DIP 1021--Argument Ownership and Function Calls--Community Review Round 1

[Mike Franklin]

On Monday, 22 July 2019 at 08:45:11 UTC, Olivier FAURE wrote:

> Assuming you compile with dip1000 and dip1021, foo and bar in 
> the above code would have to be marked `return` (that is, 
> `this` is a `return ref`) or trigger a compile error.

Indeed. That seems to work.

> The compiler would presumably be conservative and assumes that 
> they always return references to overlapping data, which is 
> usually what you want (it's what the Rust compiler does, at 
> least).

I'd rather not make any assumptions.  Walter, what say you?  
Given the code below (with added `return` lifetime annotations), 
how would DIP1021 behave?

--- someLibrary.d
module someLibrary;

import core.stdc.stdlib;

struct S
{
     int a = 1;
     int b = 2;

     ref int foo() @safe return
     {
         if ((rand() % 2) == 0)
             return a;
         else
             return b;
     }

     ref int bar() @safe return
     {
         if ((rand() % 4) == 0)
             return a;
         else
             return b;
     }
}

--- main.d
import someLibrary;

void doSomething(scope ref int a, scope ref int b) @safe
{
     // whatever...
}

void main() @safe
{
     S s;
     for(int i = 0; i < 100; i++)
     	doSomething(s.foo(), s.bar());
}

Thanks,
Mike