scanner: fix new string interpolation - nested quotes (#16345)

vlib/v/scanner/scanner.v

@@ -1326,9 +1326,15 @@ fn (mut s Scanner) ident_string() string {
 fn (s Scanner) is_valid_interpolation(start_pos int) bool {
 	mut is_valid_inter := true
 	mut has_rcbr := false
-	mut is_assign := true
+	mut inside_par := false
 	for i := start_pos; i < s.text.len - 1; i++ {
-		if s.text[i] == s.quote {
+		if s.text[i] == `(` {
+			inside_par = true
+		}
+		if s.text[i] == `)` && inside_par {
+			inside_par = false
+		}
+		if s.text[i] == s.quote && !inside_par {
 			break
 		}
 		if s.text[i] == `}` {
@@ -1337,12 +1343,12 @@ fn (s Scanner) is_valid_interpolation(start_pos int) bool {
 		}
 		if s.text[i] == `=` && (s.text[i - 1] in [`!`, `>`, `<`] || s.text[i + 1] == `=`) {
 			// `!=` `>=` `<=` `==`
-			is_assign = false
+			return true
 		}
-		if (s.text[i] == `=` && is_assign) || (s.text[i] == `:` && s.text[i + 1].is_space()) {
+		if s.text[i] == `=` || (s.text[i] == `:` && s.text[i + 1].is_space()) {
 			// We reached the end of the line or string without reaching "}".
 			// Also if there's "=", there's no way it's a valid interpolation expression:
-			// e.g. `println("{a.b = 42}")` `println('{foo:bar}')`
+			// e.g. `println("{a.b = 42}")` `println('{foo: bar}')`
 			is_valid_inter = false
 			break
 		}

vlib/v/tests/string_new_interpolation_test.v

@@ -16,13 +16,11 @@ fn test_string_new_interpolation() {
 	println('{a}{{{{{b}}}}}')
 	assert '{a}{{{{{b}}}}}' == '1{{{{2}}}}'
 
-	// vfmt off
 	s := 'hello'
-	println('{s == "hello"}')
-	assert '{s == "hello"}' == 'true'
-	println('{s != "hello"}')
-	assert '{s != "hello"}' == 'false'
-	// vfmt on
+	println('{s == 'hello'}')
+	assert '{s == 'hello'}' == 'true'
+	println('{s != 'hello'}')
+	assert '{s != 'hello'}' == 'false'
 
 	n := 22
 	println('{n >= 10}')
@@ -39,4 +37,21 @@ fn test_string_new_interpolation() {
 
 	println('{@FILE}')
 	assert '{@FILE}'.contains('string_new_interpolation_test.v')
+
+	ret := foo()
+	println(ret)
+	assert ret == r'[]T{aaa, bbb, ccc}'
+}
+
+fn foo() string {
+	match true {
+		true {
+			fields := ['aaa', 'bbb', 'ccc']
+			return '[]T{{fields.join(', ')}}'
+		}
+		else {
+			fields := ['aaa', 'bbb', 'ccc']
+			return 'const ({fields.join(' ')})'
+		}
+	}
 }