cast(string) for mutable array

[H. S. Teoh]

On Fri, Jul 31, 2026 at 08:10:39AM -0700, H. S. Teoh via Digitalmars-d-learn wrote:
> > On Friday, 31 July 2026 at 13:52:12 UTC, Vindex9 wrote:
> > > Is this code normal? Why can't it be marked with `@safe`? Can only
> > > `idup` be used for `@safe` in such cases?
> > > 
> > > ```d
> > > string fn() @trusted pure {
> > >     char[] arr;
> > >     // some manipulations...
> > >     return cast(string) arr;
> > > }
> > > ```
> > > 
> > > Where is the line between cases where the `@trusted` tag can be
> > > used and cases where the `@system` tag can be used?
> [...]
> 
> If the function is pure, you should be able to just return char[] and
> the caller will be able to assign it to string.  No need for a cast.
> Just specify the return type as `char[]`.
[...]

Proof of concept:

```
import std;

char[] pureStringFunc() pure @safe {	// N.B. returns mutable char[]
	char[] arr;
	arr ~= "abc";
	foreach (i; 0 .. 5) {
		arr ~= i.to!(char[]);
	}
	return arr;
}

void main() @safe {
	string s = pureStringFunc();	// N.B. assigns to (immutable) string
	writeln(s);
}
```

Output:
```
abc01234
```


T

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