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Date:   Sat, 17 Apr 2021 14:29:15 +0100
From:   Wedson Almeida Filho <wedsonaf@google.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Peter Zijlstra <peterz@infradead.org>, ojeda@kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 00/13] [RFC] Rust support
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On Fri, Apr 16, 2021 at 04:03:07PM +0100, Matthew Wilcox wrote:
> Well, we could do that in C too.
> 
> struct unlocked_inode {
> 	spinlock_t i_lock;
> };
> 
> struct locked_inode {
> 	spinlock_t i_lock;
> 	unsigned short i_bytes;
> 	blkcnt_t i_blocks;
> };
> 
> struct locked_inode *lock_inode(struct unlocked_inode *inode)
> {
> 	spin_lock(&inode->i_lock);
> 	return (struct locked_inode *)inode;
> }

Indeed you can do this kind of thing in C, but as I said before (apologies if
I'm too repetitive on this) Rust forces you to do it the right way, whereas the
lack of enforcement in C leaves room for mistakes.

If you do add extensions to C to add some of these restrictions (and I encourage
you to pursue such extensions as we all benefit from better C), it is likely not
sufficient to reach the level of compile-time guarantee that Rust offers because
you need a whole slew of restrictions/enforcements.

I also note that academics have a formalisation of [a subset of] Rust that show
the soundness of these guarantees and the requirements on unsafe to compose
safely. So we're not talking about guesswork, there are formal machine-checked
proofs published about this (see for example
https://people.mpi-sws.org/~dreyer/papers/safe-sysprog-rust/paper.pdf).