2 files changed, 26 insertions, 4 deletions

diff --git a/crates/ra_ide_db/src/line_index.rs b/crates/ra_ide_db/src/line_index.rs
index 00ba95913..212cb7b5b 100644
--- a/crates/ra_ide_db/src/line_index.rs
+++ b/crates/ra_ide_db/src/line_index.rs
@@ -8,7 +8,9 @@ use superslice::Ext;
 
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub struct LineIndex {
+    /// Offset the the beginning of each line, zero-based
     pub(crate) newlines: Vec<TextSize>,
+    /// List of non-ASCII characters on each line
     pub(crate) utf16_lines: FxHashMap<u32, Vec<Utf16Char>>,
 }
 
@@ -22,7 +24,9 @@ pub struct LineCol {
 
 #[derive(Clone, Debug, Hash, PartialEq, Eq)]
 pub(crate) struct Utf16Char {
+    /// Start offset of a character inside a line, zero-based
     pub(crate) start: TextSize,
+    /// End offset of a character inside a line, zero-based
     pub(crate) end: TextSize,
 }
 
@@ -120,7 +124,7 @@ impl LineIndex {
     fn utf16_to_utf8_col(&self, line: u32, mut col: u32) -> TextSize {
         if let Some(utf16_chars) = self.utf16_lines.get(&line) {
             for c in utf16_chars {
-                if col >= u32::from(c.start) {
+                if col > u32::from(c.start) {
                     col += u32::from(c.len()) - 1;
                 } else {
                     // From here on, all utf16 characters come *after* the character we are mapping,
@@ -226,8 +230,10 @@ const C: char = \"メ メ\";
         // UTF-16 to UTF-8
         assert_eq!(col_index.utf16_to_utf8_col(1, 15), TextSize::from(15));
 
-        assert_eq!(col_index.utf16_to_utf8_col(1, 18), TextSize::from(20));
-        assert_eq!(col_index.utf16_to_utf8_col(1, 19), TextSize::from(23));
+        // メ UTF-8: 0xE3 0x83 0xA1, UTF-16: 0x30E1
+        assert_eq!(col_index.utf16_to_utf8_col(1, 17), TextSize::from(17)); // first メ at 17..20
+        assert_eq!(col_index.utf16_to_utf8_col(1, 18), TextSize::from(20)); // space
+        assert_eq!(col_index.utf16_to_utf8_col(1, 19), TextSize::from(21)); // second メ at 21..24
 
         assert_eq!(col_index.utf16_to_utf8_col(2, 15), TextSize::from(15));
     }
diff --git a/crates/rust-analyzer/src/main_loop.rs b/crates/rust-analyzer/src/main_loop.rs
index 3bc2e0a46..401fae755 100644
--- a/crates/rust-analyzer/src/main_loop.rs
+++ b/crates/rust-analyzer/src/main_loop.rs
@@ -666,6 +666,10 @@ fn apply_document_changes(
     mut line_index: Cow<'_, LineIndex>,
     content_changes: Vec<TextDocumentContentChangeEvent>,
 ) {
+    // Remove when https://github.com/rust-analyzer/rust-analyzer/issues/4263 is fixed.
+    let backup_text = old_text.clone();
+    let backup_changes = content_changes.clone();
+
     // The changes we got must be applied sequentially, but can cross lines so we
     // have to keep our line index updated.
     // Some clients (e.g. Code) sort the ranges in reverse. As an optimization, we
@@ -693,7 +697,19 @@ fn apply_document_changes(
                 }
                 index_valid = IndexValid::UpToLine(range.start.line);
                 let range = range.conv_with(&line_index);
-                old_text.replace_range(Range::<usize>::from(range), &change.text);
+                let mut text = old_text.to_owned();
+                match std::panic::catch_unwind(move || {
+                    text.replace_range(Range::<usize>::from(range), &change.text);
+                    text
+                }) {
+                    Ok(t) => *old_text = t,
+                    Err(e) => {
+                        eprintln!("Bug in incremental text synchronization. Please report the following output on https://github.com/rust-analyzer/rust-analyzer/issues/4263");
+                        dbg!(&backup_text);
+                        dbg!(&backup_changes);
+                        std::panic::resume_unwind(e);
+                    }
+                }
             }
             None => {
                 *old_text = change.text;