Teensy 4.x build fails with Arm GNU 15.2 on WireIMXRT.cpp

[zackees]

Summary

FastLED's Teensy 4.1 Blink build now gets past the earlier missing-framework-library issue, but still fails under fbuild before linking. The failure is in PlatformIO's cached framework-arduinoteensy 1.160.0 WireIMXRT.cpp when fbuild compiles it with Arm GNU 15.2.1.

This looks like a fbuild Teensy toolchain compatibility issue, not a FastLED source/file-layout issue: the same generated PlatformIO project builds successfully when run through PlatformIO, using the same Teensy framework version but PlatformIO's Teensy-pinned GCC 11.3.1 toolchain.

Related previous issue: #163, which was closed for the missing SPI/OctoWS2811 library-resolution problem. This appears to be the next blocker after that fix.

Reproduction

From FastLED master on Windows, April 22, 2026:

bash compile teensy41 --examples Blink

FastLED routes teensy41 through fbuild because ci/compiler/fbuild_boards.py includes teensy40 and teensy41.

Observed fbuild invocation:

C:\Users\niteris\dev\fastled10\.venv\Scripts\fbuild.EXE C:\Users\niteris\dev\fastled10\.build\pio\teensy41 build -e teensy41

fbuild version:

fbuild 2.2.0

Failure

Board: Teensy 4.1 / IMXRT1062 @ 600MHz
Memory: 7.75MB Flash, 512.00KB RAM
Toolchain: arm-none-eabi-gcc 15.2.1
...
build error: build failed: compilation failed for C:\Users\niteris\.fbuild\prod\cache\platforms\dl-framework-arduinoteensy-1.160.0\1b0675c97b0fd8d7\1.160.0\libraries\Wire\WireIMXRT.cpp:
...
cores/teensy4/imxrt.h:1356:35: error: 'reinterpret_cast' from integer to pointer
#define IMXRT_CCM (*(IMXRT_REGISTER32_t *)IMXRT_CCM_ADDRESS)
...
libraries/Wire/WireIMXRT.cpp:425:9: note: in expansion of macro 'CCM_CCGR2'
        CCM_CCGR2, CCM_CCGR2_LPI2C1(CCM_CCGR_ON),
...
libraries/Wire/WireIMXRT.cpp:448:9: note: in expansion of macro 'CCM_CCGR6'
        CCM_CCGR6, CCM_CCGR6_LPI2C4_SERIAL(CCM_CCGR_ON),

The failing source is defining constexpr TwoWire::I2C_Hardware_t objects initialized with CCM_CCGR2 / CCM_CCGR6, which expand through IMXRT_CCM in the Teensy core headers.

Control: PlatformIO succeeds

Using the same generated project:

uv run pio run --project-dir .build\pio\teensy41 -e teensy41

Result:

SUCCESS Took 196.77 seconds

PlatformIO package set:

framework-arduinoteensy @ 1.160.0 (1.60)
toolchain-gccarmnoneeabi-teensy @ 1.110301.0 (11.3.1)

So the important observed difference is fbuild's generic Arm GNU 15.2.1 toolchain versus PlatformIO's Teensy toolchain 11.3.1.

Suspected Cause

The fbuild Teensy orchestrator currently uses the generic ArmToolchain:

• crates/fbuild-build/src/teensy/orchestrator.rs calls fbuild_packages::toolchain::ArmToolchain::new(...)
• crates/fbuild-packages/src/toolchain/arm.rs hard-codes Arm GNU Toolchain 15.2.Rel1

Teensyduino framework-arduinoteensy 1.160.0 appears to expect the PlatformIO/PJRC-compatible Teensy GCC 11.3.1 package. GCC 15 rejects code that GCC 11.3.1 accepts in WireIMXRT.cpp / imxrt.h.

Expected Behavior

fbuild build -e teensy41 should compile the same generated FastLED Blink project that PlatformIO compiles, or fbuild should clearly pin/use a Teensy-compatible toolchain for Teensy 4.x instead of the generic newest Arm GNU toolchain.

A likely fix is to add/use a Teensy-specific ARM GCC package matching PlatformIO's toolchain-gccarmnoneeabi-teensy @ 1.110301.0, at least for framework-arduinoteensy 1.160.0.

[zackees]

FastLED-side tracking issue updated here: FastLED/FastLED#2365