fmt: handle printing sentinel terminated arrays

[bangbangsheshotmedown]

I came up with all that casting by trial&error, but something tells me that there should be a much simpler solution..

[alexrp]

test
+- test-modules
   +- test-std
      +- run test std-wasm32-wasi.0.1...0.2.2-musl-generic-Debug
         +- zig test Debug wasm32-wasi 1 errors
/home/ci/actions-runner5/_work/zig/zig/lib/std/fmt.zig:668:90: error: @ptrCast increases pointer alignment
/home/ci/actions-runner5/_work/zig/zig/lib/std/fmt.zig:668:99: note: '*const anyopaque' has alignment '1'
/home/ci/actions-runner5/_work/zig/zig/lib/std/fmt.zig:668:90: note: '*const u16' has alignment '2'
/home/ci/actions-runner5/_work/zig/zig/lib/std/fmt.zig:668:90: note: use @alignCast to assert pointer alignment

[rohlem]

With #22505 merged, the new implementation should just use .sentinel(), which takes care of all the casting. Something like (untested):

if (info.sentinel()) |s| {
  const index = std.mem.indexOfSentinel(info.child, s, value[0..]);
  return formatBuf(value[0..index], options, writer);
}

[Vexu]

You do not need to build Zig to test changes to the standard library, you just need to point --zig-lib-dir at your modified lib directory.

https://github.com/ziglang/zig/wiki/Contributing#directly-testing-the-standard-library-with-zig-test