyone using a custom or pre-release board will hit the same wall.

Context: the failing platformio.ini

For reference, the (working under pio, failing under fbuild) config is:

[platformio]
src_dir = examples/Blink

[env]
platform = https://github.com/zackees/platform-nxplpc-arduino.git
framework = arduino

[env:lpc845brk]
board = lpc845brk
board_build.ldscript = \${platformio.packages_dir}/framework-arduino-lpc8xx/linker_scripts/gcc/lpc845_flash.ld

with boards/lpc845brk.json sitting at the project root. PlatformIO finds it; fbuild doesn't.

Related

• Native support for NXP LPC8xx (LPC845-BRK, LPCXpresso804, LPCXpresso845-MAX) #513 — broader LPC8xx tracking issue. This local-board-resolution gap is a finer-grained, more generally useful fix.
• Workflow source: https://github.com/zackees/ArduinoCore-LPC8xx/blob/main/.github/workflows/_fbuild.yml

cc @zackees