fix(build): unbreak ESP32 sketch builds — gcc cwd/-o path consistency (closes #282)

[zackees]

Summary

Every ESP32 sketch build on main has been failing in CI since 2026-05-28 with HWCDC.cpp: fatal error: opening dependency file tests/platform/esp32dev/.fbuild/build/esp32dev/quick/core/HWCDC_57cf.cpp.d: No such file or directory (and the same for ColorFormat.c and other framework core/ sources). Closes #282.

Root cause: latent asymmetry in compile_source. When the CLI is invoked with a relative project_dir (which CI does — fbuild build tests/platform/esp32dev ...), the relative output reaches compile_source and hits zccache::compile_cwd_from_output (which canonicalizes the workspace to absolute) paired with zccache::path_arg_for_compile_cwd (which short-circuits on relative paths and returns the raw relative string). gcc then ran with absolute CWD plus a relative -o, resolving to a doubled path (/.../fbuild/tests/platform/esp32dev/tests/platform/esp32dev/.fbuild/...) whose core/ parent was never create_dir_all'd.

The bug landed weeks ago (#191/#193) but stayed hidden because the soldr build cache restored .o files on every CI run, skipping the cold-compile dispatch. The setup-soldr@v0 floating tag picking up soldr 0.7.33 → 0.7.42 changed the toolchain-cache hash, invalidated every prior .o, and the latent bug surfaced on every cold framework core/ source.

Fix

Promote source and output to absolute paths at the top of compile_source via a small absolute_from_cwd helper (equivalent in intent to std::path::absolute, hand-written because clippy enforces MSRV 1.75 and that function is stable since 1.79). With absolute inputs, compile_cwd_from_output and path_arg_for_compile_cwd produce a consistent (cwd, -o) pair that joins back to the original physical file.

Test plan

• soldr cargo test -p fbuild-build --lib compiler:: — 97 passed, 0 failed, including 3 new tests:
  • absolute_from_cwd_is_identity_on_absolute_paths
  • absolute_from_cwd_promotes_relative_paths
  • compile_path_contract_pairs_cwd_and_output_arg_for_282
• soldr cargo test -p fbuild-build --lib — 543 passed, 0 failed (no regressions).
• Build ESP32 Dev (and the other 5 currently-red ESP32 board workflows) pass on this PR — this is the regression guarantee for the actual CI scenario; once green, ESP32 sketch builds fail when project_dir is relative — compile_cwd vs -o path mismatch (missing core/ parent dir) #282 is verified end-to-end.

Does NOT require pinning zackees/setup-soldr — soldr can keep floating on v0.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Fixed an issue where output paths were not resolved correctly during compilation, which could cause build failures in certain directory configuration scenarios.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

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Run ID: d653f3a4-c8c3-4924-ad1b-ed1cd0912494

📥 Commits

Reviewing files that changed from the base of the PR and between f9fe196 and 16e044c.

📒 Files selected for processing (1)

• crates/fbuild-build/src/compiler.rs

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📝 Walkthrough

Walkthrough

This PR fixes build failures in cold-cache ESP32 compilation when project_dir is relative. A relative output path combined with an absolute compile cwd causes gcc to write dependency files to a non-existent doubled path. The fix normalizes source and output to absolute paths before computing the zccache compile cwd, ensuring consistent path resolution.

Changes

Compilation Path Normalization

Layer / File(s)	Summary
Path normalization helper crates/fbuild-build/src/compiler.rs	absolute_from_cwd converts relative paths to absolute using std::env::current_dir() without canonicalizing symlinks, with fallback to original path on error.
Compile source path resolution crates/fbuild-build/src/compiler.rs	compile_source calls absolute_from_cwd on both source and output before computing compile_cwd and -o arguments to prevent doubled paths when resolving relative output against absolute cwd.
Regression and unit tests crates/fbuild-build/src/compiler.rs	Issue #282 regression test validates cwd.join(path_arg_for_compile_cwd(abs_output, cwd)) resolves to the same absolute output path; unit tests verify absolute_from_cwd is identity for absolute paths and correctly promotes relative inputs.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• FastLED/fbuild#193: Adjusts zccache compile cwd and path normalization in the same compiler.rs file to ensure output/source paths resolve correctly relative to computed compile cwd.
• FastLED/fbuild#273: Modifies path handling that feeds into path_arg_for_compile_cwd, directly overlapping in the same path-resolution logic for compile output paths.

Poem

🐇 A rabbit hops through paths absolute and true,
No more doubled trails when cwd meets -o,
From relative tangles, a helper breaks through,
Normalize early, let the cold compiles flow! 🏗️

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately and specifically describes the main fix: restoring ESP32 sketch builds by ensuring gcc receives consistent absolute paths for both cwd and -o arguments, directly addressing issue #282.
Linked Issues check	✅ Passed	The PR fully implements the narrow fix preferred in issue #282: normalizing output and source to absolute paths in compiler.rs before computing compile_cwd, ensuring consistent cwd/-o path resolution. Regression tests for issue #282 are included.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to fixing the cwd/-o path mismatch in compile_source and adding regression tests for issue #282; no unrelated modifications are present.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/compile-cwd-relative-project-dir-282

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