[Issue 23318] New: GCAllocator should not implement deallocate

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23318

          Issue ID: 23318
           Summary: GCAllocator should not implement deallocate
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: snarwin+bugzilla at gmail.com

Currently, a container instantiated using GCAllocator is not usable in @safe
code:

---
/+dub.sdl:
dependency "emsi_containers" version="~>0.9.0"
+/
import containers.dynamicarray;
import std.experimental.allocator.gc_allocator;

void main() @safe
{
    auto a = DynamicArray!(int, GCAllocator)();
}
---

Attempting to compile the above program produces the following error message:

---
onlineapp.d(9,10): Error: `@safe` function `D main` cannot call `@system`
destructor `containers.dynamicarray.DynamicArray!(int, GCAllocator,
false).DynamicArray.~this`
---

The destructor is @system because it calls GCAllocator.deallocate, which in
turn calls GC.free [1]--in other words, because the container is freeing its
memory manually, rather than relying on the GC to free it automatically.

Since GCAllocator is intended to be std.experimental.allocator's interface to
D's built-in GC, it should match the behavior of the GC as closely as possible.
In particular, a container instantiated with GCAllocator should behave the same
way as a container that allocates with `new` directly w.r.t. memory management.
Such a container would *not* free its memory manually on destruction.

To resolve this inconsistency, GCAllocator.deallocate should be removed.

[1]
https://github.com/dlang/phobos/blob/v2.100.1/std/experimental/allocator/gc_allocator.d#L89-L93

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