diff --git a/vlib/builtin/utf8.v b/vlib/builtin/utf8.v
index a9078e7c5c..462cfa05bf 100644
--- a/vlib/builtin/utf8.v
+++ b/vlib/builtin/utf8.v
@@ -16,32 +16,34 @@ pub fn utf32_to_str(code u32) string {
 
 pub fn utf32_to_str_no_malloc(code u32, buf voidptr) string {
 	icode := int(code) // Prevents doing casts everywhere
+	mut res := ''
 	unsafe {
 		mut buffer := byteptr(buf)
 		if icode <= 127 { /* 0x7F */
 			buffer[0] = byte(icode)
-			return tos(buffer, 1)
+			res = tos(buffer, 1)
 		}
 		if icode <= 2047 { /* 0x7FF */
 			buffer[0] = 192 | byte(icode>>6)  /* 0xC0 - 110xxxxx */
 			buffer[1] = 128 | byte(icode & 63) /* 0x80 - 0x3F - 10xxxxxx */
-			return tos(buffer, 2)
+			res = tos(buffer, 2)
 		}
 		if icode <= 65535 { /* 0xFFFF */
 			buffer[0] = 224 | byte(icode>>12)/* 0xE0 - 1110xxxx */
 			buffer[1] = 128 | (byte(icode>>6) & 63) /* 0x80 - 0x3F - 10xxxxxx */
 			buffer[2] = 128 | byte(icode & 63) /* 0x80 - 0x3F - 10xxxxxx */
-			return tos(buffer, 3)
+			res = tos(buffer, 3)
 		}
 		if icode <= 1114111/* 0x10FFFF */ {
 			buffer[0] = 240 | byte(icode>>18)  /* 0xF0 - 11110xxx */
 			buffer[1] = 128 | (byte(icode>>12) & 63) /* 0x80 - 0x3F - 10xxxxxx */
 			buffer[2] = 128 | (byte(icode>>6) & 63) /* 0x80 - 0x3F - 10xxxxxx */
 			buffer[3] = 128 | byte(icode & 63) /* 0x80 - 0x3F - 10xxxxxx */
-			return tos(buffer, 4)
+			res = tos(buffer, 4)
 		}
 	}
-	return ''
+	res.is_lit = 1 // let autofree know this string doesn't have to be freed
+	return res
 }
 
 // Convert utf8 to utf32
@@ -181,10 +183,10 @@ fn utf8_str_visible_length(s string) int {
 			// diacritical marks supplement 0xe1b780 - 0xe1b880
 			// diacritical marks for symbols 0xe28390 - 0xe28480
 			// half marks 0xefb8a0 - 0xefb8b0
-			if (r >= 0xe1aab0 && r < 0xe1ac80) 
-			|| (r >= 0xe1b780 && r < 0xe1b880) 
-			|| (r >= 0xe28390 && r < 0xe28480) 
-			|| (r >= 0xefb8a0 && r < 0xefb8b0) { 
+			if (r >= 0xe1aab0 && r < 0xe1ac80)
+			|| (r >= 0xe1b780 && r < 0xe1b880)
+			|| (r >= 0xe28390 && r < 0xe28480)
+			|| (r >= 0xefb8a0 && r < 0xefb8b0) {
 				l--
 			}
 		}
diff --git a/vlib/v/gen/cgen.v b/vlib/v/gen/cgen.v
index 78f730e58a..d1674746a3 100644
--- a/vlib/v/gen/cgen.v
+++ b/vlib/v/gen/cgen.v
@@ -1551,8 +1551,8 @@ fn (mut g Gen) gen_assign_stmt(assign_stmt ast.AssignStmt) {
 		else {}
 	}
 	// Free the old value assigned to this string var (only if it's `str = [new value]`)
-	mut af := g.pref.autofree && assign_stmt.op == .assign && assign_stmt.left_types.len == 1 &&
-		assign_stmt.left_types[0] == table.string_type && assign_stmt.left[0] is ast.Ident
+	mut af := g.pref.autofree && !g.is_builtin_mod && assign_stmt.op == .assign && assign_stmt.left_types.len ==
+		1 && assign_stmt.left_types[0] == table.string_type && assign_stmt.left[0] is ast.Ident
 	mut sref_name := ''
 	if af {
 		ident := assign_stmt.left[0] as ast.Ident