[Issue 8471] std.stdio.readf should be @trusted

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=8471

--- Comment #6 from Jakub Łabaj <uaaabbjjkl at gmail.com> ---
Currently I see one way to break the safety, which is to not pass a real
pointer, but a structure with unary '*' overloaded:

@safe unittest
{
    struct Unsafe
    {
        int* x;
        ref int opUnary(string s)() if (s == "*")
        {
            int y;
            // int* ptr = &y; // not @safe
            return *x;
        }
    }
    static int x;
    static Unsafe unsafe;
    unsafe.x = &x;
    string text = "10";
    formattedRead(text, "%d ", unsafe); // called by readf
    assert(*unsafe.x == 10);
}

Probably I can't mess up assignment operator nor constructor, because only
builtin types are parsable (constrained by function unformatValue). So I think
making formattedRead / readf accepting only pointers to builtin types is a way
to make them @trusted.

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