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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Intervention needed?
Date: Mon, 18 Mar 2019 18:36:15 -0500
Organization: JSA Research & Innovation
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Xref: reader01.eternal-september.org comp.lang.ada:55893
Date: 2019-03-18T18:36:15-05:00
List-Id: <comp.lang.ada>

"Optikos" <optikos@verizon.net> wrote in message 
news:6e1977a5-701e-4b4f-a937-a1b89d9127f0@googlegroups.com...
On Tuesday, March 12, 2019 at 5:53:04 PM UTC-4, Randy Brukardt wrote:
>ARG, please leave an immense paper trail of what-conflicts-with-what in
>Ada as defined to achieve a Rust-esque borrow checker in its full 
>perfect-pointer form.

Read the proposals for AI12-0240-1 through AI12-0240-5. The main problem is 
massive amounts of complexity.

>Randy, would Tucker's new ownership proposal come close to 100% 
>feature-parity
>with Rust's borrow checker?

Surely not the latest one, since lots of stuff was dropped from it.

But a better question is whether the Rust borrow checker allows building 
proper ADTs for most data structures. Most of Tucker's proposals didn't have 
a safe way to build typical data structures like a doubly-linked list or the 
parent pointer of a tree structure. Leaving out these backward pointers 
means adding a substantial performance degradation for (possibly) common 
operations like node deletion. Depending on what you're doing, that could be 
a non-starter. I haven't had a chance to actually look at Rust's actual 
rules; Ada is hard enough and as we're in the home stretch for Ada 2020, I 
literally don't have time for much else. (Probably shouldn't be answering 
this message...)

Tucker's latest proposal does address the back pointer problem. So at least 
that can be done with checks.

In any case, an Ada proposal has to be able to build the containers, and 
since the containers are designed with possibly dangling cursors, a totally 
safe system would not be usable with them. The containers do include checks 
that would detect many ownership problems, however, so any lack of 
compile-time safety is definitely mitigated. (It's not 100% safe because of 
the possiblity that a cursor outlive the owner object; in that case, the 
checks are unreliable.)

                          Randy.