scanner: fix new string interpolation println('{a}{b}{c}{d}') (#16258)

cmd/tools/vtest-cleancode.v

@@ -47,6 +47,7 @@ const verify_known_failing_exceptions = [
 	'vlib/builtin/int_test.v' /* special number formatting that should be tested */,
 	// TODOs and unfixed vfmt bugs
 	'vlib/v/gen/js/tests/js.v', /* local `hello` fn, gets replaced with module `hello` aliased as `hl` */
+	'vlib/v/tests/string_new_interpolation_test.v', /* new string interpolation */
 ]
 
 const vfmt_verify_list = [

vlib/v/scanner/scanner.v

@@ -1214,7 +1214,7 @@ fn (mut s Scanner) ident_string() string {
 				u_escapes_pos << s.pos - 1
 			}
 		}
-		// ${var} (ignore in vfmt mode) (skip \$)
+		// '${var}' (ignore in vfmt mode) (skip \$)
 		if prevc == `$` && c == `{` && !is_raw
 			&& s.count_symbol_before(s.pos - 2, scanner.backslash) % 2 == 0 {
 			s.is_inside_string = true
@@ -1223,7 +1223,7 @@ fn (mut s Scanner) ident_string() string {
 			s.pos -= 2
 			break
 		}
-		// $var
+		// '$var'
 		if prevc == `$` && util.is_name_char(c) && !is_raw
 			&& s.count_symbol_before(s.pos - 2, scanner.backslash) % 2 == 0 {
 			s.is_inside_string = true
@@ -1231,7 +1231,7 @@ fn (mut s Scanner) ident_string() string {
 			s.pos -= 2
 			break
 		}
-		// {var} (ignore in vfmt mode) (skip \{)
+		// '{var}' (ignore in vfmt mode) (skip \{)
 		if c == `{` && util.is_name_char(s.text[s.pos + 1]) && prevc != `$` && !is_raw
 			&& s.count_symbol_before(s.pos - 1, scanner.backslash) % 2 == 0 {
 			// Detect certain strings with "{" that are not interpolation:
@@ -1258,6 +1258,33 @@ fn (mut s Scanner) ident_string() string {
 				break
 			}
 		}
+		// '{var1}{var2}'
+		if prevc == `{` && util.is_name_char(c) && s.text[s.pos - 2] !in [`$`, `{`] && !is_raw
+			&& s.count_symbol_before(s.pos - 2, scanner.backslash) % 2 == 0 {
+			// Detect certain strings with "{" that are not interpolation:
+			// e.g. "{init: " (no "}" at the end)
+			mut is_valid_inter := true
+			for i := s.pos; i < s.text.len; i++ {
+				if s.text[i] == `}` {
+					// No } in this string, so it's not a valid `{x}` interpolation
+					break
+				}
+				if s.text[i] in [`=`, `:`, `\n`, s.inter_quote] {
+					// 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}')`
+					is_valid_inter = false
+					break
+				}
+			}
+			if is_valid_inter {
+				s.is_inside_string = true
+				s.is_enclosed_inter = true
+				// so that s.pos points to $ at the next step
+				s.pos -= 2
+				break
+			}
+		}
 		if c != scanner.backslash {
 			backslash_count = 0
 		}

vlib/v/tests/string_new_interpolation_test.v

@@ -0,0 +1,9 @@
+fn test_string_new_interpolation() {
+	a, b, c, d := 1, 2, 3, 4
+
+	println('{a}{b}{c}{d}')
+	assert '{a}{b}{c}{d}' == '1234'
+
+	println('{a} {b} {c} {d}')
+	assert '{a} {b} {c} {d}' == '1 2 3 4'
+}