autofree: fix frees before returns

vlib/v/gen/c/cgen.v

@@ -4721,27 +4721,30 @@ fn (mut g Gen) return_statement(node ast.Return) {
 			if expr is ast.Ident {
 				g.returned_var_name = expr.name
 			}
-			g.autofree_scope_vars(node.pos.pos - 1, node.pos.line_nr, true)
 		}
 		// free := g.is_autofree && !g.is_builtin_mod // node.exprs[0] is ast.CallExpr
 		// Create a temporary variable for the return expression
-		free := !g.is_builtin_mod // node.exprs[0] is ast.CallExpr
+		mut gen_tmp_var := !g.is_builtin_mod // node.exprs[0] is ast.CallExpr
 		mut tmp := ''
-		if free {
+		if gen_tmp_var {
 			// `return foo(a, b, c)`
 			// `tmp := foo(a, b, c); free(a); free(b); free(c); return tmp;`
 			// Save return value in a temp var so that all args (a,b,c) can be freed
 			// Don't use a tmp var if a variable is simply returned: `return x`
 			if node.exprs[0] !is ast.Ident {
 				tmp = g.new_tmp_var()
+				g.write('/*tmp return var*/ ')
 				g.write(g.typ(g.fn_decl.return_type))
 				g.write(' ')
 				g.write(tmp)
 				g.write(' = ')
 			} else {
+				gen_tmp_var = false
+				g.autofree_scope_vars(node.pos.pos - 1, node.pos.line_nr, true)
 				g.write('return ')
 			}
 		} else {
+			g.autofree_scope_vars(node.pos.pos - 1, node.pos.line_nr, true)
 			g.write('return ')
 		}
 		if expr0.is_auto_deref_var() {
@@ -4755,10 +4758,11 @@ fn (mut g Gen) return_statement(node ast.Return) {
 		} else {
 			g.expr_with_cast(node.exprs[0], node.types[0], g.fn_decl.return_type)
 		}
-		if free {
+		if gen_tmp_var {
 			g.writeln(';')
 			has_semicolon = true
 			if tmp != '' {
+				g.autofree_scope_vars(node.pos.pos - 1, node.pos.line_nr, true)
 				g.write('return $tmp')
 				has_semicolon = false
 			}

vlib/v/tests/valgrind/1.strings_and_arrays.v

@@ -1,4 +1,5 @@
 import os
+import strings
 
 // This program is built and run via Valgrind to ensure there are no leaks with -autofree
 fn simple() {
@@ -341,6 +342,12 @@ fn comp_if() {
 fn anon_fn() {
 }
 
+fn return_sb_str() string {
+	mut sb := strings.new_builder(100)
+	sb.write_string('hello')
+	return sb.str() // sb should be freed, but only after .str() is called
+}
+
 fn main() {
 	println('start')
 	simple()
@@ -364,6 +371,7 @@ fn main() {
 	free_before_return()
 	free_before_return_bool()
 	free_before_break()
+	s := return_sb_str()
 	// free_map()
 	// loop_map()
 	free_array_except_returned_element()