This is great! Thanks for sharing, it is a cool idea to try bringing QoL improvements from Rust to C++. Got a question:
Does this prevent the RVO/NRVO compiler optimizations for return values? And when those fail, same question for the move-construction for types that do have a move constructor.
Have you considered using the immediately invoked function expression (IIFE) pattern as a standards-compliant alternative to gcc statement expressions?
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Please leave that Frankenstein of a language alone. Stop bolting features on it, it's already a monster.
https://libfn.org/ also is worth a look.