`in` parameters made useful

[Andrei Alexandrescu]

On 8/21/20 1:20 AM, Araq wrote:
> On Thursday, 20 August 2020 at 22:19:16 UTC, Andrei Alexandrescu wrote:
>> On 8/20/20 1:31 PM, IGotD- wrote:
>>> This is interesting on a general level as well and true for several 
>>> programming languages. Let the compiler optimize the parameter 
>>> passing unless the programmer explicitly ask for a certain way (copy 
>>> object, pointer/reference etc.).
>>
>> This has been discussed a few times. If mutation is allowed, aliasing 
>> is a killer:
>>
>> void fun(ref S a, const compiler_chooses S b) {
>>     ... mutate a, read b ...
>> }
>>
>> S x;
>> fun(x, x); // oops
>>
>> The problem now is that the semantics of fun depends on whether the 
>> compiler chose pass by value vs. pass by reference.
> 
> True but in practice it doesn't happen very often. The benefits far 
> outweigh this minor downside.

That seems quite worrisome. A bug rare and subtle that can become 
devastating. Something the Hindenburg captain might have said.

> Plus there are known ways to prevent this 
> form of aliasing at compile-time.

Not if you have globals and/or separate compilation.