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Newsgroups: comp.lang.ada
Date: Tue, 24 Sep 2019 04:23:28 -0700 (PDT)
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Subject: Re: Implementing Rust's borrow checked pointers
From: Optikos <optikos@verizon.net>
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Xref: reader01.eternal-september.org comp.lang.ada:57179
Date: 2019-09-24T04:23:28-07:00
List-Id: <comp.lang.ada>

On Tuesday, September 24, 2019 at 4:05:45 AM UTC-5, Lucretia wrote:
> Hi,
> 
> Been talking to someone on Telegraph and he was saying Ada should implement this, just wondering
> whether Ada could? I posed a slight change to access type specification to do this, what do people
> think?
> 
> type P is restricted access X;
> 
> Restricted in this case would mean that once assigned it cannot be re-assigned into or out of with out
> some sort of move operation, which could be implemented as an attribute on the access type.
> 
> A : P := L'Access;
> B : P := A'Move; --  A cannot no longer be used.
> 
> begin
>    A.all ... ; -- raises exception.

No, to be as useful as Rust's borrow checker, instead of raising exception, it needs to be a compile-time error.  The compiler needs to maintain a whole-program directed graph at compile-time, not defer a detection-based localized analysis to run-time.

> I don't know enough about all this to put a complete proposal together, but I think I've got the basics
> understood.
> 
> Luke.