teensy41 on Windows host: GCC LTO temp-file 'mv' strips backslashes from C:\ paths

[zackees]

Summary

When running bash compile teensy41 --examples Ports/PJRCSpectrumAnalyzer on a Windows host with fbuild 2.2.4, the teensy41 toolchain's GCC LTO post-link step shells out to MSYS mv with a Windows path whose backslashes have been stripped, causing mv to fail and collect2.exe to abort the link.

Symptom

End of the build (after compile + LTO):

/usr/bin/mv: cannot stat 'C:UsersniterisccfpQR27.ltrans0.o.tem': No such file or directory
make: [C:\Users\niteris\ccfpQR27.ltrans0.ltrans.o] Error 1 (ignored)
/usr/bin/mv: cannot stat 'C:UsersniterisccfpQR27.ltrans1.o.tem': No such file or directory
make: [C:\Users\niteris\ccfpQR27.ltrans1.ltrans.o] Error 1 (ignored)
collect2.exe: error: ld returned 1 exit status

Compilation finished in 03m:24s with 1 error(s):
  - Board teensy41 failed

The make rule target keeps the backslashes (C:\Users\niteris\ccfpQR27.ltrans0.ltrans.o), but the argument that reaches /usr/bin/mv has them dropped (C:UsersniterisccfpQR27.ltrans0.o.tem). Classic MSYS shell-escape collapse.

Environment

• Host: Windows 10 Pro (10.0.19045)
• Shell: Git Bash / MSYS-based mv
• fbuild: 2.2.4
• Board: teensy41
• Example: Ports/PJRCSpectrumAnalyzer (exercises LTO across enough TUs that -flto partitions into 2+ ltrans pieces, which is what triggers the temp-file mv)

Why this matters

CI runs on Linux so the Linux GCC driver handles LTO temp files internally without shelling out to a separate mv. This bug is host-Windows-only and bites contributors developing locally. Pairs poorly with the otherwise-fixed CMSIS-DSP link (#257) — the prerequisite link errors are gone, so the build now gets all the way to LTO and falls over on the temp-file rename instead.

Root cause guess

GCC's LTO driver (lto-wrapper) generates a make rule like:

$(target): $(temp)
	mv $(temp) $(target)

When the temp path contains literal backslashes (C:\Users\...), passing it through $(temp) into a make recipe that's executed via MSYS bash loses the backslashes because make treats \ as a line-continuation/escape character inside recipe lines on POSIX shells. The build dir for LTO temps is %TEMP% or %USERPROFILE%, both of which are Windows-native paths.

Possible fixes (in fbuild's teensy41 toolchain config)

1. Set TMPDIR to a POSIX-style path before invoking the link step so LTO temps land somewhere with forward slashes (e.g. /tmp/fbuild-lto). This is what arduino-cli does on Windows. Probably the lowest-effort fix.
2. Quote temp paths in the LTO wrapper's emitted make rules. Out of scope for fbuild — would need an upstream GCC patch.
3. Disable LTO for teensy41 on Windows hosts (workaround, would hurt binary size).

Option 1 is what I'd recommend — set TMPDIR=$(cygpath -u "$TEMP") (or equivalent POSIX-style override) in fbuild's teensy41 environment on Windows.

Reproduce

bash compile teensy41 --examples Ports/PJRCSpectrumAnalyzer

on a Windows host with Git Bash / MSYS shell. The example is large enough to force GCC to partition LTO; smaller examples may not trip the bug because they fit in a single ltrans partition that doesn't need the temp-file dance.

Not affected

• Linux CI build (uses GCC's native temp-file rename, no shell-out to mv)
• macOS host (same reason)

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