sx/issues/0047-run-output-on-stderr-runtime-on-stdout.md

Symptom

#run print(...) output lands on stderr; runtime print(...) output lands on stdout. The test runner captures both via 2>&1 so they appear interleaved in snapshots, but for a human running sx run foo.sx and piping stdout somewhere — or in any context that separates the streams — the compile-time output silently goes to a different place than the runtime output.

The recently-landed --- build done --- delimiter (commit 2993072) makes the boundary visible in test logs but doesn't fix the underlying split.

Reproduction

#import "modules/std.sx";

configure :: () {
    print("hello from #run\n");
}
#run configure();

main :: () -> s32 {
    print("hello from runtime\n");
    return 0;
}

$ ./zig-out/bin/sx run repro.sx > /tmp/stdout.txt 2> /tmp/stderr.txt
$ cat /tmp/stdout.txt
hello from runtime
$ cat /tmp/stderr.txt
hello from #run

Expected (most consistent): both on stdout — both are print calls in user code; the user doesn't distinguish "build-time print" from "runtime print" at the call site.

Root cause

The interp accumulates print output into an internal buffer (Interpreter.output: []u8) via the out builtin (src/ir/interp.zig:1678-1683). After the interp returns, the buffer is flushed to stderr via std.debug.print from src/core.zig:187 and src/core.zig:190.

The JIT-executed runtime print lowers through BuiltinId.out in emit_llvm.zig:2936-2954 which emits write(1, ptr, len) — directly to fd 1 (stdout).

So the split is at the flush call in core.zig, not at the print mechanism itself.

Investigation prompt

For a fresh session picking this up:

The fix is small in code but touches a few callers. The two std.debug.print("{s}", .{interp.output.items}) sites at src/core.zig:187 and :190 should write to stdout instead.

Options for the stdout write:

1. libc write(1, ...) via std.c.write(1, ptr, len). Direct, no Zig std buffering, interleaves correctly with --- build done --- (also direct to stderr).
2. std.posix.write(std.posix.STDOUT_FILENO, ...) — Zig'''s typed wrapper. Same effect.
3. std.fs.File.stdout() + writeAll — more idiomatic but may bring buffering complexity.

Whichever route, the order of writes vs the --- build done --- delimiter (which currently goes to stderr) matters:

• If both #run output AND delimiter both go to stdout: ordering preserved within stdout's buffer.
• If delimiter stays on stderr and #run goes to stdout: the delimiter might appear out-of-order in mixed-stream captures.

Cleanest: move BOTH to stdout. The compile-error path (in renderErrors) stays on stderr — only successful #run output moves.

Other call sites that might leak #run output to stderr:

• core.zig:187 — invokeByFuncId error path.
• core.zig:190 — invokeByFuncId success path.
• Grep for interp.output to find others.

Verification

After the fix, the repro above should produce:

$ ./zig-out/bin/sx run repro.sx > /tmp/stdout.txt 2> /tmp/stderr.txt
$ cat /tmp/stdout.txt
hello from #run
--- build done ---
hello from runtime
$ cat /tmp/stderr.txt
(empty)

Test snapshots for the 7 tests with top-level #run will need to be re---updated, but the visible content is the same — just on the right stream now.

Full suite + zig build test must still pass.