DIP1000 scope inference

[Paul Backus]

On Tuesday, 25 October 2022 at 01:35:28 UTC, Steven Schveighoffer 
wrote:
> Does the inferred `scope` make it so that the compiler is 
> *allowed* to allocate the `[4, 5, 6]` literal on the stack? 
> Keep in mind that I never put `scope` here, this is something 
> the compiler did on its own.

No, it does not. This capability was added only for array 
literals, and only for variable initialization:

DMD PR: https://github.com/dlang/dmd/pull/14562
Spec PR (pending): https://github.com/dlang/dlang.org/pull/3442

However, this thread raises an important point: changing the way 
existing language constructs allocate memory in the presence of 
`scope` may cause `@trusted` code which relied on the original 
behavior to become unsound.

For example, the `@trusted` function below is memory safe when 
using the current compiler release, but will become unsafe when 
compiled with DMD 2.101:

```d
@trusted int[] example()
{
     scope example = [1, 2, 3];
     return example;
}
```

The worst part is that the potential memory corruption is 
introduced silently. Users who upgrade to DMD 2.101 will have no 
idea that the ground has shifted beneath their feet until their 
code invokes UB at runtime.