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digitalmars.D.learn - dip1000, perhaps annotate with return, and vibe-d

reply aliak <something something.com> writes:
Trying to get dip1000 flag in use. I have this error:

Error: returning Optional(null, false).this(value) escapes a 
reference to parameter value, perhaps annotate with return

in this function:

public auto some(T)(auto ref T value) {
     return Optional!T(value); // <-- error on this line
}

And it's instantiated from here:

static Optional!T fromRepresentation(Json value) {
   if (value == Json.undefined) {
     return no!T;
   }
   return some(deserializeJson!T(value)); // <-- instantiated from 
here
}

I put the qualifier "return" on the parameter to some, but did 
nothing. And I put it on Optional.this() as well, but I'm not 
sure where I'm supposed to put it.

The constructor for Optional takes an auto ref T ... I'm not sure 
that makes sense. Does it make sense to take a constructor 
parameter that is copied in to the struct by auto ref if you're 
not calling .move on it explicitly?

And as for vibe-d, I get this when compiling with dip1000:

Also, vibe-d gives me this: 
../../../.dub/packages/vibe-core-1.6.2/vibe-core/source/vibe/inter
al/traits.d(391,2): Error: static assert:  "FileStream does not implement
method "read" of type  safe ulong(scope ubyte[] dst, IOMode mode)"

Has anyone seen that or used vibed with dip1000?
Jul 24 2019
parent reply Paul Backus <snarwin gmail.com> writes:
On Wednesday, 24 July 2019 at 12:54:51 UTC, aliak wrote:

 Trying to get dip1000 flag in use. I have this error:

 Error: returning Optional(null, false).this(value) escapes a 
 reference to parameter value, perhaps annotate with return

 in this function:

 public auto some(T)(auto ref T value) {
     return Optional!T(value); // <-- error on this line
 }

 And it's instantiated from here:

 static Optional!T fromRepresentation(Json value) {
   if (value == Json.undefined) {
     return no!T;
   }
   return some(deserializeJson!T(value)); // <-- instantiated 
 from here
 }

 I put the qualifier "return" on the parameter to some, but did 
 nothing. And I put it on Optional.this() as well, but I'm not 
 sure where I'm supposed to put it.


It should go on the constructor's parameter; i.e.,

this(auto return ref T value) { /* ... */ }

Under the hood, a constructor actually returns the constructed 
value by reference, so the actual signature of the above 
constructor seen by the lifetime checker is:

ref Optional!T __ctor(auto return ref T value)

You can see this for yourself with something like `pragma(msg, 
typeof(Optional!T.__ctor))`.
Jul 24 2019
parent aliak <something something.com> writes:
On Wednesday, 24 July 2019 at 16:23:48 UTC, Paul Backus wrote:

 On Wednesday, 24 July 2019 at 12:54:51 UTC, aliak wrote:

 [...]


 It should go on the constructor's parameter; i.e.,

 this(auto return ref T value) { /* ... */ }

 Under the hood, a constructor actually returns the constructed 
 value by reference, so the actual signature of the above 
 constructor seen by the lifetime checker is:

 ref Optional!T __ctor(auto return ref T value)

 You can see this for yourself with something like `pragma(msg, 
 typeof(Optional!T.__ctor))`.


Thanks! The under the hood stuff was good to know!

I was putting it in the right place but it seems to still have 
been complaining. Ah well. I guess an auto ref on a constructor 
doesn't really make sense anyway.
Jul 25 2019