From d9c2b8859181cdf81732de50adf9fdc85fa47f3f Mon Sep 17 00:00:00 2001
From: Matthew Lugg <mlugg@mlugg.co.uk>
Date: Sat, 26 Sep 2026 10:51:37 +0100
Subject: [PATCH] incremental: don't map ZIR instructions when the tag changes

I don't think this technically used to matter for instructions other
than `struct_decl`/`union_decl`/`enum_decl`/`opaque_decl` (which were
already handled correctly), but it now does because of the recent
improvements to DWARF's line number tracking.

Resolves: https://codeberg.org/ziglang/zig/issues/36963
---
 src/Zcu.zig                                   | 36 ++++++++++++++++---
 .../struct_reification_becomes_anon_init      | 19 ++++++++++
 2 files changed, 51 insertions(+), 4 deletions(-)
 create mode 100644 test/incremental/struct_reification_becomes_anon_init

diff --git a/src/Zcu.zig b/src/Zcu.zig
index 56118073660df5a55f77517a459fe5f666c1d442..81684fad8e4d22bd26de82f1b6680a213ad4d609 100644
--- a/src/Zcu.zig
+++ b/src/Zcu.zig
@@ -3509,9 +3509,23 @@ pub fn mapOldZirToNew(
             for (
                 old_contents.other.items[0..num_other],
                 new_contents.other.items[0..num_other],
-            ) |old_inst, new_inst| {
+            ) |old_inst_index, new_inst_index| {
                 // These instructions don't have declarations, so we just modify `inst_map` directly.
-                inst_map.putAssumeCapacity(old_inst, new_inst);
+
+                // But first: a mapping must not change an instruction's tag, so ignore any
+                // candidates which would.
+                const old_inst = old_zir.instructions.get(@backingInt(old_inst_index));
+                const new_inst = new_zir.instructions.get(@backingInt(new_inst_index));
+                if (old_inst.tag != new_inst.tag) {
+                    continue;
+                }
+                if (old_inst.tag == .extended and
+                    old_inst.data.extended.opcode != new_inst.data.extended.opcode)
+                {
+                    continue;
+                }
+
+                inst_map.putAssumeCapacity(old_inst_index, new_inst_index);
             }
         }
 
@@ -3602,9 +3616,23 @@ pub fn mapOldZirToNew(
             for (
                 old_contents.other.items[0..num_other],
                 new_contents.other.items[0..num_other],
-            ) |old_inst, new_inst| {
+            ) |old_inst_index, new_inst_index| {
                 // These instructions don't have declarations, so we just modify `inst_map` directly.
-                inst_map.putAssumeCapacity(old_inst, new_inst);
+
+                // But first: a mapping must not change an instruction's tag, so ignore any
+                // candidates which would.
+                const old_inst = old_zir.instructions.get(@backingInt(old_inst_index));
+                const new_inst = new_zir.instructions.get(@backingInt(new_inst_index));
+                if (old_inst.tag != new_inst.tag) {
+                    continue;
+                }
+                if (old_inst.tag == .extended and
+                    old_inst.data.extended.opcode != new_inst.data.extended.opcode)
+                {
+                    continue;
+                }
+
+                inst_map.putAssumeCapacity(old_inst_index, new_inst_index);
             }
 
             if (old_contents.func_decl) |old_func_inst| {
diff --git a/test/incremental/struct_reification_becomes_anon_init b/test/incremental/struct_reification_becomes_anon_init
new file mode 100644
index 0000000000000000000000000000000000000000..7099685ba2c2e464607039c5eb461fa500422d69
--- /dev/null
+++ b/test/incremental/struct_reification_becomes_anon_init
@@ -0,0 +1,19 @@
+#update=initial version
+#file=main.zig
+const Foo = @Struct(.auto, null, &.{"reified_field"}, &.{void}, &.{.{}});
+pub fn main(init: std.process.Init) !void {
+    const field_name = @typeInfo(Foo).@"struct".field_names[0];
+    try std.Io.File.stdout().writeStreamingAll(init.io, field_name ++ "\n");
+}
+const std = @import("std");
+#expect_stdout="reified_field\n"
+
+#update=change to `@TypeOf` anonymous init
+#file=main.zig
+const Foo = @TypeOf(.{ .anon_field = 123 });
+pub fn main(init: std.process.Init) !void {
+    const field_name = @typeInfo(Foo).@"struct".field_names[0];
+    try std.Io.File.stdout().writeStreamingAll(init.io, field_name ++ "\n");
+}
+const std = @import("std");
+#expect_stdout="anon_field\n"
-- 
2.54.0