[Issue 23021] New: [dip1000] infer return scope from pure nothrow

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23021

          Issue ID: 23021
           Summary: [dip1000] infer return scope from pure nothrow
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: dkorpel at live.nl

Currently, you can pass a scope pointer to a non-scope parameter if:
- the called function is pure nothrow
- there are no parameters with mutable indirections that can store pointers
- the called function's return type has no pointers / `ref`

```
@safe:
bool deref(int* x) pure nothrow;

void main() 
{
    int x;
    deref(&x); // allowed to pass scope pointer
}
```

However, this doesn't work, because the third condition isn't met:
```
@safe:
int* identity(int* x) pure nothrow;

void main() 
{
    int x;
    int* y = identity(&x); // y is now a scope pointer
}
```

Since the first two conditions are met, the compiler could infer `return scope`
on `identity` instead of `scope`.

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