From 6cbb2aa7ca92dad49622e09a1286e3b6979777c5 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Tue, 21 Apr 2026 17:16:17 +0300
Subject: [PATCH] cgen: fix autofree memory leak on reused assignment (fixes
 #18408)

---
 vlib/v/gen/c/assign.v                         | 32 ++++++++++++++++---
 .../c/testdata/autofree_reused.c.must_have    |  8 +++--
 vlib/v/gen/c/testdata/autofree_reused.out     |  1 +
 vlib/v/gen/c/testdata/autofree_reused.vv      |  7 ++++
 4 files changed, 41 insertions(+), 7 deletions(-)

diff --git a/vlib/v/gen/c/assign.v b/vlib/v/gen/c/assign.v
index 8d65df44f..1d6309282 100644
--- a/vlib/v/gen/c/assign.v
+++ b/vlib/v/gen/c/assign.v
@@ -627,8 +627,9 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 	}
 	// Free the old value assigned to this string var (only if it's `str = [new value]`
 	// or `x.str = [new value]` )
-	mut af := g.is_autofree && !g.is_builtin_mod && !g.is_autofree_tmp && node.op == .assign
-		&& node.left_types.len == 1 && node.left[0] in [ast.Ident, ast.SelectorExpr]
+	mut af := g.is_autofree && !g.is_builtin_mod && !g.is_autofree_tmp && node.left_types.len == 1
+		&& node.left[0] in [ast.Ident, ast.SelectorExpr] && (node.op == .assign
+		|| (node.op == .plus_assign && node.left_types[0] == ast.string_type))
 	if af && node.right.len == 1 && node.right[0] is ast.CallExpr {
 		call_expr := node.right[0] as ast.CallExpr
 		if call_expr.is_method && call_expr.left is ast.CallExpr {
@@ -640,7 +641,8 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 	if af {
 		first_left_type := node.left_types[0]
 		first_left_sym := g.table.sym(node.left_types[0])
-		if first_left_type == ast.string_type || first_left_sym.kind == .array {
+		if first_left_type == ast.string_type
+			|| (node.op == .assign && first_left_sym.kind == .array) {
 			type_to_free = if first_left_type == ast.string_type { 'string' } else { 'array' }
 			mut ok := true
 			left0 := node.left[0]
@@ -723,7 +725,10 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 				val_type = g.unwrap_generic(resolved_right_type)
 			}
 		}
-		val := node.right[i]
+		mut val := node.right[i]
+		mut str_add_rhs_tmp := ''
+		mut str_add_rhs_needs_free := false
+		mut skip_str_add_rhs_clone := false
 		if is_decl && g.cur_concrete_types.len > 0 && val is ast.CallExpr
 			&& val.return_type_generic != 0 {
 			mut resolved_val_type := g.resolve_return_type(val).clear_option_and_result()
@@ -1587,6 +1592,22 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 				is_mut_arg_pointer_rebind = true
 			}
 			if node.op == .plus_assign && unaliased_right_sym.kind == .string {
+				if g.is_autofree && !g.is_builtin_mod && !g.is_autofree_tmp
+					&& val !in [ast.Ident, ast.StringLiteral, ast.SelectorExpr, ast.ComptimeSelector] {
+					str_add_rhs_tmp = '_str_add_rhs_${node.pos.pos}_${i}'
+					g.writeln(g.autofree_tmp_arg_init_stmt('string ${str_add_rhs_tmp} = ', val))
+					val = ast.Expr(ast.Ident{
+						mod:  g.cur_mod.name
+						name: str_add_rhs_tmp
+					})
+					str_add_rhs_needs_free = true
+					skip_str_add_rhs_clone = true
+					defer(fn) {
+						if str_add_rhs_needs_free {
+							g.writeln('builtin__string_free(&${str_add_rhs_tmp});')
+						}
+					}
+				}
 				if mut left is ast.IndexExpr {
 					if g.table.sym(left.left_type).kind == .array_fixed {
 						// strs[0] += str2 => `strs[0] = _string__plus(strs[0], str2)`
@@ -1901,7 +1922,8 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 			mut cloned := false
 			if g.is_autofree {
 				if right_sym.kind in [.array, .string] && !unwrapped_val_type.has_flag(.shared_f) {
-					if g.gen_clone_assignment(var_type, val, unwrapped_val_type, false) {
+					if !skip_str_add_rhs_clone
+						&& g.gen_clone_assignment(var_type, val, unwrapped_val_type, false) {
 						cloned = true
 					}
 				} else if right_sym.info is ast.Interface && var_type != ast.error_type {
diff --git a/vlib/v/gen/c/testdata/autofree_reused.c.must_have b/vlib/v/gen/c/testdata/autofree_reused.c.must_have
index d609b996e..2c93b61af 100644
--- a/vlib/v/gen/c/testdata/autofree_reused.c.must_have
+++ b/vlib/v/gen/c/testdata/autofree_reused.c.must_have
@@ -4,6 +4,10 @@ VV_LOC void main__main(void) {
 	string _t1 = Array_u8_str(s); builtin__println(_t1); builtin__string_free(&_t1);
 	;
 	byteptr bb = ((byteptr)("a"));
-	string ss = builtin__byteptr_vstring(bb);
-	builtin__println(ss);
+string ss = builtin__byteptr_vstring(bb);
+builtin__println(ss);
+string _str_add_rhs_
+s = builtin__string__plus(s, _str_add_rhs_
+builtin__string_free(&_str_add_rhs_
+builtin__string_free(&_sref
 }
diff --git a/vlib/v/gen/c/testdata/autofree_reused.out b/vlib/v/gen/c/testdata/autofree_reused.out
index 30ec3724e..589902b74 100644
--- a/vlib/v/gen/c/testdata/autofree_reused.out
+++ b/vlib/v/gen/c/testdata/autofree_reused.out
@@ -1,2 +1,3 @@
 [97]
 a
+ab
diff --git a/vlib/v/gen/c/testdata/autofree_reused.vv b/vlib/v/gen/c/testdata/autofree_reused.vv
index a4593f05d..4689e8349 100644
--- a/vlib/v/gen/c/testdata/autofree_reused.vv
+++ b/vlib/v/gen/c/testdata/autofree_reused.vv
@@ -1,4 +1,10 @@
 // vtest vflags: -autofree
+fn use_plus_assign() {
+	mut s := u8(`a`).ascii_str()
+	s += u8(`b`).ascii_str()
+	println(s)
+}
+
 fn main() {
 	b := byteptr(c'a')
 	s := unsafe { b.vbytes(1) }
@@ -7,4 +13,5 @@ fn main() {
 	bb := byteptr(c'a')
 	ss := unsafe { bb.vstring() }
 	println(ss)
+	use_plus_assign()
 }
-- 
2.39.5