[Issue 17927] [scope] scope input return value can be escaped

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=17927

--- Comment #2 from Walter Bright <bugzilla at digitalmars.com> ---
Changing the @trusted to @safe makes the first example fail to compile with:

  test.d(6): Error: pointer slicing not allowed in safe functions

Changing String to:

  struct String {
    inout(char)[] opSlice() inout scope @safe {
        return ptr[];
    }

    char[] ptr;
  }

And it now compiles, as it should. Will look at the rest.

--- Comment #3 from Walter Bright <bugzilla at digitalmars.com> ---
Back to the process of stripping things down to the essentials:

--------------------
const(char)* foo1(scope const(char)* ptr) @safe { return ptr; }

inout(char)* foo2(scope inout(char)* ptr) @safe { return ptr; }
--------------------

Produces the expected error messages:

  test.d(1): Error: scope variable ptr may not be returned
  test.d(3): Error: scope variable ptr may not be returned

So add in a bit of complexity:

--------------
struct String {
    const(char)* mem1() const scope @safe { return ptr; }

    inout(char)* mem2() inout scope @safe { return ptr; }

    char* ptr;
}
--------------

Produces:

  test.d(2): Error: scope variable this may not be returned

The message for mem2() is not generated, so the issue is with the 'inout' on
the 'this' parameter.

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