fix: don't ICE on LineOverflow when the line contains multibyte characters

Author: shulaoda

src/formatting.rs

@@ -354,7 +354,9 @@ impl FormattingError {
     // (space, target)
     pub(crate) fn format_len(&self) -> (usize, usize) {
         match self.kind {
-            ErrorKind::LineOverflow(found, max) => (max, found - max),
+            ErrorKind::LineOverflow(_, _, byte_start) => {
+                (byte_start, self.line_buffer.len() - byte_start)
+            }
             ErrorKind::TrailingWhitespace
             | ErrorKind::DeprecatedAttr
             | ErrorKind::BadAttr
@@ -564,8 +566,10 @@ impl<'a> FormatLines<'a> {
             }
 
             // Check for any line width errors we couldn't correct.
-            let error_kind = ErrorKind::LineOverflow(self.line_len, self.config.max_width());
-            if self.line_len > self.config.max_width()
+            let max = self.config.max_width();
+            let error_kind =
+                ErrorKind::LineOverflow(self.line_len, max, self.byte_offset_at_col(max));
+            if self.line_len > max
                 && !self.is_skipped_line()
                 && self.should_report_error(kind, &error_kind)
             {
@@ -600,6 +604,22 @@ impl<'a> FormatLines<'a> {
         }
     }
 
+    /// Inverse of `Self::char`'s column accounting: walk `line_buffer` with
+    /// the same rule (tab = `tab_spaces` cols, every other char = 1 col) and
+    /// return the byte offset where the accumulated column count first reaches
+    /// `target_col`. Returns `line_buffer.len()` if the line is shorter.
+    fn byte_offset_at_col(&self, target_col: usize) -> usize {
+        let tab_spaces = self.config.tab_spaces();
+        let mut col = 0;
+        for (idx, ch) in self.line_buffer.char_indices() {
+            if col >= target_col {
+                return idx;
+            }
+            col += if ch == '\t' { tab_spaces } else { 1 };
+        }
+        self.line_buffer.len()
+    }
+
     fn push_err(&mut self, kind: ErrorKind, is_comment: bool, is_string: bool) {
         self.errors.push(FormattingError {
             line: self.cur_line,

src/lib.rs

@@ -106,12 +106,13 @@ pub(crate) mod visitor;
 /// these can currently be propagated to clients.
 #[derive(Error, Debug)]
 pub enum ErrorKind {
-    /// Line has exceeded character limit (found, maximum).
+    /// Line has exceeded character limit (found, maximum, byte offset within
+    /// the line where the overflow region begins).
     #[error(
         "line formatted, but exceeded maximum width \
          (maximum: {1} (see `max_width` option), found: {0})"
     )]
-    LineOverflow(usize, usize),
+    LineOverflow(usize, usize, usize),
     /// Line ends in whitespace.
     #[error("left behind trailing whitespace")]
     TrailingWhitespace,

tests/cargo-fmt/source/issue_6850/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "issue_6850"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

tests/cargo-fmt/source/issue_6850/src/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    "☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃"
+}

tests/rustfmt/main.rs

@@ -203,6 +203,27 @@ fn dont_emit_ICE() {
     }
 }
 
+#[test]
+fn dont_panic_on_line_overflow_with_multibyte_chars() {
+    // See also https://github.com/rust-lang/rustfmt/issues/6850
+    let args = [
+        "--config",
+        "error_on_line_overflow=true,error_on_unformatted=true",
+        "tests/cargo-fmt/source/issue_6850/src/main.rs",
+    ];
+
+    let (_stdout, stderr) = rustfmt(&args);
+    assert!(stderr.contains(
+        "line formatted, but exceeded maximum width (maximum: 100 (see `max_width` option), found: 126)"
+    ));
+
+    let panic_re = regex::Regex::new("thread.*panicked").unwrap();
+    assert!(
+        !panic_re.is_match(&stderr),
+        "rustfmt panicked instead of reporting line overflow:\n{stderr}"
+    );
+}
+
 #[test]
 fn rustfmt_emits_error_when_control_brace_style_is_always_next_line() {
     // See also https://github.com/rust-lang/rustfmt/issues/5912