https://github.com/Rust-GCC/gccrs is in progress, but I believe it works enough to be worth playing with. Note that it doesn't implement the borrow checker - the plan is to reuse rustc's borrow checker as a compiler plugin. For now, as I understand it, gccrs itself assumes that the code is valid (i.e., would be accepted by rustc).

https://thephilbert.io/ is the primary developer's blog and has status updates.