From 1cd3c9cb315a4199767de4b48dc082b4fe623a7b Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Thu, 26 Feb 2026 10:37:14 +0300
Subject: [PATCH] cgen: fix modifying a struct pointer by passing it as mut to
 a function (fixes #17788)

---
 vlib/v/gen/c/assign.v                         | 13 +++++-
 .../mut_arg_struct_pointer_rebind_test.v      | 42 +++++++++++++++++++
 2 files changed, 53 insertions(+), 2 deletions(-)
 create mode 100644 vlib/v/tests/mut_arg_struct_pointer_rebind_test.v

diff --git a/vlib/v/gen/c/assign.v b/vlib/v/gen/c/assign.v
index 913710c02..52ecec01b 100644
--- a/vlib/v/gen/c/assign.v
+++ b/vlib/v/gen/c/assign.v
@@ -678,6 +678,13 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 			mut op_expected_right := ast.no_type
 			is_shared_re_assign := !is_decl && node.left_types[i].has_flag(.shared_f)
 				&& left is ast.Ident && left_sym.kind in [.array, .map, .struct]
+			mut is_mut_arg_pointer_rebind := false
+			// Keep pointer traversal assignments on auto-deref vars as local pointer rebinds.
+			if !is_decl && !is_shared_re_assign && !var_type.has_flag(.option)
+				&& var_type.is_any_kind_of_pointer() && unwrapped_val_type.is_any_kind_of_pointer()
+				&& left.is_auto_deref_var() && !val.is_auto_deref_var() && node.op == .assign {
+				is_mut_arg_pointer_rebind = true
+			}
 			if node.op == .plus_assign && unaliased_right_sym.kind == .string {
 				if mut left is ast.IndexExpr {
 					if g.table.sym(left.left_type).kind == .array_fixed {
@@ -872,7 +879,7 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 						g.op_arg(left, op_expected_left, var_type)
 					} else {
 						if !is_decl && !is_shared_re_assign && left.is_auto_deref_var()
-							&& !var_type.has_flag(.option) {
+							&& !var_type.has_flag(.option) && !is_mut_arg_pointer_rebind {
 							g.write('*')
 						}
 						if node_.op == .assign && var_type.has_flag(.option_mut_param_t) {
@@ -1167,7 +1174,9 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 						if op_overloaded {
 							g.op_arg(val, op_expected_right, val_type)
 						} else {
-							exp_type := if var_type.is_ptr()
+							exp_type := if is_mut_arg_pointer_rebind {
+								var_type
+							} else if var_type.is_ptr()
 								&& (left.is_auto_deref_var() || var_type.has_flag(.shared_f)) {
 								var_type.deref()
 							} else {
diff --git a/vlib/v/tests/mut_arg_struct_pointer_rebind_test.v b/vlib/v/tests/mut_arg_struct_pointer_rebind_test.v
new file mode 100644
index 000000000..ae7e0376d
--- /dev/null
+++ b/vlib/v/tests/mut_arg_struct_pointer_rebind_test.v
@@ -0,0 +1,42 @@
+module main
+
+@[heap]
+struct Node {
+mut:
+	children []&Node
+}
+
+@[heap]
+struct Tree {
+mut:
+	root &Node
+}
+
+fn find_leaf(mut node Node) &Node {
+	for {
+		if node.children.len == 0 {
+			return node
+		}
+		node = node.children[0]
+	}
+	return node
+}
+
+fn test_mut_struct_arg_passed_as_pointer_keeps_original_tree_root() {
+	mut n1 := &Node{}
+	mut t := &Tree{
+		root: n1
+	}
+
+	mut n2 := &Node{}
+	t.root.children << n2
+	mut n3 := &Node{}
+	t.root.children[0].children << n3
+
+	found := find_leaf(mut t.root)
+	assert found == n3
+	assert t.root.children.len == 1
+	assert t.root.children[0] == n2
+	assert t.root.children[0].children.len == 1
+	assert t.root.children[0].children[0] == n3
+}
-- 
2.39.5