From c698e55754748fa9e97fed4fe279c604d3b381cd Mon Sep 17 00:00:00 2001
From: Matthew Lugg <mlugg@mlugg.co.uk>
Date: Wed, 27 May 2026 16:28:52 +0100
Subject: [PATCH] Compilation: don't update C inputs on incremental updates

...at least, for now, so that the use case of performing incremental
updates to the *Zig* source code works without crashing the linker.
This works around https://codeberg.org/ziglang/zig/issues/32081.
---
 src/Compilation.zig | 20 ++++++++++++++++----
 src/link.zig        |  3 +++
 2 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/src/Compilation.zig b/src/Compilation.zig
index 180d41f08ce99369294308dc2739d90e8a46b22d..9b187614a76bfc115f244004fb055f9d38eb321a 100644
--- a/src/Compilation.zig
+++ b/src/Compilation.zig
@@ -3008,10 +3008,19 @@ pub fn update(comp: *Compilation, main_progress_node: std.Progress.Node) UpdateE
 
     // For compiling C objects, we rely on the cache hash system to avoid duplicating work.
     // Add a Job for each C object.
-    try comp.c_object_work_queue.ensureUnusedCapacity(gpa, comp.c_object_table.count());
-    for (comp.c_object_table.keys()) |c_object| {
-        comp.c_object_work_queue.pushBackAssumeCapacity(c_object);
-        try comp.appendFileSystemInput(try .fromUnresolved(arena, comp.dirs, &.{c_object.src.src_path}));
+    if (comp.bin_file != null and comp.bin_file.?.post_prelink) {
+        assert(comp.config.incremental);
+        // TODO: this indicates that we are using incremental compilation and this is not the first
+        // incremental update. The incremental linkers do not (currently?) support updating C inputs
+        // incrementally. The frontend needs to learn to trigger a full rebuild if a C link input
+        // changes. For now, to avoid crashing the linker in this case, don't kick off C object
+        // updates if we've done prelink already. https://codeberg.org/ziglang/zig/issues/32081
+    } else {
+        try comp.c_object_work_queue.ensureUnusedCapacity(gpa, comp.c_object_table.count());
+        for (comp.c_object_table.keys()) |c_object| {
+            comp.c_object_work_queue.pushBackAssumeCapacity(c_object);
+            try comp.appendFileSystemInput(try .fromUnresolved(arena, comp.dirs, &.{c_object.src.src_path}));
+        }
     }
 
     for (comp.link_inputs) |input| if (input.path()) |path| {
@@ -7595,6 +7604,9 @@ pub fn queuePrelinkTaskMode(comp: *Compilation, path: Cache.Path, must_link: boo
 
 /// Only valid to call during `update`.
 pub fn queuePrelinkTasks(comp: *Compilation, tasks: []const link.PrelinkTask) Io.Cancelable!void {
+    if (tasks.len > 0) {
+        if (comp.bin_file) |lf| assert(!lf.post_prelink);
+    }
     comp.link_prog_node.increaseEstimatedTotalItems(tasks.len);
     try comp.link_queue.enqueuePrelink(comp, tasks);
 }
diff --git a/src/link.zig b/src/link.zig
index 2c8a4f7aa5ea02dd6fe9047593fc016a0a5a700a..728d2d4f245d83902ef319881b7db128fbf1e85f 100644
--- a/src/link.zig
+++ b/src/link.zig
@@ -1205,6 +1205,7 @@ pub const File = struct {
 
     pub fn loadInput(base: *File, input: Input) anyerror!void {
         if (base.tag == .lld) return;
+        assert(!base.post_prelink);
         switch (base.tag) {
             inline .elf, .elf2, .wasm => |tag| {
                 dev.check(tag.devFeature());
@@ -1424,6 +1425,8 @@ pub fn doPrelinkTask(comp: *Compilation, task: PrelinkTask) void {
         return;
     };
 
+    assert(!base.post_prelink);
+
     var timer = comp.startTimer();
     defer if (timer.finish(io)) |ns| {
         comp.mutex.lockUncancelable(io);
-- 
2.54.0