Build_opt.h only used when compiled with LTO

[bbc25185]

Describe the bug
After testing, I noticed that the Build_opt.h file is used only if, as a compilation optimization, one with LTO is chosen.

To Reproduce
The Build_opt.h file contains the following code:
-DSERIAL_RX_BUFFER_SIZE=512 -DSERIAL_TX_BUFFER_SIZE=512
-DMAVLINK_COMM_NUM_BUFFERS=4
-DPIN_SERIAL1_RX=PB7 -DPIN_SERIAL1_TX=PB6
-DENABLE_HWSERIAL1
-DPIN_SERIAL2_RX=PA3 -DPIN_SERIAL2_TX=PA2
-DENABLE_HWSERIAL2
-DPIN_SERIAL3_RX=PB9 -DPIN_SERIAL3_TX=PB8
-DENABLE_HWSERIAL3
-DPIN_SERIAL4_RX=PA1 -DPIN_SERIAL4_TX=PA0
-DENABLE_HWSERIAL4

Steps to reproduce the behavior:
Selected Board: STM32G0 Series
Board Part Number: Generic G070KBTx
Usart Support: Enabled (No generic Serial)
Compiled with Optimize - Smallest: No error and Ram usage dont change with Buffer size change
Compiled with Optimize - Smallest with LTO: Various error reporting "note: 'SerialX' was previously declared here" and Buffer Size changing with Buffer Size setting.

Expected behavior
The file Build_opt.h always working

Desktop (please complete the following information):

• Windows11
• Arduino IDE version: 2.3.4
• STM32 core version: 2.10.1
• Tools menu settings if not the default: Usart Support: Enabled (no generic Serial)
• Upload method: SWD

Board (please complete the following information):

• Name: STM32G0 Series
• Hardware Revision: Generic G070KBTx

Additional context
To solve the problem, I normally compiled with LTO but due to errors that appeared when I started implementing FreeRtos.
Without LTO the options in the Build_opt.h file are not applied for my project.

[fpistm]

Hi @bbc25185
Here my result:

Default config::

Sketch uses 19996 bytes (15%) of program storage space. Maximum is 131072 bytes.
Global variables use 1432 bytes (3%) of dynamic memory, leaving 35432 bytes for local variables. Maximum is 36864 bytes.

With your build_opt.h:

Sketch uses 20304 bytes (15%) of program storage space. Maximum is 131072 bytes.
Global variables use 6084 bytes (16%) of dynamic memory, leaving 30780 bytes for local variables. Maximum is 36864 bytes.

Warning message:

warning: "ENABLE_HWSERIAL4" redefined
   84 |       #define ENABLE_HWSERIAL4
      |               ^~~~~~~~~~~~~~~~

That is normal as the default Serial for the GenericG070KBTx is the Serial4. So as you define it again in the build_opt.h....
Maybe you got an Arduino cache issue but there is no issue with build_opt.h.

[bbc25185]

This is my setting:

This is compiled without the build_opt.h file:

And this is compiled with the build_opt.h file:

Am I missing something that the build_opt.h file doesn't work for me unless I enable LTO on the build Optimization?