From 916b1b41df6d4a787b8b2cc5b6994b9545ea441e Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Tue, 14 Apr 2026 12:45:25 +0300
Subject: [PATCH] scanner: v parser should provide more accurate error msg
 (fixes #25309)

---
 vlib/v/scanner/scanner.v                      | 20 +++++++++++++++++++
 .../string_interpolation_missing_rcbr_err.out |  7 +++++++
 .../string_interpolation_missing_rcbr_err.vv  |  6 ++++++
 3 files changed, 33 insertions(+)
 create mode 100644 vlib/v/scanner/tests/string_interpolation_missing_rcbr_err.out
 create mode 100644 vlib/v/scanner/tests/string_interpolation_missing_rcbr_err.vv

diff --git a/vlib/v/scanner/scanner.v b/vlib/v/scanner/scanner.v
index 06354b1a1..2b1debeb3 100644
--- a/vlib/v/scanner/scanner.v
+++ b/vlib/v/scanner/scanner.v
@@ -812,6 +812,16 @@ pub fn (mut s Scanner) text_scan() token.Token {
 				return s.new_token(.question, '?', 1)
 			}
 			single_quote, double_quote {
+				if s.is_likely_unclosed_string_interpolation(c) {
+					s.error_with_pos('expected `}` to close string interpolation', token.Pos{
+						len:       1
+						line_nr:   s.line_nr
+						pos:       s.pos
+						col:       u16_col(s.current_column() - 1)
+						file_idx:  s.file_idx
+						last_line: s.line_nr
+					})
+				}
 				s.str_helper_tokens << c
 				start_line := s.line_nr
 				ident_string := s.ident_string()
@@ -1857,6 +1867,16 @@ fn (s Scanner) str_quote() u8 {
 	return 255
 }
 
+@[direct_array_access; inline]
+fn (s &Scanner) is_likely_unclosed_string_interpolation(current_quote u8) bool {
+	if current_quote != s.quote || s.str_helper_tokens.len == 0 || s.str_quote() != 255
+		|| s.all_tokens.len == 0 {
+		return false
+	}
+	prev_tok := s.all_tokens[s.all_tokens.len - 1]
+	return prev_tok.kind in [.number, .string, .chartoken, .rpar, .rsbr, .rcbr]
+}
+
 @[inline]
 fn u16_col(col int) u16 {
 	return if col < 0 { u16(0) } else { u16(col) }
diff --git a/vlib/v/scanner/tests/string_interpolation_missing_rcbr_err.out b/vlib/v/scanner/tests/string_interpolation_missing_rcbr_err.out
new file mode 100644
index 000000000..f84799026
--- /dev/null
+++ b/vlib/v/scanner/tests/string_interpolation_missing_rcbr_err.out
@@ -0,0 +1,7 @@
+vlib/v/scanner/tests/string_interpolation_missing_rcbr_err.vv:4:23: error: expected `}` to close string interpolation
+    2 |
+    3 | fn main() {
+    4 |     broken := '${foo()' + '${bar})'
+      |                       ^
+    5 |     _ = '\t${broken}'
+    6 | }
diff --git a/vlib/v/scanner/tests/string_interpolation_missing_rcbr_err.vv b/vlib/v/scanner/tests/string_interpolation_missing_rcbr_err.vv
new file mode 100644
index 000000000..7dc212060
--- /dev/null
+++ b/vlib/v/scanner/tests/string_interpolation_missing_rcbr_err.vv
@@ -0,0 +1,6 @@
+module main
+
+fn main() {
+    broken := '${foo()' + '${bar})'
+    _ = '\t${broken}'
+}
-- 
2.39.5