Tell us your DIP1000 woes

[Paul Backus]

On Sunday, 25 August 2024 at 13:10:22 UTC, Mike Parker wrote:
> If you've had problems using DIP1000, please post here with a 
> description of what you encountered and any details about it 
> you think we may find helpful. Please provide example code 
> where possible.

Here's an issue I ran into when working on my proof-of-concept 
allocator/container library.

When writing exception-safe code, a common pattern is to save the 
original value of a variable before performing an operation that 
might throw, and then restore that value if the operation fails.

Unfortunately, it is impossible to do this in `@safe` code if the 
variable you want to save and restore is `scope`, because the 
compiler will treat the saved copy as having a shorter lifetime 
than the original.

For example, this code fails to compile when using 
`-preview=dip1000`:

     @safe void example(ref scope int[] var)
     {
         int[] savedValue = var;
         scope (failure) var = savedValue;

         // Do stuff with a that might throw...
     }

...with the following error message:

     onlineapp.d(4): Error: scope variable `savedValue` assigned 
to `ref` variable `var` with longer lifetime

Ideally, the compiler should be able to infer that `savedValue` 
has the *exact same* lifetime as `var` in this code, and allow 
the assignment.