flush
A buffered file or stream writer, written to and never flushed.
var buf: [256]u8 = undefined;
var fw = std.Io.File.stdout().writerStreaming(io, &buf);
try fw.interface.print("hello from greet\n", .{});
The writer collects what it is given in buf, and only flush (or end, for a file) sends it on. Here the function returns, fw goes with it, and the line is never written. It compiles clean in every build mode, prints nothing and reports nothing. A command that prints why it failed to a buffered stderr and then returns 1 exits with the right status and no message; one that prints more than the buffer holds loses only the last part. Code in the shape of the old API, where std.io.getStdOut().writer() wrote straight through, reads the same.
The fix is try fw.interface.flush(); after the last write, or try fw.end(); for a file.
What flush reports
A var local made by a call to a method named writer or writerStreaming, or by File.Writer.init or .initStreaming, whose last argument, the buffer, is not &.{}, and written through its interface later: a call on fw.interface, or on a local made with &fw.interface, other than flush. It is reported at the declaration when nothing after it in its block flushes it (fw.interface.flush(), w.flush() through such a local, or the writer’s own fw.flush() or fw.end(), in a defer or not) or hands it on: &fw, &fw.interface or such a local passed to a call, stored, or returned.
Not reported: an unbuffered writer (&.{}), which writes straight through; Allocating and fixed writers, whose buffer is the result; a reader; and a writer handed to a call, which only the callee can judge.
Found in real code
Over Zig 0.17.0’s standard library and compiler sources and the application repos, flush reports one, real, in the compiler’s build runner, which saves a fuzz crash input through a buffered writer it never flushes. Zig 0.17.1 fixes it, flushing that writer, and flush reports nothing in its sources. Over fourteen well-known public Zig projects, four in a terminal emulator: three real (two commands whose error message is lost before they exit 1, and one whose plain output loses what is left in its 4 KB buffer at the end) and one a test that writes a file it only deletes.