Performance issue in struct initialization

[Basile B. via Digitalmars-d]

On Saturday, 23 April 2016 at 13:37:31 UTC, Andrei Alexandrescu 
wrote:
> https://issues.dlang.org/show_bug.cgi?id=15951. I showed a few 
> obvious cases, but finding the best code in general is tricky. 
> Ideas? -- Andrei

A new "@noinit" attribute could solve this issue and other cases 
where the initializer is a handicap:

The runtime would skip the copy of the initializer when
1- @noinit is an attribute of an aggregate.
2- a ctor that takes at least one parameter is present.
3- the default ctor is disabled (only a condition for the structs 
or the new free form unions)

// OK
@noinit struct Foo
{
    uint a;
    @disable this();
    this(uint a){}
}

// not accepted because a ctor with parameters misses
@noinit struct Foo
{
    @disable this();
}

// Ok but a warning will be emitted...
@noinit struct Foo
{
    uint a = 1; // ...because this value is ignored
    @disable this();
    this(uint a){}
}

// not accepted because there's a default ctor
@noinit struct Foo
{
    this(){}
}

The rationale is that when there's a constructor that takes 
parameters it's really suposed to initialize the aggregate. At 
least that would be the contract, the "postulate', put by the 
usage of @noinit.