P5.4: migrate all callers to on_build; delete set_post_link_callback

on_build is now the sole post-build callback mechanism. Migrated the 9 callers
(0602/0603/1611/1614/1615/1616 + the platform bundle_main) from
opts.set_post_link_callback(cb) to on_build(cb), giving each callback the
(opt: BuildOptions) param. Deleted set_post_link_callback from build.sx,
compiler_lib (bound_fns + handleSetPostLinkCallback), and the VM arm.

Reworked the P5 smoke tests for the new semantics: an on_build override REPLACES
the build (must emit+link or delegate), unlike the old post-link callback which
ran after the auto-link. 1662 (queries) + 1664 (override+List-grow) now delegate
to default_pipeline for the real build; deleted 1661/1663 (the primitives are now
exercised by every AOT build). bundle_main invoked with pass_options=true.

Benign 37-.ir churn (build.sx shrank). 703/0 both gates.

examples/0602-comptime-interp-cast-ptr-cmp.sx

@@ -18,7 +18,7 @@ bug :: (cmd: [:0]u8) -> ?R {
     R.{ x = 1 }
 }
 
-post_link :: () -> bool {
+post_link :: (opt: BuildOptions) -> bool abi(.compiler) {
     if r := bug("echo hi") { puts("ok"); } else { puts("null"); }
     true
 }
@@ -28,7 +28,7 @@ post_link :: () -> bool {
 // `set_post_link_callback()` compiler-API calls are permitted.
 configure :: () abi(.compiler) {
     opts := build_options();
-    opts.set_post_link_callback(post_link);
+    on_build(post_link);
 }
 #run configure();
 

examples/0603-comptime-interp-variadic-any.sx

@@ -10,7 +10,7 @@
 
 puts :: (s: [:0]u8) -> i32 extern libc;
 
-cb :: () -> bool {
+cb :: (opt: BuildOptions) -> bool abi(.compiler) {
     a := format("{}", "x");
     puts("1-arg ok");
     b := format("{} {}", "x", "y");
@@ -25,7 +25,7 @@ cb :: () -> bool {
 // `set_post_link_callback()` compiler-API calls are permitted.
 configure :: () abi(.compiler) {
     opts := build_options();
-    opts.set_post_link_callback(cb);
+    on_build(cb);
 }
 #run configure();
 

examples/1611-platform-post-link-callback.sx

@@ -10,14 +10,14 @@
 
 puts :: (s: [:0]u8) -> i32 extern libc;
 
-post_link :: () -> bool {
+post_link :: (opt: BuildOptions) -> bool abi(.compiler) {
     puts("[post-link] callback fired");
     true
 }
 
 configure :: () abi(.compiler) {
     opts := build_options();
-    opts.set_post_link_callback(post_link);
+    on_build(post_link);
 }
 #run configure();
 

examples/1614-platform-macos-bundle.sx

@@ -11,7 +11,7 @@ configure :: () abi(.compiler) {
     opts := build_options();
     opts.set_bundle_path("HelloApp.app");
     opts.set_bundle_id("co.example.hello");
-    opts.set_post_link_callback(bundle_main);
+    on_build(bundle_main);
 }
 #run configure();
 

examples/1615-platform-ios-sim-bundle.sx

@@ -13,7 +13,7 @@ configure :: () abi(.compiler) {
     opts := build_options();
     opts.set_bundle_path("IosSimApp.app");
     opts.set_bundle_id("co.example.iossim");
-    opts.set_post_link_callback(bundle_main);
+    on_build(bundle_main);
 }
 #run configure();
 

examples/1616-platform-ios-device-bundle.sx

@@ -32,7 +32,7 @@ configure :: () abi(.compiler) {
     opts.set_bundle_id("co.swipelab.sxprobe");
     opts.set_codesign_identity("Apple Development: Alexandru Agrapine (DC8VVHJ9W4)");
     opts.set_provisioning_profile("/Users/agra/Downloads/SwipeS32DevProfile.mobileprovision");
-    opts.set_post_link_callback(bundle_main);
+    on_build(bundle_main);
 }
 #run configure();
 

examples/1661-platform-post-link-vm-list.sx

@@ -1,33 +0,0 @@
-#import "modules/std.sx";
-#import "modules/build.sx";
-
-// P5.1 smoke test — the post-link build driver runs on the comptime VM,
-// not the legacy interpreter.
-//
-// The callback allocates and GROWS a `List` (three appends). Comptime List
-// growth works only on the VM; the legacy interp fails it (issue 0141:
-// `struct_get: base has no fields`). So this build SUCCEEDS only because
-// `core.invokeByFuncId` routes the post-link callback through the VM.
-//
-// AOT note: the corpus snapshots an AOT example's *executed binary* streams,
-// and the build step's own stdout is discarded — so the callback's success is
-// observed via the build EXIT CODE. On the VM the callback returns true, the
-// build links cleanly (exit 0), and the binary runs → "runtime main". If the
-// driver regressed to the legacy interp the callback would bail, the build
-// would exit non-zero, and this snapshot would fail.
-
-post_link :: () -> bool abi(.compiler) {
-    names := List(string).{};
-    names.append("alpha");
-    names.append("beta");
-    names.append("gamma");
-    return names.len == 3;
-}
-
-configure :: () abi(.compiler) {
-    opts := build_options();
-    opts.set_post_link_callback(post_link);
-}
-#run configure();
-
-main :: () { print("runtime main\n"); }

examples/1662-platform-build-pipeline-queries.sx

@@ -2,16 +2,12 @@
 #import "modules/build.sx";
 #import "modules/compiler.sx";
 
-// P5.2 smoke test — the build-pipeline query primitives (`c_object_paths` /
-// `link_libraries` / `emit_object`) run on the comptime VM: the list queries
-// return a `List(string)` the VM builds in flat memory; `emit_object` returns
-// the compiler-emitted object path. Registered as a post-link callback (which
-// runs on the VM — core.invokeByFuncId), so the build state is live there.
-//
-// The `#import c` source compiles to one `.o`, so `c_object_paths()` must return
-// a one-element list whose single string is the (non-empty) object path. AOT
-// snapshots the executed binary, so success is observed via the build exit code:
-// the callback returns true only if the VM-built list is well-formed.
+// P5.4 — the build-pipeline query primitives on the comptime VM. A custom
+// `on_build` override inspects the build metadata (`c_object_paths` returns the
+// VM-built `List(string)` of C companion objects), then DELEGATES to the stdlib
+// `default_pipeline` for the real emit + link. The `#import c` source compiles to
+// one object, so `c_object_paths()` must be a one-element list of a non-empty
+// path. AOT runs the produced binary → "runtime main: 42".
 
 #import c {
     #source "1662-platform-build-pipeline-queries.c";
@@ -19,25 +15,18 @@
 
 c_marker :: () -> i64 extern;
 
-check :: () -> bool abi(.compiler) {
+build :: (opt: BuildOptions) -> bool abi(.compiler) {
     objs := c_object_paths();
-    libs := link_libraries();
-    obj := emit_object();              // the compiler-emitted sx object path
-    if obj.len == 0 { return false; }
     if objs.len != 1 { return false; }
     if objs.items[0].len == 0 { return false; }
-    // `link_libraries` must be a well-formed (possibly empty) list — touch each
-    // entry so a malformed fat pointer would fault rather than pass silently.
+    // link_libraries must be a well-formed (possibly empty) list — touch each.
+    libs := link_libraries();
     sum : i64 = 0;
     i : i64 = 0;
     while i < libs.len { sum += libs.items[i].len; i += 1; }
-    return sum >= 0;
+    if sum < 0 { return false; }
+    return default_pipeline(opt);   // the real emit + link
 }
-
-configure :: () abi(.compiler) {
-    opts := build_options();
-    opts.set_post_link_callback(check);
-}
-#run configure();
+#run on_build(build);
 
 main :: () { print("runtime main: {}\n", c_marker()); }

examples/1663-platform-build-pipeline-link.sx

@@ -1,31 +0,0 @@
-#import "modules/std.sx";
-#import "modules/build.sx";
-#import "modules/compiler.sx";
-
-// P5.2b smoke test — the `link` build-pipeline ACTION runs on the comptime VM,
-// dispatching through the host-installed linker hook (the VM can't link itself).
-// The post-link callback (which runs on the VM — core.invokeByFuncId) re-links
-// the build's own objects into a temp output via the sx `link` primitive. A link
-// failure bails → the build fails; success → the callback returns true and the
-// build's binary runs ("runtime main"). AOT snapshots the binary, so the link's
-// success is observed via the build exit code.
-
-relink :: () -> bool abi(.compiler) {
-    objs := List(string).{};
-    cobjs := c_object_paths();
-    i : i64 = 0;
-    while i < cobjs.len { objs.append(cobjs.items[i]); i += 1; }
-    objs.append(emit_object());
-    fws := List(string).{};
-    flags := List(string).{};
-    link(objs, ".sx-tmp/1663-link-out", link_libraries(), fws, flags, "");
-    return true;
-}
-
-configure :: () abi(.compiler) {
-    opts := build_options();
-    opts.set_post_link_callback(relink);
-}
-#run configure();
-
-main :: () { print("runtime main\n"); }

examples/1664-platform-on-build-callback.sx

@@ -1,17 +1,20 @@
 #import "modules/std.sx";
 #import "modules/build.sx";
-#import "modules/compiler.sx";
 
-// P5.3 smoke test — `on_build(build)` registers the build callback (the Phase 5
-// form), and the compiler invokes it post-codegen WITH the BuildOptions handle
-// as an arg (`build :: (opt: BuildOptions) -> bool`). This exercises the VM
-// entry that passes an explicit arg after the implicit context (runEntryArgs).
-// The callback uses the build-pipeline primitives (no opt accessors yet) and
-// returns true; AOT observes success via the build exit code.
+// P5.4 — the `on_build` override. A user `#run on_build(build);` replaces the
+// stdlib `default_pipeline` as the build driver; the compiler invokes it after
+// codegen with the `BuildOptions` handle. This build GROWS a `List` on the build
+// VM (comptime List growth works only on the VM — issue 0141; the legacy interp
+// fails it) then DELEGATES to `default_pipeline` for the real emit + link, so a
+// working binary is still produced → "runtime main".
 
 build :: (opt: BuildOptions) -> bool abi(.compiler) {
-    obj := emit_object();
-    return obj.len > 0;
+    names := List(string).{};
+    names.append("alpha");
+    names.append("beta");
+    names.append("gamma");
+    if names.len != 3 { return false; }
+    return default_pipeline(opt);
 }
 #run on_build(build);
 

examples/expected/1202-ffi-cc-c-large-aggregate.ir

@@ -689,9 +689,6 @@ declare ptr @BuildOptions.jni_main_java_source_at(i64, i64) #0
 ; Function Attrs: nounwind
 declare i64 @build_options() #0
 
-; Function Attrs: nounwind
-declare void @set_post_link_callback(i64, ptr) #0
-
 ; Function Attrs: nounwind
 declare void @on_build(ptr) #0
 

examples/expected/1309-ffi-objc-class-method-lowering.ir

@@ -1191,9 +1191,6 @@ declare ptr @BuildOptions.jni_main_java_source_at(i64, i64) #0
 ; Function Attrs: nounwind
 declare i64 @build_options() #0
 
-; Function Attrs: nounwind
-declare void @set_post_link_callback(i64, ptr) #0
-
 ; Function Attrs: nounwind
 declare void @on_build(ptr) #0
 

examples/expected/1314-ffi-objc-class-dealloc-roundtrip.ir

@@ -1207,9 +1207,6 @@ declare ptr @BuildOptions.jni_main_java_source_at(i64, i64) #0
 ; Function Attrs: nounwind
 declare i64 @build_options() #0
 
-; Function Attrs: nounwind
-declare void @set_post_link_callback(i64, ptr) #0
-
 ; Function Attrs: nounwind
 declare void @on_build(ptr) #0
 

examples/expected/1329-ffi-objc-call-03-selector-sharing.ir

@@ -1183,9 +1183,6 @@ declare ptr @BuildOptions.jni_main_java_source_at(i64, i64) #0
 ; Function Attrs: nounwind
 declare i64 @build_options() #0
 
-; Function Attrs: nounwind
-declare void @set_post_link_callback(i64, ptr) #0
-
 ; Function Attrs: nounwind
 declare void @on_build(ptr) #0
 

examples/expected/1347-ffi-objc-dsl-07-mangling-table.ir

@@ -1209,9 +1209,6 @@ declare ptr @BuildOptions.jni_main_java_source_at(i64, i64) #0
 ; Function Attrs: nounwind
 declare i64 @build_options() #0
 
-; Function Attrs: nounwind
-declare void @set_post_link_callback(i64, ptr) #0
-
 ; Function Attrs: nounwind
 declare void @on_build(ptr) #0
 

examples/expected/1402-ffi-jni-call-03-methodid-sharing.ir

@@ -1185,9 +1185,6 @@ declare ptr @BuildOptions.jni_main_java_source_at(i64, i64) #0
 ; Function Attrs: nounwind
 declare i64 @build_options() #0
 
-; Function Attrs: nounwind
-declare void @set_post_link_callback(i64, ptr) #0
-
 ; Function Attrs: nounwind
 declare void @on_build(ptr) #0
 

examples/expected/1403-ffi-jni-call-04-jint-return.ir

@@ -1183,9 +1183,6 @@ declare ptr @BuildOptions.jni_main_java_source_at(i64, i64) #0
 ; Function Attrs: nounwind
 declare i64 @build_options() #0
 
-; Function Attrs: nounwind
-declare void @set_post_link_callback(i64, ptr) #0
-
 ; Function Attrs: nounwind
 declare void @on_build(ptr) #0
 

examples/expected/1404-ffi-jni-call-05-jlong-return.ir

@@ -1183,9 +1183,6 @@ declare ptr @BuildOptions.jni_main_java_source_at(i64, i64) #0
 ; Function Attrs: nounwind
 declare i64 @build_options() #0
 
-; Function Attrs: nounwind
-declare void @set_post_link_callback(i64, ptr) #0
-
 ; Function Attrs: nounwind
 declare void @on_build(ptr) #0
 

examples/expected/1405-ffi-jni-call-06-jdouble-return.ir

@@ -1183,9 +1183,6 @@ declare ptr @BuildOptions.jni_main_java_source_at(i64, i64) #0
 ; Function Attrs: nounwind
 declare i64 @build_options() #0
 
-; Function Attrs: nounwind
-declare void @set_post_link_callback(i64, ptr) #0
-
 ; Function Attrs: nounwind
 declare void @on_build(ptr) #0
 

examples/expected/1406-ffi-jni-call-07-jboolean-return.ir

@@ -1183,9 +1183,6 @@ declare ptr @BuildOptions.jni_main_java_source_at(i64, i64) #0
 ; Function Attrs: nounwind
 declare i64 @build_options() #0
 
-; Function Attrs: nounwind
-declare void @set_post_link_callback(i64, ptr) #0
-
 ; Function Attrs: nounwind
 declare void @on_build(ptr) #0
 

examples/expected/1407-ffi-jni-call-08-jobject-return.ir

@@ -1183,9 +1183,6 @@ declare ptr @BuildOptions.jni_main_java_source_at(i64, i64) #0
 ; Function Attrs: nounwind
 declare i64 @build_options() #0
 
-; Function Attrs: nounwind
-declare void @set_post_link_callback(i64, ptr) #0
-
 ; Function Attrs: nounwind
 declare void @on_build(ptr) #0
 

examples/expected/1408-ffi-jni-call-09-static.ir

@@ -1183,9 +1183,6 @@ declare ptr @BuildOptions.jni_main_java_source_at(i64, i64) #0
 ; Function Attrs: nounwind
 declare i64 @build_options() #0
 
-; Function Attrs: nounwind
-declare void @set_post_link_callback(i64, ptr) #0
-
 ; Function Attrs: nounwind
 declare void @on_build(ptr) #0
 

examples/expected/1418-ffi-jni-class-08-call.ir

@@ -1183,9 +1183,6 @@ declare ptr @BuildOptions.jni_main_java_source_at(i64, i64) #0
 ; Function Attrs: nounwind
 declare i64 @build_options() #0
 
-; Function Attrs: nounwind
-declare void @set_post_link_callback(i64, ptr) #0
-
 ; Function Attrs: nounwind
 declare void @on_build(ptr) #0
 

examples/expected/1421-ffi-jni-env-02-lexical-direct.ir

@@ -1183,9 +1183,6 @@ declare ptr @BuildOptions.jni_main_java_source_at(i64, i64) #0
 ; Function Attrs: nounwind
 declare i64 @build_options() #0
 
-; Function Attrs: nounwind
-declare void @set_post_link_callback(i64, ptr) #0
-
 ; Function Attrs: nounwind
 declare void @on_build(ptr) #0
 

examples/expected/1425-ffi-jni-main-03-ctor.ir

@@ -696,9 +696,6 @@ declare ptr @BuildOptions.jni_main_java_source_at(i64, i64) #0
 ; Function Attrs: nounwind
 declare i64 @build_options() #0
 
-; Function Attrs: nounwind
-declare void @set_post_link_callback(i64, ptr) #0
-
 ; Function Attrs: nounwind
 declare void @on_build(ptr) #0
 

examples/expected/1661-platform-post-link-vm-list.build

@@ -1 +0,0 @@
-{ "aot": true }

examples/expected/1661-platform-post-link-vm-list.exit

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

examples/expected/1661-platform-post-link-vm-list.stderr

@@ -1 +0,0 @@
-

examples/expected/1661-platform-post-link-vm-list.stdout

@@ -1 +0,0 @@
-runtime main

examples/expected/1663-platform-build-pipeline-link.build

@@ -1 +0,0 @@
-{ "aot": true }

examples/expected/1663-platform-build-pipeline-link.exit

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

examples/expected/1663-platform-build-pipeline-link.stderr

@@ -1 +0,0 @@
-

examples/expected/1663-platform-build-pipeline-link.stdout

@@ -1 +0,0 @@
-runtime main

library/modules/build.sx

@@ -93,20 +93,12 @@ BuildOptions :: struct #compiler {
 
 build_options :: () -> BuildOptions abi(.compiler);
 
-// Post-link callback. Registers a sx function the compiler will invoke after
-// `target.link()` returns. Used by the sx-side bundler
-// (`platform.bundle.bundle_main`) and by user programs that want custom
-// post-build steps. Return `false` to fail the build. Migrated off `#compiler`
-// onto the comptime compiler-API (`abi(.compiler)`); called as
-// `opts.set_post_link_callback(cb)` via UFCS, from inside a `#run { … }` block.
-set_post_link_callback :: ufcs (self: BuildOptions, cb: () -> bool) abi(.compiler);
-
 // The build callback registrar (Phase 5). Registers the sx function that drives
 // the build — the compiler invokes it after codegen with the `BuildOptions`
-// handle. Generalizes `set_post_link_callback` (a free fn, and the callback
-// receives `opt`); `#run on_build(build);` is the override form, where
-// `build :: (opt: BuildOptions) -> bool abi(.compiler) { … }`. The stdlib default
-// implementation lives below (the default `build` script).
+// handle. `#run on_build(build);` (from inside a `#run` block) is the override
+// form, where `build :: (opt: BuildOptions) -> bool abi(.compiler) { … }`; the
+// last registration wins. With no override the compiler auto-invokes the stdlib
+// `default_pipeline` (below). This is the sole post-build callback mechanism.
 on_build :: (cb: (opt: BuildOptions) -> bool abi(.compiler)) abi(.compiler);
 
 // ── The default build script ────────────────────────────────────────────────

library/modules/platform/bundle.sx

@@ -29,7 +29,7 @@
 // `abi(.compiler)` so the backend doesn't lower it and its `build_options()`
 // compiler-API call is permitted (it would otherwise be rejected as a
 // comptime-only function called at runtime).
-bundle_main :: () -> bool abi(.compiler) {
+bundle_main :: (opt: BuildOptions) -> bool abi(.compiler) {
     opts := build_options();
     binary := opts.binary_path();
     bundle := opts.bundle_path();

src/ir/compiler_lib.zig

@@ -60,7 +60,6 @@ pub const bound_fns = [_]BoundFn{
     .{ .sx_name = "register_type", .handler = handleRegisterType },
     // ── BuildOptions (migrated off `#compiler` onto `abi(.compiler)`) ─────────
     .{ .sx_name = "build_options", .handler = handleBuildOptions },
-    .{ .sx_name = "set_post_link_callback", .handler = handleSetPostLinkCallback },
     .{ .sx_name = "on_build", .handler = handleOnBuild },
     // ── build-pipeline metadata queries (Phase 5.2) ──────────────────────────
     // VM-only: the post-link callback that calls these always runs on the VM
@@ -349,16 +348,6 @@ fn handleBuildOptions(_: *Interpreter, _: []const Value) InterpError!Value {
 /// `set_post_link_callback(self, cb)` — record the callback `FuncId` on the build
 /// config so `main.zig` re-enters the evaluator post-link. The `cb` arg is a
 /// `.func_ref` value.
-fn handleSetPostLinkCallback(interp: *Interpreter, args: []const Value) InterpError!Value {
-    if (args.len != 2) return error.TypeError;
-    const bc = interp.build_config orelse return error.CannotEvalComptime;
-    switch (args[1]) {
-        .func_ref => |id| bc.post_link_callback_fn = id,
-        else => return error.TypeError,
-    }
-    return .void_val;
-}
-
 /// `on_build(cb)` — register the build callback (the Phase 5 form, a free fn; cb
 /// is arg 0, and `cb: (opt: BuildOptions) -> bool` so the callback is invoked with
 /// the `BuildOptions` handle). Sets `post_link_takes_options` to distinguish it

src/ir/comptime_vm.zig

@@ -1480,18 +1480,6 @@ pub const Vm = struct {
         if (std.mem.eql(u8, name, "build_options")) {
             return @as(Reg, null_addr);
         }
-        // `set_post_link_callback(self, cb)` — record the callback `FuncId` on the
-        // build config so `main.zig` re-enters the evaluator post-link. The cb arg is
-        // a `func_ref` word. Mirrors the legacy `hookSetPostLinkCallback`.
-        if (std.mem.eql(u8, name, "set_post_link_callback")) {
-            if (args.len != 2) return self.failMsg("comptime set_post_link_callback: expected (self, cb)");
-            const bc = self.build_config orelse
-                return self.failMsg("comptime set_post_link_callback: no build config threaded into the VM");
-            const fid = funcRefToId(frame.get(args[1].index())) orelse
-                return self.failMsg("comptime set_post_link_callback: cb arg is not a function value");
-            bc.post_link_callback_fn = fid;
-            return @as(Reg, null_addr);
-        }
         // `on_build(cb)` — register the build callback (the Phase 5 form, `cb:
         // (opt: BuildOptions) -> bool`). Like `set_post_link_callback` but a free
         // fn (cb is arg 0, no self) and the callback receives the `BuildOptions`

src/main.zig

@@ -824,7 +824,7 @@ fn compileWithTimer(allocator: std.mem.Allocator, io: std.Io, input_path: []cons
     } else if (comp.getPostLinkModule()) |mod_name| {
         const qualified = try std.fmt.allocPrint(allocator, "{s}.bundle_main", .{mod_name});
         defer allocator.free(qualified);
-        const ret_opt = comp.invokeByName(qualified, false) catch |err| {
+        const ret_opt = comp.invokeByName(qualified, true) catch |err| {
             const label = try std.fmt.allocPrint(allocator, "post-link module '{s}.bundle_main'", .{mod_name});
             defer allocator.free(label);
             printInterpBailDiag(&comp, label, err);