Binary size

RMK has included many optimizations by default to of binary size. But there are still some tricks to reduce the binary size more. If you got linker error like:

= note: rust-lld: error: 
        ERROR(cortex-m-rt): The .text section must be placed inside the FLASH memory.
        Set _stext to an address smaller than 'ORIGIN(FLASH) + LENGTH(FLASH)'

or some errors occur when writing configs to flash, that means that your microcontroller's internal flash is not big enough.

There are several approaches to solve the problem:

Change DEFMT_LOG level

Logging is quite useful when debugging the firmware, but it requires a lot of flash. You can change the default logging level to error at .cargo/config.toml, to print only error messages and save flash:

# .cargo/config.toml

[env]
- DEFMT_LOG = "debug"
+ DEFMT_LOG = "error"

Use panic-halt

By default, RMK uses panic-probe to print error messages if panic occurs. But panic-probe actually takes lots of flash because the panic call can not be optimized. The solution is to use panic-halt instead of panic-probe:

# In your binary's Cargo.toml

- panic-probe = { version = "0.3", features = ["print-defmt"] }
+ panic-halt = "0.2"

The in main.rs, use panic-halt instead:

// src/main.rs

- use panic_probe as _;
+ use panic_halt as _;

Remove defmt-rtt

You can also remove the entire defmt-rtt logger to save flash.

# In your binary's Cargo.toml 
- defmt-rtt = "0.4"

In this case, you have to implement an empty defmt logger.

# src/main.rs
- use defmt_rtt as _;

+ #[defmt::global_logger]
+ struct Logger;
+ 
+ unsafe impl defmt::Logger for Logger {
+     fn acquire() {}
+     unsafe fn flush() {}
+     unsafe fn release() {}
+     unsafe fn write(_bytes: &[u8]) {}
+ }

Enable unstable feature

According to embassy's doc, you can set the following in your .cargo/config.toml

[unstable]
build-std = ["core"]
build-std-features = ["panic_immediate_abort"]

And then compile your project with nightly Rust:

cargo +nightly build --release
# Or
cargo +nightly size --release

This config will reduce about 4-6kb of binary size furthermore.

After applying all above approaches, total binary size of stm32h7 example can be reduced from about 93KB to 54KB, which means the binary size decreases about 42%!

Make storage optional

Making storage feature optional and marking sequential-storage dependency as optional could also reduce the binary size a lot.

This work is not done yet, if there is still binary size issue for your microcontroller, please fire an issue at https://github.com/HaoboGu/rmk/issues and let us know! We'll improve the priority of this feature if we got sufficient feedback.

Any PRs are also welcomed.