test(asm): Phase 0.1 — corpus ir-only branch for cross-target examples

When a `.build` target doesn't match the host, the runner can't execute the
example here, so it verifies via `sx ir --target` only: asserts exit + the `.ir`
snapshot (stdout) + diagnostics (stderr), never `.stdout`. An `.ir` snapshot is
REQUIRED in ir-only mode — its absence is a loud failure, never a silent pass.

- corpus_run.test.zig: ir_only flag (target set & !hostMatchesTarget); first
  dispatch arm runs `sx ir`, sets act_exit/act_err/act_ir; skip stdout in both
  update and verify modes; require ir_raw.
- lock fixture 1639-platform-target-cross (asm-free main, target x86_64-linux,
  checked-in .ir). Verified: corrupt .ir => IR mismatch; delete .ir => require
  failure.

Test-infra only; no compiler code. zig build test green (647 corpus, 444 unit).

current/CHECKPOINT-ASM.md

@@ -6,39 +6,34 @@ commit, one step at a time per the cadence rule (no commit may both add a test
 and make it pass).
 
 ## Last completed step
-**0.0** — corpus runner target-gating + `<name>.build` JSON config. Added
-`BuildConfig` (`std.json.parseFromSliceLeaky` → `struct { aot, target }`,
-unknown-key ⇒ `error.UnknownField`) replacing the standalone `.aot` marker;
-migrated the 2 existing `.aot` markers (1226/1227) to `{ "aot": true }`; threaded
-`--target` into the `run`/`build`/`ir` spawns via `withTarget`; added
-`hostMatchesTarget` (shorthand-expand + arch/os token match, `arm64`→`aarch64`)
-gating the execute path. Cross-target mismatch fails **loudly** (placeholder until
-0.1's ir-only branch) — verified the bail fires (`target=linux,
-host=aarch64-macos`). Locked with `examples/1638-platform-target-host.sx`
-(`.build` `{ "target": "macos" }`, runs natively + asserts stdout) + unit tests
-for the JSON parse and `hostMatchesTarget`. `zig build test` green (646 corpus, 0
-failed; 444 unit, 0 failed). Files: `src/corpus_run.test.zig`,
-`examples/1638-*`, `examples/expected/{1226,1227,1638}-*`.
+**0.1** — corpus runner **ir-only branch** for cross-target examples. Replaced
+0.0's loud placeholder bail: when `cfg.target` doesn't match the host (`ir_only`),
+`sweepRoot` skips run/build/exec and verifies via `sx ir --target` only —
+asserting `.exit` (ir cmd) + `.ir` (normalized stdout) + `.stderr`, never
+`.stdout` (write skipped in update mode, assertion skipped in verify mode). An
+`.ir` snapshot is **required** in ir-only mode — its absence is a loud failure
+("needs an .ir snapshot for ir-only mode"). Locked with
+`examples/1639-platform-target-cross.sx` (asm-free `main :: () -> i64 { return 0;
+}`), `.build` `{ "target": "x86_64-linux" }`, + checked-in `.ir`. Verified both
+guards fire: corrupting the `.ir` → IR mismatch; deleting it → the require-failure.
+`zig build test` green (647 corpus, 0 failed; 444 unit). Files:
+`src/corpus_run.test.zig`, `examples/1639-*`.
 
 ## Current state
-Phase 0 step 0.0 landed (test-infra only — no compiler code touched). The corpus
-runner now reads `expected/<name>.build` and threads/gates `--target`; an
-arch-pinned example whose target matches the host **executes**, a mismatch
-currently **bails loudly** ("cross-target ir-only mode not yet implemented"). Phase
-A–E feasibility already confirmed against the live tree (`LLVMGetInlineAsm` /
-`LLVMBuildCall2` / `LLVMAppendModuleInlineAsm` in LLVM@19 `Core.h`; ERR-stream
-`extractvalue`→tuple machinery in `emit_llvm.zig:726-927`; lib-less `extern`, 60
-sites; `--target` a global CLI flag).
+Phase 0 steps 0.0 + 0.1 landed (test-infra only — no compiler code). The corpus
+runner reads `expected/<name>.build` (JSON `{ aot, target }`), threads `--target`,
+and gates on host arch+os: a matching target **executes** (full exit/stdout/stderr
++ optional `.ir`); a mismatch runs **ir-only** (`sx ir --target`, asserting
+exit+ir+stderr, `.ir` required). Phase A–E feasibility already confirmed against
+the live tree (`LLVMGetInlineAsm` / `LLVMBuildCall2` / `LLVMAppendModuleInlineAsm`
+in LLVM@19 `Core.h`; ERR-stream `extractvalue`→tuple in `emit_llvm.zig:726-927`;
+lib-less `extern`, 60 sites; `--target` a global CLI flag).
 
 ## Next step
-**0.1** — implement the **mismatch ⇒ ir-only** branch in `sweepRoot` (replace the
-loud placeholder bail): when `cfg.target` doesn't match the host, skip
-run/build/exec and assert only `.exit`+`.ir`+`.stderr` from `sx ir --target`;
-require an `.ir` snapshot (loud failure if absent). Lock with
-`examples/16xx-platform-target-cross.sx` (asm-free `() -> i64 { return 0; }`),
-`.build` `{ "target": "x86_64-linux" }`, + a checked-in `.ir` snapshot. Then 0.2
-(CLAUDE.md §Testing/§Test-layout docs for `.build`), then Phase A (`kw_asm`). See
-`PLAN-ASM.md` Phase 0.
+**0.2** — docs: update CLAUDE.md §"Testing"/§"Test layout" to document the
+`<name>.build` JSON config (`aot` + `target` keys, ir-only gating) replacing the
+standalone `.aot` marker prose (~lines 435, 492). Then Phase A (`kw_asm` keyword +
+lex test). See `PLAN-ASM.md` Phase 0 / Phase A.
 
 ## Log
 - (init) Plan + design doc written; ASM stream opened.
@@ -46,6 +41,9 @@ require an `.ir` snapshot (loud failure if absent). Lock with
   marker), `--target` threading, `hostMatchesTarget` execute-gate, loud
   cross-target placeholder bail. Migrated 1226/1227 `.aot`→`.build`; locked with
   1638 fixture + unit tests. `zig build test` green.
+- (0.1) ir-only branch: cross-target examples verify via `sx ir --target` only
+  (exit+ir+stderr, no stdout; `.ir` required). Locked with 1639 fixture; verified
+  corrupt-.ir → mismatch and missing-.ir → loud failure. `zig build test` green.
 
 ## Known issues
 None yet.

examples/1639-platform-target-cross.sx

@@ -0,0 +1,7 @@
+// Phase 0 (ASM stream) test-infra lock: exercises the corpus runner's
+// CROSS-TARGET ir-only path. The `.build` pins `x86_64-linux`, which does NOT
+// match this aarch64 host, so the runner skips run/build/exec and verifies via
+// `sx ir --target x86_64-linux` only — asserting exit + the `.ir` snapshot +
+// stderr (no `.stdout`). Asm-free on purpose: it locks the harness gating, not
+// any inline-asm lowering (that arrives in Phase A+).
+main :: () -> i64 { return 0; }

examples/expected/1639-platform-target-cross.build

@@ -0,0 +1 @@
+{ "target": "x86_64-linux" }

examples/expected/1639-platform-target-cross.exit

@@ -0,0 +1 @@
+0

examples/expected/1639-platform-target-cross.ir

@@ -0,0 +1,6 @@
+
+; Function Attrs: nounwind
+define i32 @main() #0 {
+entry:
+  ret i32 0
+}

examples/expected/1639-platform-target-cross.stderr

@@ -0,0 +1 @@
+

src/corpus_run.test.zig

@@ -314,20 +314,36 @@ fn sweepRoot(
         const is_aot = cfg.aot;
 
         // An example pinned to a non-host target cannot execute here; it routes
-        // to ir-only mode (Phase 0.1). Until that lands, a mismatch must fail
-        // loudly — never silently pass.
-        if (cfg.target) |t| {
-            if (!hostMatchesTarget(t)) {
-                try failures.append(fail_gpa, try std.fmt.allocPrint(fail_gpa, "{s}: cross-target ir-only mode not yet implemented (target={s}, host={s}-{s})", .{ name, t, @tagName(builtin.cpu.arch), @tagName(builtin.os.tag) }));
-                continue;
-            }
-        }
+        // to ir-only mode (verify via `sx ir` only — see the first arm below).
+        const ir_only = if (cfg.target) |t| !hostMatchesTarget(t) else false;
 
         var act_exit: u32 = undefined;
         var act_out: []const u8 = undefined;
         var act_err: []const u8 = undefined;
+        var act_ir: ?[]const u8 = null;
 
-        if (is_aot) {
+        if (ir_only) {
+            // Cross-target: cannot run on this host. Verify via `sx ir` only —
+            // exit code, the IR snapshot (stdout), and diagnostics (stderr). An
+            // .ir snapshot is REQUIRED: without it an arch-pinned example would
+            // assert nothing. Its absence is a loud failure, never a silent pass.
+            if (ir_raw == null) {
+                try failures.append(fail_gpa, try std.fmt.allocPrint(fail_gpa, "{s}: cross-target example (target={s}) needs an .ir snapshot for ir-only mode", .{ name, cfg.target.? }));
+                continue;
+            }
+            const ir_res = std.process.run(a, io, .{
+                .argv = try withTarget(a, &.{ corpus_paths.sx_exe, "ir", rel_path }, cfg.target),
+                .cwd = .{ .path = repo_root },
+                .timeout = deadline(io),
+            }) catch |err| {
+                try failures.append(fail_gpa, try std.fmt.allocPrint(fail_gpa, "{s}: `sx ir` {s}{s}", .{ name, @errorName(err), if (err == error.Timeout) " (>10s)" else "" }));
+                continue;
+            };
+            act_exit = termCode(ir_res.term);
+            act_out = ""; // stdout carries IR (asserted via .ir), not a separate stream
+            act_err = trimNl(try normalizeStd(a, ir_res.stderr));
+            act_ir = trimNl(try normalizeIr(a, ir_res.stdout));
+        } else if (is_aot) {
             // Build a native executable, then run it. The build's own stderr
             // ("compiled: <path>") is intentionally discarded — only the built
             // program's streams are snapshotted. A build failure (e.g. an
@@ -383,10 +399,10 @@ fn sweepRoot(
             act_err = trimNl(try normalizeStd(a, run_res.stderr));
         }
 
-        // --- sx ir (only when a snapshot already exists; mirrors the shell's
-        // `$has_ir` gate — update mode never CREATES new .ir files) ---
-        var act_ir: ?[]const u8 = null;
-        if (ir_raw != null) {
+        // --- sx ir (execute-mode only; ir-only produced act_ir above). Runs
+        // when a snapshot already exists; mirrors the shell's `$has_ir` gate —
+        // update mode never CREATES new .ir files. ---
+        if (!ir_only and ir_raw != null) {
             const ir_res = std.process.run(a, io, .{
                 .argv = try withTarget(a, &.{ corpus_paths.sx_exe, "ir", rel_path }, cfg.target),
                 .cwd = .{ .path = repo_root },
@@ -404,7 +420,7 @@ fn sweepRoot(
         // --- update mode: overwrite snapshots with freshly-normalized output ---
         if (corpus_paths.update_goldens) {
             try writeGolden(io, a, exp_dir, name, "exit", try std.fmt.allocPrint(a, "{d}", .{act_exit}));
-            try writeGolden(io, a, exp_dir, name, "stdout", act_out);
+            if (!ir_only) try writeGolden(io, a, exp_dir, name, "stdout", act_out);
             try writeGolden(io, a, exp_dir, name, "stderr", act_err);
             if (act_ir) |ir| try writeGolden(io, a, exp_dir, name, "ir", ir);
             updated += 1;
@@ -422,7 +438,7 @@ fn sweepRoot(
         var diag: std.ArrayList(u8) = .empty;
         if (act_exit != exp_exit)
             try diag.appendSlice(a, try std.fmt.allocPrint(a, "  exit: expected={d} actual={d}\n", .{ exp_exit, act_exit }));
-        if (!std.mem.eql(u8, act_out, exp_out))
+        if (!ir_only and !std.mem.eql(u8, act_out, exp_out))
             try appendDiff(a, &diag, "stdout", exp_out, act_out);
         if (!std.mem.eql(u8, act_err, exp_err))
             try appendDiff(a, &diag, "stderr", exp_err, act_err);