https://github.com/nickcoutsos/keymap-editor

Some reading material: https://mcyoung.xyz/2021/06/01/linker-script/
Most commented script ever: https://github.com/wntrblm/Castor_and_Pollux/blob/main/firmware/scripts/samd21g18a.ld (mirror: https://gist.github.com/den-mentiei/5e14f054b670885e6ac47dd6f3273090)

Is it more convenient to use it as compared to arm-none-eabi toolchain?

Check: https://github.com/rust-embedded/cargo-binutils

nice!nano's bootloader can be easily flashed with a uf2 file (https://github.com/microsoft/uf2).

Refer to:
https://github.com/zmkfirmware/zmk/blob/bf2fc68070520a350266875075c912e863fcbf7c/app/boards/arm/nice_nano/CMakeLists.txt

Some ideas from https://getreuer.info/posts/keyboards/symbol-layer/index.html

https://github.com/qmk/qmk_firmware/blob/master/users/callum/readme.md#oneshot-modifiers

https://getreuer.info/posts/keyboards/layer-lock/index.html

Configure the required Rust toolchain via rust-toolchain.toml.

https://github.com/knurling-rs/defmt

Use probe-run as a custom Cargo runner.

Something like this to .cargo/config:

[target.'cfg(all(target_arch = "arm", target_os = "none"))']
runner = "probe-run --chip nRF52840_xxAA"

SoftDevice is the proprietary low-energy Bluetooth implementation from the Nordic Semiconductor.

memory.x needs to be updated to map it, so it is available.

Expose serial over USB for logging/basic controls.

https://github.com/nrf-rs/nrf-hal/blob/master/examples/usb/src/bin/serial.rs

https://github.com/embedded-graphics/embedded-graphics

Custom build.rs might be added, so we always rebuild on any memory.x change.

Refer to https://github.com/rust-embedded/cortex-m-quickstart/blob/master/build.rs

https://getreuer.info/posts/keyboards/caps-word/index.html

Typing heatmap sounds useless but funny :) Refer to https://github.com/qmk/qmk_firmware/blob/652d1d8a6d0a98ab2cff285dc46b91b702adedc6/quantum/rgb_matrix/animations/typing_heatmap_anim.h for the general idea.

A host tool to control the keyboard things.

Use:
https://crates.io/crates/hidapi