Below, the mem.cpy segfaults as the ptr in the buffer[0..] slice results as 0x0 (seems to only be visible in debuggers while printing out with std.debug.warn gives the correct address). Other code shows the slice ptr being the first byte of the address and not necessarily 0x0.

const std = @import("std");
threadlocal var buffer: [42]u8 = undefined;
pub fn main() void {
    std.mem.copy(u8, buffer[0..], "Hello world");
}

Was not able to reproduce on windows, macosx, and linux with --single-threaded. Getting the slice using the method below appears to provide the correct address however.

const buf = @intToPtr([*]u8, @ptrToInt(&buffer[0]))[0..buffer.len];