[rust-dev] Fwd: &self/&mut self in traits considered harmful(?)

Cameron Zwarich zwarich at mozilla.com
Mon Jun 16 15:17:16 PDT 2014


On Jun 16, 2014, at 3:03 PM, Cameron Zwarich <zwarich at mozilla.com> wrote:

> On Jun 16, 2014, at 2:24 PM, Patrick Walton <pcwalton at mozilla.com> wrote:
> 
>> On 6/16/14 1:04 PM, Sebastian Gesemann wrote:
>>> Am 16.06.2014 19:36, schrieb Patrick Walton:
>>>> On 6/16/14 7:32 AM, Sebastian Gesemann wrote:
>>>>> Assuming this RFC is accepted: How would I have to implement Add for a
>>>>> custom type T where moving doesn't make much sense and I'd rather use
>>>>> immutable references to bind the operands?
>>>> 
>>>> You don't implement Add for those types.
>>> 
>>> As far as I'm concerned that's anything but a satisfactory answer. I
>>> really don't see the point of your RFC. If anything, it seems to make
>>> things worse from my perspective. That's why I asked you for some
>>> clarifications. Of course, you don't owe me anything. And I hope you
>>> don't take this personally.
>> 
>> I don't see much of a use case for `Add` without move semantics. Does anyone have any?
>> 
>> Strings and vectors, perhaps, but I would argue that having to call `.clone()` on the LHS or RHS (as appropriate) is an improvement, because cloning strings and vectors can be very expensive.
> 
> This applies to Mul rather than Add, but if you are multiplying matrices then you want the destination to not alias either of the sources for vectorization purposes, so passing by reference is preferred.

I stated the right case, but the wrong reason. It’s not for vectorization, it’s because it’s not easy to reuse the storage of a matrix while multiplying into it.

Cameron


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