I've seen that before - is there an implementation, and also, how do the usual users of Ada feel about it?
I feel like one of the attractions to Ada for this sort of use case is its maturity, and "here's a proposal for a complicated feature, inspired by Rust, a language which young and rapidly evolving and in particular switched to a new borrow checker implementation less than two years ago and shipped it with known unsoundnesses and is working on a new borrow checker implementation as we speak" may not be attractive to Ada's usual users. :) (To be clear, I'm fine with the risk profile of Rust's borrow checker, but in part that's because I'm fine with the risk profile of using the latest stable Rust and picking up any fixes every six weeks. I'm fine with a more powerful feature that lets me develop code quickly and I'm fine with upgrading my compiler every six weeks to pick up any bug fixes; I imagine that people writing airplane control software are probably going to prefer something less powerful that hasn't changed in decades.)
Honestly, I think one of the questions here is community. I don't really know who the Ada community is, beyond press releases from Adacore about planes and trains. I suspect that community simply does not care about safe dynamic allocation, even if it were available as a supported option, so they aren't going to be excited about testing out this feature. Is there a user community who would use this, in production (doesn't have to be a company, the "serious hobbyist" open source dev is fine and probably even preferable here), within the next five years or so? Are there folks writing stuff like high-performance grep replacements in Ada or hardened HTML 5 implementations in Ada?
I'd love to see (or help with!) such a comparison, but I get the sense that Ada's and Rust's use cases are so different that writing a fair apples-to-apples comparison is hard and even finding someone to do it would be hard.