[Issue 12857] Don't allow declaring @system function inside @safe block

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=12857

Kenji Hara <k.hara.pg at gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|accepts-invalid             |
           Severity|major                       |enhancement

--- Comment #3 from Kenji Hara <k.hara.pg at gmail.com> ---
OK, this is not a corruption of @safe concept. The original code can be
rewritten as follows:

int* ptr;

static void foo() @system {
    ptr = cast(int*)1;  // stomp memory
}

auto func(int n) @safe {
    if (!ptr)
        ptr = new int(n);
    return &foo;
}

void main() { ... }

And func cannot assume the pointer value won't be corrupted, because someone
can corrupt it.

But, in original code, the static variable `ptr` is declared inside the safe
function. Therefore anyone cannot stomp it from outside of foo.

I think that everything inside @safe function should be safe or trusted. From
the point of view, declaring @system function inside @safe is much dangerous.
By disallowing it, we can stop writing error-prone code.

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