Note: This doesn't work on macOS 10.13+ (High Sierra and up). See Rust's issue #28794.

Live code reloading is very useful during development. However, Emacs does not support unloading modules. Live reloading thus requires a custom module loader, e.g. emacs-rs-module, which is itself a dynamic module.

To use it, load it in Emacs:

(require 'rs-module)

Then use it to load other modules instead of require or module-load:

;; Will unload the old version of the module first.
(rs-module/load "full/path/to/module.so")

cargo doesn't support installing dynamic libs yet, so you have to include emacs-rs-module as a dev dependency to compile it on your own:

[dev-dependencies]
emacs-rs-module = { version = "0.7.0" }

magit-libgit2 is an example of how to set this all up, to have live-reloading on-save.

A future version will have tighter integration with cargo.