fix(nrf52): map PIO nRF52DK variant to Adafruit pca10056

[zackees]

Summary

Build nRF52840 DK fails on every main run with:

fatal error: variant.h: No such file or directory
cores/nRF5/Uart.h:27:10
    27 | #include "variant.h"

Reproducible: every recent run of the workflow.

Root cause

fbuild ships one nRF52 framework — Adafruit's framework-arduinoadafruitnrf52 (URL hardcoded in crates/fbuild-packages/src/library/nrf52_core.rs).

The nrf52840_dk board JSON declares build.variant = "nRF52DK" (matching PIO upstream — the board-validator agrees). But Adafruit's framework doesn't have a variants/nRF52DK/ directory. It has variants/pca10056/ — PCA10056 is Nordic's product code for the nRF52840-DK, so it's the right hardware. Verified via GitHub API: GET /repos/adafruit/Adafruit_nRF52_Arduino/contents/variants?ref=1.6.1 lists pca10056, no nRF52DK.

variants/nRF52DK/ is the name used in the other nRF52 Arduino framework — sandeepmistry's arduino-nRF5 (the one PIO actually selects for this board upstream). fbuild doesn't ship that one.

PlatformIO papers over the difference invisibly by picking the right framework per board. fbuild can't (yet), so the immediate failure is the variant compile's include path pointing at a nonexistent variants/nRF52DK/.

Fix

Same shape as #319's SAMD build.core = "adafruit" → cores/arduino/ fallback. Add a tiny per-framework alias table so PIO-matching board JSONs still resolve to the right Adafruit-named directory:

fn nrf52_variant_alias(variant_name: &str) -> Option<&'static str> {
    match variant_name {
        // nRF52840-DK: PIO/sandeepmistry name -> Adafruit/PCA product-code name.
        "nRF52DK" => Some("pca10056"),
        _ => None,
    }
}

fn resolve_nrf52_variant_dir(variants_dir: &Path, variant_name: &str) -> PathBuf {
    let primary = variants_dir.join(variant_name);
    if primary.is_dir() { return primary; }
    if let Some(aliased) = nrf52_variant_alias(variant_name) {
        let candidate = variants_dir.join(aliased);
        if candidate.is_dir() { return candidate; }
    }
    primary
}

Wired into Nrf52Cores::get_variant_dir. Both pre-existing Adafruit boards (adafruit_feather_nrf52840_sense, nice_nano_nrf52840, etc.) continue to resolve literally — alias only kicks in when the literal dir is missing.

Why not just install sandeepmistry's framework?

Considered (and recommended in the issue as the "ideal" fix), but it's a substantial new package + resolver + per-board framework selection. The alias table is a one-line fix per affected board and unblocks CI immediately. We can revisit the framework-set decision when more sandeepmistry-only boards land.

Test plan

• cargo test --lib -p fbuild-packages nrf52 — 11/11 passing (4 new tests: alias resolves, literal-when-present wins, no-match returns literal, literal-takes-precedence-over-alias)
• cargo test --lib -p fbuild-packages — 410+/0 passing
• CI Build nRF52840 DK workflow turns green on next main push
• Sanity-check Build Adafruit Feather NRF52840 Sense continues to pass (its board JSON uses Adafruit-native feather_nrf52840_sense so the alias path is never entered)

Fixes #321.

[coderabbitai]

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Run ID: 68707846-f936-4af8-8581-8888cdd1db1c

📥 Commits

Reviewing files that changed from the base of the PR and between b7560aa and 5078e0b.

📒 Files selected for processing (1)

• crates/fbuild-packages/src/library/nrf52_core.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/nrf52-variant-alias-321

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