According to the following proof by @veykril, Rust's decl macros _are_ Turing-complete!
The Little Book of Rust Macros:
https://lukaswirth.dev/tlborm/decl-macros/minutiae/turing-completeness.html
Also a somewhat related blog post by Shea Leffler, who proofs Rust's type system is Turing-complete by implementing another Turing-complete language (Smallfuck) with its type system:
https://sdleffler.github.io/RustTypeSystemTuringComplete/
⚠️ I haven't verified any of this!
My meditation time over the past couple of months was to do some #Rust in the evenings. Very calming to have the compiler yell at me, and then trying to get it to compile w/o errors and warnings.
After having started the #RustMacro level, I tried the #RustLifetime level. However, combining the lifetimes with async closures is still something I cannot fathom.
Jan 
Linus Gasser