Warn about using the GC

[Richard (Rikki) Andrew Cattermole]

On 14/01/2024 10:25 PM, Walter Bright wrote:
> Instead of using @nogc, we could have a compiler switch "warn on use of 
> GC" which you can turn on if you like, or not, or turn on the -w switch 
> to treat the gc warnings as errors.

We already have a way of finding out where all the GC allocations is as 
a compiler switch.

This does not need to be a warning.

You want the guarantee in hot loops that a GC allocation will not take 
place there accidentally.

The primary source of this is closure creation. Where it is being stored 
is not scope (such as a function parameter).

I.e. ``@localnogc`` or ``@nogc(scope)``

```d
void hot(float[] a, float b) /* @localnogc */ {
	size_t last;
	accept(() { // error: will allocate closure context with GC
		if (last > a.length)
			return float.init;
		return a[last++] + b;
	});// ok, accept uses GC, this call is not checked.
}

void accept(float delegate() del) {

}
```

The transitive nature of ``@nogc`` where it checks function calls is the 
only behavior that needs to be disabled. Everything else is good for 
such an addition. Although AA mutation and ``.length =`` should probably 
be kept. Too easy to do them without realizing it.

The goal here is to prevent unnecessary work, that is almost certain to 
have been done by accident.