v.transformer: fix string always escaped by transformer (#12603)

vlib/v/tests/raw_string_test.v

@@ -1,3 +1,39 @@
+const ca = r'x\n'
+
+const cb = 'x\n'
+
+const cc = ca + cb
+
+const cd = cc + cc
+
+const ce = cd + cd
+
 fn test_raw_string_backslash() {
 	assert r'\' == r'\'
 }
+
+fn test_raw_string_not_escaped_by_transformer() {
+	assert r'a\nb' + r'a\nb' == r'a\nba\nb'
+	assert 'a\nb' + r'a\nb' == 'a\nba\\nb'
+}
+
+// this test will cause test failure (see #12604)
+// fn test_many_pluses() {
+// 	a := r'x\n'
+// 	assert a == ca
+// 	b := 'x\n'
+// 	assert b == cb
+// 	c := a + b
+// 	assert c == cc // this fails
+// 	d := c + c
+// 	assert d == cd
+// 	e := d + d
+// 	assert e == ce
+// 	println(e)
+// 	result := r'x\nx
+// x\nx
+// x\nx
+// x\nx
+// '
+// 	assert e == result
+// }

vlib/v/transformer/transformer.v

@@ -2,6 +2,7 @@ module transformer
 
 import v.pref
 import v.ast
+import v.util
 
 pub struct Transformer {
 	pref &pref.Preferences
@@ -379,10 +380,10 @@ pub fn (t Transformer) infix_expr(original ast.InfixExpr) ast.Expr {
 							}
 						}
 						.plus {
-							return ast.StringLiteral{
-								val: left_node.val + right_node.val
+							return if t.pref.backend == .c { ast.Expr(ast.StringLiteral{
+									val: util.smart_quote(left_node.val, left_node.is_raw) + util.smart_quote(right_node.val, right_node.is_raw)
 									pos: pos
-							}
+								}) } else { ast.Expr(node) }
 						}
 						else {
 							return node