Optimize binary size

Adjust build config (.cargo/config.toml)

Here is an example of a binary size-optimized .cargo/config.toml. If you use rktk-template, you don't need modify it, as it is already included in the template.

[profile.release]
debug = true
opt-level = "z"
lto = "fat"
panic = "abort"
rustflags = ["-Zlocation-detail=none"]
codegen-units = 1

[unstable]
build-std = ["core", "alloc"]
build-std-features = ["panic_immediate_abort", "optimize_for_size"]

• debug = true

  This enables debug info. This make ELF binary big, but the final binary flashed to mcu (UF2 file) is not affected. So it is recommended to enable this.

• opt-level = "z"

  This enables size optimization of compiler. This is the most important option to reduce binary size. For other options, see cargo book. The documents suggets that "s" sometimes produces smaller binaries than "z", but in my experience, "z" is always smaller than "s".

• lto = "fat"

  This enables link time optimization. This is also important to reduce binary size.

• panic = "abort" and rustflags = ["-Zlocation-detail=none"]

  These two lines allow the program to simply stop at panic. This reduces the binary size. However, you will no longer receive messages in case of panic.

  Note that second line is only available in nightly rust.

• build-std and build-std-features

  By default, program will be compiled with precompiled core and alloc crates. By enabling this, these crates are compiled from source code with size optimization.

  These options are only available in nightly rust. Note that these options cannot be used with per-package-target unstable feature