Languages like Rust show us how important memory safety is today, and the planned DIP69 works for @safe code, this means now in D it becomes more important to use @safe functions in most cases.

There are still some cases where you can't use @safe even if they should be safe, some of them are shown here, but Phobos/druntime is getting better, and they will decrease:


void main() @safe {
    import std.stdio, std.algorithm, std.bigint, std.typecons, std.array;
    [1, 2].sort!("a < b", SwapStrategy.stable);
    auto r = [1, 2].sort().release;
    writeln;
    BigInt a;
    a = a + a;
    alias Foo = Tuple!int;
    Foo[] data;
    data.remove!(x => x == Foo());
    int[] b;
    auto c = b.capacity;
    b.schwartzSort!(x => x);
    const r2 = cartesianProduct([1], [1]).array;
    [Typedef!int(1)].array;
}


So perhaps it's a good idea to have @safe functions by default. This is how it could be done:
Step 1) Introduce a "-safe" compiler switch that gives a warning where a function unmarked with @system/@trusted calls a @system/@trusted function or performs memory-unsafe operations. This will help D developers improve Phobos.
Step 2) The functions defined above generate a warning if the -wi/-w switches are used (even if no -safe switch is used).
Step 3) The functions defined above generate a deprecation message (even if no -safe switch is used). The compiler -safe switch is still allowed, but it's not documented in the options help message of the compiler.
Step 4) The functions defined above give an error, and the -safe switch is removed from the compiler (it's not recognized any more).

See also Issue 12941