.h files are now preprocessed

[fpistm]

Describe the bug
STM32 core allows user to define some value by adding a build_opt.h file at sketch level.
This file is not a standard header file as it is used by the gcc command line with @file option.

https://github.com/stm32duino/wiki/wiki/Customize-build-options-using-build_opt.h

To achieve this a simple recipe hooks prebuild is used. When no file exists at sketch level it creates an empty one.
It works fine until Arduino 1.8.13. Since 1.8.15 it is failed.
.h file extension has been used because Arduino 1.8.x does not allow to create file without a specific extension (ino, c, cpp, h, hpp).

I guess it is linked to #1224

For example a build_opt.h with:
-DHAL_CRC_MODULE_ENABLED

is converted to:

#line 1 "c:\\Users\\<username>\\AppData\\Local\\Temp\\.arduinoIDE-unsaved2021410-37444-aykylf.uyhtw\\sketch_may10a\\build_opt.h"
-DHAL_CRC_MODULE_ENABLED

Below the build result due to the #line addition:
Using command:

$ arduino-cli compile -b STMicroelectronics:stm32:Nucleo_144:pnum=NUCLEO_H743ZI2 C:\\STM32\\arduino\\arduino-1.8.15\\portable\\sketchbook\\blink_minimal\\blink_minimal.ino
arm-none-eabi-g++: error: #line: No such file or directory
arm-none-eabi-g++: error: 1: No such file or directory

Error during build: exit status 1

To Reproduce
Steps to reproduce the behaviour:

1. Install STM32 core version 2.0.0 (https://github.com/stm32duino/BoardManagerFiles/raw/master/package_stmicroelectronics_index.json)
2. Add a build_opt.h file containing the line -DHAL_CRC_MODULE_ENABLED to an example
3. Build the example:
   arduino-cli compile -b STMicroelectronics:stm32:Nucleo_144:pnum=NUCLEO_H743ZI2 C:\\STM32\\arduino\\arduino-1.8.15\\portable\\sketchbook\\blink_minimal\\blink_minimal.ino
   Expected behavior
   Do not preprocess .h file and able to build.

Desktop (please complete the following information):

• CLI version (output of arduino-cli version): arduino-cli.exe alpha Version: 0.18.3 Commit: d710b64 Date: 2021-05-14T12:36:58Z
• OS and platform: all

Additional context

Issue originally reported by an user here:
stm32duino/Arduino_Core_STM32#1387
arduino/arduino-ide#363

Maybe a fix would be to add an extra file extension to avoid to parse it. Ex: .opt or simply .txt.

[matthijskooijman]

@fpistm Would it not be better to fix this by using a different extension that .h? Since build_opt.h does not actually contain C/C++ code, but compiler commandline options, so .h does not seem appropriate? Or is it needed to ensure that the file is copied into the temp directory when compiling a sketch?

[fpistm]

@matthijskooijman
Yes we agree, as mentioned:

.h file extension has been used because Arduino 1.8.x does not allow to create file without a specific extension (ino, c, cpp, h, hpp).

And this is exactly what I've suggested at the end of my OP:

Maybe a fix would be to add an extra file extension to avoid to parse it. Ex: .opt or simply .txt.

For me ".opt" is fine and more explicit: build.opt. And is inline with gcc documention: Read command-line options
https://gcc.gnu.org/onlinedocs/gcc/Overall-Options.html

About the copy I don't yet think about it but it can always be done by the prebuild recipe.

[silvanocerza]

Just chiming in to say that am not ignoring this. We're focusing on developing the Pluggable Discovery and it's taking most of our time, I want to take a look at this as soon as we're done with the Pluggable. No ETA for now.