Those cases are definitely supported. Here's a reasonably faithful re-interpretation of Duff's device which generates the right assembly. If you'll take my word for it, code and assembly equivalent to goto in the middle of a loop body isn't an issue either. The only thing you're losing is the unreadable syntactic interleaving you can do in C.

    pub fn copy(T: type, noalias dst: []T, noalias src: []const T, unroll: comptime_int) void {
        if (unroll < 2)
            @compileError("No-op unrolls not supported");
        std.debug.assert(dst.len == src.len);
        std.debug.assert(dst.len > 0);

        var n: usize = (src.len + unroll - 1) / unroll;
        var i: usize = 0;
        duff: switch (src.len % unroll) {
            inline 0,2...unroll => |x| {
                dst[i] = src[i];
                i += 1;
                continue :duff (unroll + x - 1) % unroll;
            },
            inline 1 => {
                dst[i] = src[i];
                i += 1;
                n -= 1;
                if (n == 0)
                    break :duff;
                continue :duff 0;
            },
            else => unreachable,
        }
    }