[rust-dev] Another idea on requiring "mut" prefix at call site

Artella Coding artella.coding at googlemail.com
Wed Apr 30 01:34:31 PDT 2014

Ah think I found it :

On Wed, Apr 30, 2014 at 9:28 AM, Artella Coding <
artella.coding at googlemail.com> wrote:

> Hi, do you have a link to the reddit article, so I can have a read? Thanks
> On Wed, Apr 30, 2014 at 8:33 AM, Noam Yorav-Raphael <noamraph at gmail.com>wrote:
>> Hi,
>> I had a bug caused by a function mutating its arguments, and it had
>> occurred to me that it may be a good idea if rust would require a "mut"
>> prefix in that case. I asked on reddit, and was referred to this thread:
>> https://mail.mozilla.org/pipermail/rust-dev/2014-January/007670.html
>> In the above message, Patrick shows a few examples which show that it's
>> hard to come up with rules on which arguments should be prefixed by "mut"
>> that will be sound and complete. I have an idea which may be. The idea is
>> to not look at function arguments but at uses of a variable. Here's a rule:
>> Whenever a variable which was declared with "let mut" is being used in a
>> way that would have been illegal have it not been declared with "let mut",
>> it should be prefixed by "mut", unless it's obvious from the context that
>> it has to be mutable.
>> I think it's quite simple and says exactly what should be the rules in
>> Patrick's examples. What's not well-defined is the "obvious from the
>> context" part. Certainly when a variable is on the left hand side of an
>> assignment there would be no need for "mut" annotation, as well as when
>> it's being prefixed by "&mut". I don't know if there are other cases.
>> (If you're interested in the bug: I had to use a function solve(A, b)
>> which gets a matrix A and a vector b and returns a vector x such that Ax=b.
>> It does Gauss elimination, and for efficiency it modified A and b instead
>> of allocating new arrays. I used it like x = solve(A, b) and then used A
>> again. It was in Fortran, so the arguments A and b were annotated as being
>> "in out", but of course it didn't stop my perfectly looking function from
>> having a hidden bug.)
>> What do you think?
>> Noam
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