16 files changed, 221 insertions, 146 deletions

diff --git a/crates/ide/src/lib.rs b/crates/ide/src/lib.rs
index 89e7bef7d..a2c8db505 100644
--- a/crates/ide/src/lib.rs
+++ b/crates/ide/src/lib.rs
@@ -95,7 +95,7 @@ pub use ide_db::{
     },
     call_info::CallInfo,
     label::Label,
-    line_index::{LineCol, LineIndex},
+    line_index::{LineCol, LineColUtf16, LineIndex},
     search::{ReferenceAccess, SearchScope},
     source_change::{FileSystemEdit, SourceChange},
     symbol_index::Query,
diff --git a/crates/ide_db/src/line_index.rs b/crates/ide_db/src/line_index.rs
index 41226305e..8e9d8cca2 100644
--- a/crates/ide_db/src/line_index.rs
+++ b/crates/ide_db/src/line_index.rs
@@ -15,11 +15,19 @@ pub struct LineIndex {
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
-pub struct LineCol {
+pub struct LineColUtf16 {
     /// Zero-based
     pub line: u32,
     /// Zero-based
-    pub col_utf16: u32,
+    pub col: u32,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+pub struct LineCol {
+    /// Zero-based
+    pub line: u32,
+    /// Zero-based utf8 offset
+    pub col: u32,
 }
 
 #[derive(Clone, Debug, Hash, PartialEq, Eq)]
@@ -92,14 +100,21 @@ impl LineIndex {
         let line = partition_point(&self.newlines, |&it| it <= offset) - 1;
         let line_start_offset = self.newlines[line];
         let col = offset - line_start_offset;
-
-        LineCol { line: line as u32, col_utf16: self.utf8_to_utf16_col(line as u32, col) as u32 }
+        LineCol { line: line as u32, col: col.into() }
     }
 
     pub fn offset(&self, line_col: LineCol) -> TextSize {
-        //FIXME: return Result
-        let col = self.utf16_to_utf8_col(line_col.line, line_col.col_utf16);
-        self.newlines[line_col.line as usize] + col
+        self.newlines[line_col.line as usize] + TextSize::from(line_col.col)
+    }
+
+    pub fn to_utf16(&self, line_col: LineCol) -> LineColUtf16 {
+        let col = self.utf8_to_utf16_col(line_col.line, line_col.col.into());
+        LineColUtf16 { line: line_col.line, col: col as u32 }
+    }
+
+    pub fn to_utf8(&self, line_col: LineColUtf16) -> LineCol {
+        let col = self.utf16_to_utf8_col(line_col.line, line_col.col);
+        LineCol { line: line_col.line, col: col.into() }
     }
 
     pub fn lines(&self, range: TextRange) -> impl Iterator<Item = TextRange> + '_ {
diff --git a/crates/ide_db/src/line_index/tests.rs b/crates/ide_db/src/line_index/tests.rs
index 05f7484e8..09f3bca62 100644
--- a/crates/ide_db/src/line_index/tests.rs
+++ b/crates/ide_db/src/line_index/tests.rs
@@ -3,24 +3,29 @@ use super::*;
 #[test]
 fn test_line_index() {
     let text = "hello\nworld";
+    let table = [
+        (00, 0, 0),
+        (01, 0, 1),
+        (05, 0, 5),
+        (06, 1, 0),
+        (07, 1, 1),
+        (08, 1, 2),
+        (10, 1, 4),
+        (11, 1, 5),
+        (12, 1, 6),
+    ];
+
     let index = LineIndex::new(text);
-    assert_eq!(index.line_col(0.into()), LineCol { line: 0, col_utf16: 0 });
-    assert_eq!(index.line_col(1.into()), LineCol { line: 0, col_utf16: 1 });
-    assert_eq!(index.line_col(5.into()), LineCol { line: 0, col_utf16: 5 });
-    assert_eq!(index.line_col(6.into()), LineCol { line: 1, col_utf16: 0 });
-    assert_eq!(index.line_col(7.into()), LineCol { line: 1, col_utf16: 1 });
-    assert_eq!(index.line_col(8.into()), LineCol { line: 1, col_utf16: 2 });
-    assert_eq!(index.line_col(10.into()), LineCol { line: 1, col_utf16: 4 });
-    assert_eq!(index.line_col(11.into()), LineCol { line: 1, col_utf16: 5 });
-    assert_eq!(index.line_col(12.into()), LineCol { line: 1, col_utf16: 6 });
+    for &(offset, line, col) in &table {
+        assert_eq!(index.line_col(offset.into()), LineCol { line, col });
+    }
 
     let text = "\nhello\nworld";
+    let table = [(0, 0, 0), (1, 1, 0), (2, 1, 1), (6, 1, 5), (7, 2, 0)];
     let index = LineIndex::new(text);
-    assert_eq!(index.line_col(0.into()), LineCol { line: 0, col_utf16: 0 });
-    assert_eq!(index.line_col(1.into()), LineCol { line: 1, col_utf16: 0 });
-    assert_eq!(index.line_col(2.into()), LineCol { line: 1, col_utf16: 1 });
-    assert_eq!(index.line_col(6.into()), LineCol { line: 1, col_utf16: 5 });
-    assert_eq!(index.line_col(7.into()), LineCol { line: 2, col_utf16: 0 });
+    for &(offset, line, col) in &table {
+        assert_eq!(index.line_col(offset.into()), LineCol { line, col });
+    }
 }
 
 #[test]
diff --git a/crates/rust-analyzer/src/bin/main.rs b/crates/rust-analyzer/src/bin/main.rs
index 93d0ad4ec..89482b952 100644
--- a/crates/rust-analyzer/src/bin/main.rs
+++ b/crates/rust-analyzer/src/bin/main.rs
@@ -8,7 +8,7 @@ use std::{convert::TryFrom, env, fs, path::PathBuf, process};
 
 use lsp_server::Connection;
 use project_model::ProjectManifest;
-use rust_analyzer::{cli, config::Config, from_json, Result};
+use rust_analyzer::{cli, config::Config, from_json, lsp_ext::supports_utf8, Result};
 use vfs::AbsPathBuf;
 
 #[cfg(all(feature = "mimalloc"))]
@@ -127,7 +127,11 @@ fn run_server() -> Result<()> {
             name: String::from("rust-analyzer"),
             version: Some(String::from(env!("REV"))),
         }),
-        offset_encoding: None,
+        offset_encoding: if supports_utf8(&initialize_params.capabilities) {
+            Some("utf-8".to_string())
+        } else {
+            None
+        },
     };
 
     let initialize_result = serde_json::to_value(initialize_result).unwrap();
diff --git a/crates/rust-analyzer/src/cli/analysis_bench.rs b/crates/rust-analyzer/src/cli/analysis_bench.rs
index 6735b6388..877abd12b 100644
--- a/crates/rust-analyzer/src/cli/analysis_bench.rs
+++ b/crates/rust-analyzer/src/cli/analysis_bench.rs
@@ -97,7 +97,7 @@ impl BenchCmd {
                 let offset = host
                     .analysis()
                     .file_line_index(file_id)?
-                    .offset(LineCol { line: pos.line - 1, col_utf16: pos.column });
+                    .offset(LineCol { line: pos.line - 1, col: pos.column });
                 let file_position = FilePosition { file_id, offset };
 
                 if is_completion {
diff --git a/crates/rust-analyzer/src/cli/analysis_stats.rs b/crates/rust-analyzer/src/cli/analysis_stats.rs
index 3417af687..6d6f398f4 100644
--- a/crates/rust-analyzer/src/cli/analysis_stats.rs
+++ b/crates/rust-analyzer/src/cli/analysis_stats.rs
@@ -218,9 +218,9 @@ impl AnalysisStatsCmd {
                         bar.println(format!(
                             "{}:{}-{}:{}: {}",
                             start.line + 1,
-                            start.col_utf16,
+                            start.col,
                             end.line + 1,
-                            end.col_utf16,
+                            end.col,
                             ty.display(db)
                         ));
                     } else {
@@ -250,9 +250,9 @@ impl AnalysisStatsCmd {
                                 "{} {}:{}-{}:{}: Expected {}, got {}",
                                 path,
                                 start.line + 1,
-                                start.col_utf16,
+                                start.col,
                                 end.line + 1,
-                                end.col_utf16,
+                                end.col,
                                 mismatch.expected.display(db),
                                 mismatch.actual.display(db)
                             ));
diff --git a/crates/rust-analyzer/src/config.rs b/crates/rust-analyzer/src/config.rs
index 04a77d677..556fc2eeb 100644
--- a/crates/rust-analyzer/src/config.rs
+++ b/crates/rust-analyzer/src/config.rs
@@ -23,7 +23,10 @@ use rustc_hash::FxHashSet;
 use serde::{de::DeserializeOwned, Deserialize};
 use vfs::AbsPathBuf;
 
-use crate::{caps::completion_item_edit_resolve, diagnostics::DiagnosticsMapConfig};
+use crate::{
+    caps::completion_item_edit_resolve, diagnostics::DiagnosticsMapConfig,
+    line_index::OffsetEncoding, lsp_ext::supports_utf8,
+};
 
 config_data! {
     struct ConfigData {
@@ -415,6 +418,13 @@ impl Config {
             false
         )
     }
+    pub fn offset_encoding(&self) -> OffsetEncoding {
+        if supports_utf8(&self.caps) {
+            OffsetEncoding::Utf8
+        } else {
+            OffsetEncoding::Utf16
+        }
+    }
 
     fn experimental(&self, index: &'static str) -> bool {
         try_or!(self.caps.experimental.as_ref()?.get(index)?.as_bool()?, false)
diff --git a/crates/rust-analyzer/src/from_proto.rs b/crates/rust-analyzer/src/from_proto.rs
index 6676eebf4..5b02b2598 100644
--- a/crates/rust-analyzer/src/from_proto.rs
+++ b/crates/rust-analyzer/src/from_proto.rs
@@ -1,12 +1,17 @@
 //! Conversion lsp_types types to rust-analyzer specific ones.
 use std::convert::TryFrom;
 
-use ide::{Annotation, AnnotationKind, AssistKind, LineCol, LineIndex};
+use ide::{Annotation, AnnotationKind, AssistKind, LineCol, LineColUtf16};
 use ide_db::base_db::{FileId, FilePosition, FileRange};
 use syntax::{TextRange, TextSize};
 use vfs::AbsPathBuf;
 
-use crate::{from_json, global_state::GlobalStateSnapshot, lsp_ext, Result};
+use crate::{
+    from_json,
+    global_state::GlobalStateSnapshot,
+    line_index::{LineIndex, OffsetEncoding},
+    lsp_ext, Result,
+};
 
 pub(crate) fn abs_path(url: &lsp_types::Url) -> Result<AbsPathBuf> {
     let path = url.to_file_path().map_err(|()| "url is not a file")?;
@@ -18,8 +23,17 @@ pub(crate) fn vfs_path(url: &lsp_types::Url) -> Result<vfs::VfsPath> {
 }
 
 pub(crate) fn offset(line_index: &LineIndex, position: lsp_types::Position) -> TextSize {
-    let line_col = LineCol { line: position.line as u32, col_utf16: position.character as u32 };
-    line_index.offset(line_col)
+    let line_col = match line_index.encoding {
+        OffsetEncoding::Utf8 => {
+            LineCol { line: position.line as u32, col: position.character as u32 }
+        }
+        OffsetEncoding::Utf16 => {
+            let line_col =
+                LineColUtf16 { line: position.line as u32, col: position.character as u32 };
+            line_index.index.to_utf8(line_col)
+        }
+    };
+    line_index.index.offset(line_col)
 }
 
 pub(crate) fn text_range(line_index: &LineIndex, range: lsp_types::Range) -> TextRange {
@@ -37,8 +51,8 @@ pub(crate) fn file_position(
     tdpp: lsp_types::TextDocumentPositionParams,
 ) -> Result<FilePosition> {
     let file_id = file_id(world, &tdpp.text_document.uri)?;
-    let line_index = world.analysis.file_line_index(file_id)?;
-    let offset = offset(&*line_index, tdpp.position);
+    let line_index = world.file_line_index(file_id)?;
+    let offset = offset(&line_index, tdpp.position);
     Ok(FilePosition { file_id, offset })
 }
 
@@ -48,7 +62,7 @@ pub(crate) fn file_range(
     range: lsp_types::Range,
 ) -> Result<FileRange> {
     let file_id = file_id(world, &text_document_identifier.uri)?;
-    let line_index = world.analysis.file_line_index(file_id)?;
+    let line_index = world.file_line_index(file_id)?;
     let range = text_range(&line_index, range);
     Ok(FileRange { file_id, range })
 }
@@ -78,7 +92,7 @@ pub(crate) fn annotation(
         lsp_ext::CodeLensResolveData::Impls(params) => {
             let file_id =
                 world.url_to_file_id(&params.text_document_position_params.text_document.uri)?;
-            let line_index = world.analysis.file_line_index(file_id)?;
+            let line_index = world.file_line_index(file_id)?;
 
             Ok(Annotation {
                 range: text_range(&line_index, code_lens.range),
@@ -90,7 +104,7 @@ pub(crate) fn annotation(
         }
         lsp_ext::CodeLensResolveData::References(params) => {
             let file_id = world.url_to_file_id(&params.text_document.uri)?;
-            let line_index = world.analysis.file_line_index(file_id)?;
+            let line_index = world.file_line_index(file_id)?;
 
             Ok(Annotation {
                 range: text_range(&line_index, code_lens.range),
diff --git a/crates/rust-analyzer/src/global_state.rs b/crates/rust-analyzer/src/global_state.rs
index c3bc8791d..52c249713 100644
--- a/crates/rust-analyzer/src/global_state.rs
+++ b/crates/rust-analyzer/src/global_state.rs
@@ -7,7 +7,7 @@ use std::{sync::Arc, time::Instant};
 
 use crossbeam_channel::{unbounded, Receiver, Sender};
 use flycheck::FlycheckHandle;
-use ide::{Analysis, AnalysisHost, Change, FileId};
+use ide::{Analysis, AnalysisHost, Cancelable, Change, FileId};
 use ide_db::base_db::{CrateId, VfsPath};
 use lsp_types::{SemanticTokens, Url};
 use parking_lot::{Mutex, RwLock};
@@ -22,7 +22,7 @@ use crate::{
     diagnostics::{CheckFixes, DiagnosticCollection},
     document::DocumentData,
     from_proto,
-    line_endings::LineEndings,
+    line_index::{LineEndings, LineIndex},
     main_loop::Task,
     op_queue::OpQueue,
     reload::SourceRootConfig,
@@ -271,8 +271,11 @@ impl GlobalStateSnapshot {
         file_id_to_url(&self.vfs.read().0, id)
     }
 
-    pub(crate) fn file_line_endings(&self, id: FileId) -> LineEndings {
-        self.vfs.read().1[&id]
+    pub(crate) fn file_line_index(&self, file_id: FileId) -> Cancelable<LineIndex> {
+        let endings = self.vfs.read().1[&file_id];
+        let index = self.analysis.file_line_index(file_id)?;
+        let res = LineIndex { index, endings, encoding: self.config.offset_encoding() };
+        Ok(res)
     }
 
     pub(crate) fn url_file_version(&self, url: &Url) -> Option<i32> {
diff --git a/crates/rust-analyzer/src/handlers.rs b/crates/rust-analyzer/src/handlers.rs
index 2d697c75f..4f6f250d6 100644
--- a/crates/rust-analyzer/src/handlers.rs
+++ b/crates/rust-analyzer/src/handlers.rs
@@ -5,12 +5,11 @@
 use std::{
     io::Write as _,
     process::{self, Stdio},
-    sync::Arc,
 };
 
 use ide::{
-    AnnotationConfig, FileId, FilePosition, FileRange, HoverAction, HoverGotoTypeData, LineIndex,
-    Query, RangeInfo, Runnable, RunnableKind, SearchScope, SourceChange, TextEdit,
+    AnnotationConfig, FileId, FilePosition, FileRange, HoverAction, HoverGotoTypeData, Query,
+    RangeInfo, Runnable, RunnableKind, SearchScope, SourceChange, TextEdit,
 };
 use ide_db::SymbolKind;
 use itertools::Itertools;
@@ -37,7 +36,7 @@ use crate::{
     diff::diff,
     from_proto,
     global_state::{GlobalState, GlobalStateSnapshot},
-    line_endings::LineEndings,
+    line_index::{LineEndings, LineIndex},
     lsp_ext::{self, InlayHint, InlayHintsParams},
     lsp_utils::all_edits_are_disjoint,
     to_proto, LspError, Result,
@@ -100,7 +99,7 @@ pub(crate) fn handle_syntax_tree(
 ) -> Result<String> {
     let _p = profile::span("handle_syntax_tree");
     let id = from_proto::file_id(&snap, &params.text_document.uri)?;
-    let line_index = snap.analysis.file_line_index(id)?;
+    let line_index = snap.file_line_index(id)?;
     let text_range = params.range.map(|r| from_proto::text_range(&line_index, r));
     let res = snap.analysis.syntax_tree(id, text_range)?;
     Ok(res)
@@ -122,7 +121,7 @@ pub(crate) fn handle_expand_macro(
 ) -> Result<Option<lsp_ext::ExpandedMacro>> {
     let _p = profile::span("handle_expand_macro");
     let file_id = from_proto::file_id(&snap, &params.text_document.uri)?;
-    let line_index = snap.analysis.file_line_index(file_id)?;
+    let line_index = snap.file_line_index(file_id)?;
     let offset = from_proto::offset(&line_index, params.position);
 
     let res = snap.analysis.expand_macro(FilePosition { file_id, offset })?;
@@ -135,7 +134,7 @@ pub(crate) fn handle_selection_range(
 ) -> Result<Option<Vec<lsp_types::SelectionRange>>> {
     let _p = profile::span("handle_selection_range");
     let file_id = from_proto::file_id(&snap, &params.text_document.uri)?;
-    let line_index = snap.analysis.file_line_index(file_id)?;
+    let line_index = snap.file_line_index(file_id)?;
     let res: Result<Vec<lsp_types::SelectionRange>> = params
         .positions
         .into_iter()
@@ -178,7 +177,7 @@ pub(crate) fn handle_matching_brace(
 ) -> Result<Vec<Position>> {
     let _p = profile::span("handle_matching_brace");
     let file_id = from_proto::file_id(&snap, &params.text_document.uri)?;
-    let line_index = snap.analysis.file_line_index(file_id)?;
+    let line_index = snap.file_line_index(file_id)?;
     let res = params
         .positions
         .into_iter()
@@ -200,8 +199,7 @@ pub(crate) fn handle_join_lines(
 ) -> Result<Vec<lsp_types::TextEdit>> {
     let _p = profile::span("handle_join_lines");
     let file_id = from_proto::file_id(&snap, &params.text_document.uri)?;
-    let line_index = snap.analysis.file_line_index(file_id)?;
-    let line_endings = snap.file_line_endings(file_id);
+    let line_index = snap.file_line_index(file_id)?;
     let mut res = TextEdit::default();
     for range in params.ranges {
         let range = from_proto::text_range(&line_index, range);
@@ -213,7 +211,7 @@ pub(crate) fn handle_join_lines(
             }
         }
     }
-    let res = to_proto::text_edit_vec(&line_index, line_endings, res);
+    let res = to_proto::text_edit_vec(&line_index, res);
     Ok(res)
 }
 
@@ -227,9 +225,8 @@ pub(crate) fn handle_on_enter(
         None => return Ok(None),
         Some(it) => it,
     };
-    let line_index = snap.analysis.file_line_index(position.file_id)?;
-    let line_endings = snap.file_line_endings(position.file_id);
-    let edit = to_proto::snippet_text_edit_vec(&line_index, line_endings, true, edit);
+    let line_index = snap.file_line_index(position.file_id)?;
+    let edit = to_proto::snippet_text_edit_vec(&line_index, true, edit);
     Ok(Some(edit))
 }
 
@@ -240,8 +237,7 @@ pub(crate) fn handle_on_type_formatting(
 ) -> Result<Option<Vec<lsp_types::TextEdit>>> {
     let _p = profile::span("handle_on_type_formatting");
     let mut position = from_proto::file_position(&snap, params.text_document_position)?;
-    let line_index = snap.analysis.file_line_index(position.file_id)?;
-    let line_endings = snap.file_line_endings(position.file_id);
+    let line_index = snap.file_line_index(position.file_id)?;
 
     // in `ide`, the `on_type` invariant is that
     // `text.char_at(position) == typed_char`.
@@ -269,7 +265,7 @@ pub(crate) fn handle_on_type_formatting(
     // This should be a single-file edit
     let (_, edit) = edit.source_file_edits.into_iter().next().unwrap();
 
-    let change = to_proto::text_edit_vec(&line_index, line_endings, edit);
+    let change = to_proto::text_edit_vec(&line_index, edit);
     Ok(Some(change))
 }
 
@@ -279,7 +275,7 @@ pub(crate) fn handle_document_symbol(
 ) -> Result<Option<lsp_types::DocumentSymbolResponse>> {
     let _p = profile::span("handle_document_symbol");
     let file_id = from_proto::file_id(&snap, &params.text_document.uri)?;
-    let line_index = snap.analysis.file_line_index(file_id)?;
+    let line_index = snap.file_line_index(file_id)?;
 
     let mut parents: Vec<(lsp_types::DocumentSymbol, Option<usize>)> = Vec::new();
 
@@ -535,7 +531,7 @@ pub(crate) fn handle_runnables(
 ) -> Result<Vec<lsp_ext::Runnable>> {
     let _p = profile::span("handle_runnables");
     let file_id = from_proto::file_id(&snap, &params.text_document.uri)?;
-    let line_index = snap.analysis.file_line_index(file_id)?;
+    let line_index = snap.file_line_index(file_id)?;
     let offset = params.position.map(|it| from_proto::offset(&line_index, it));
     let cargo_spec = CargoTargetSpec::for_file(&snap, file_id)?;
 
@@ -645,14 +641,12 @@ pub(crate) fn handle_completion(
         None => return Ok(None),
         Some(items) => items,
     };
-    let line_index = snap.analysis.file_line_index(position.file_id)?;
-    let line_endings = snap.file_line_endings(position.file_id);
+    let line_index = snap.file_line_index(position.file_id)?;
 
     let items: Vec<CompletionItem> = items
         .into_iter()
         .flat_map(|item| {
-            let mut new_completion_items =
-                to_proto::completion_item(&line_index, line_endings, item.clone());
+            let mut new_completion_items = to_proto::completion_item(&line_index, item.clone());
 
             if completion_config.enable_imports_on_the_fly {
                 for new_item in &mut new_completion_items {
@@ -693,8 +687,7 @@ pub(crate) fn handle_completion_resolve(
     };
 
     let file_id = from_proto::file_id(&snap, &resolve_data.position.text_document.uri)?;
-    let line_index = snap.analysis.file_line_index(file_id)?;
-    let line_endings = snap.file_line_endings(file_id);
+    let line_index = snap.file_line_index(file_id)?;
     let offset = from_proto::offset(&line_index, resolve_data.position.position);
 
     let additional_edits = snap
@@ -707,9 +700,7 @@ pub(crate) fn handle_completion_resolve(
             resolve_data.import_for_trait_assoc_item,
         )?
         .into_iter()
-        .flat_map(|edit| {
-            edit.into_iter().map(|indel| to_proto::text_edit(&line_index, line_endings, indel))
-        })
+        .flat_map(|edit| edit.into_iter().map(|indel| to_proto::text_edit(&line_index, indel)))
         .collect_vec();
 
     if !all_edits_are_disjoint(&original_completion, &additional_edits) {
@@ -738,7 +729,7 @@ pub(crate) fn handle_folding_range(
     let file_id = from_proto::file_id(&snap, &params.text_document.uri)?;
     let folds = snap.analysis.folding_ranges(file_id)?;
     let text = snap.analysis.file_text(file_id)?;
-    let line_index = snap.analysis.file_line_index(file_id)?;
+    let line_index = snap.file_line_index(file_id)?;
     let line_folding_only = snap.config.line_folding_only();
     let res = folds
         .into_iter()
@@ -775,7 +766,7 @@ pub(crate) fn handle_hover(
             None => return Ok(None),
             Some(info) => info,
         };
-    let line_index = snap.analysis.file_line_index(position.file_id)?;
+    let line_index = snap.file_line_index(position.file_id)?;
     let range = to_proto::range(&line_index, info.range);
     let hover = lsp_ext::Hover {
         hover: lsp_types::Hover {
@@ -797,7 +788,7 @@ pub(crate) fn handle_prepare_rename(
 
     let change = snap.analysis.prepare_rename(position)?.map_err(to_proto::rename_error)?;
 
-    let line_index = snap.analysis.file_line_index(position.file_id)?;
+    let line_index = snap.file_line_index(position.file_id)?;
     let range = to_proto::range(&line_index, change.range);
     Ok(Some(PrepareRenameResponse::Range(range)))
 }
@@ -857,8 +848,7 @@ pub(crate) fn handle_formatting(
     let file = snap.analysis.file_text(file_id)?;
     let crate_ids = snap.analysis.crate_for(file_id)?;
 
-    let file_line_index = snap.analysis.file_line_index(file_id)?;
-    let file_line_endings = snap.file_line_endings(file_id);
+    let line_index = snap.file_line_index(file_id)?;
 
     let mut rustfmt = match snap.config.rustfmt() {
         RustfmtConfig::Rustfmt { extra_args } => {
@@ -935,24 +925,19 @@ pub(crate) fn handle_formatting(
 
     let (new_text, new_line_endings) = LineEndings::normalize(captured_stdout);
 
-    if file_line_endings != new_line_endings {
+    if line_index.endings != new_line_endings {
         // If line endings are different, send the entire file.
         // Diffing would not work here, as the line endings might be the only
         // difference.
         Ok(Some(to_proto::text_edit_vec(
-            &file_line_index,
-            new_line_endings,
+            &line_index,
             TextEdit::replace(TextRange::up_to(TextSize::of(&*file)), new_text),
         )))
     } else if *file == new_text {
         // The document is already formatted correctly -- no edits needed.
         Ok(None)
     } else {
-        Ok(Some(to_proto::text_edit_vec(
-            &file_line_index,
-            file_line_endings,
-            diff(&file, &new_text),
-        )))
+        Ok(Some(to_proto::text_edit_vec(&line_index, diff(&file, &new_text))))
     }
 }
 
@@ -969,7 +954,7 @@ pub(crate) fn handle_code_action(
     }
 
     let file_id = from_proto::file_id(&snap, &params.text_document.uri)?;
-    let line_index = snap.analysis.file_line_index(file_id)?;
+    let line_index = snap.file_line_index(file_id)?;
     let range = from_proto::text_range(&line_index, params.range);
     let frange = FileRange { file_id, range };
 
@@ -1010,7 +995,7 @@ pub(crate) fn handle_code_action(
 fn add_quick_fixes(
     snap: &GlobalStateSnapshot,
     frange: FileRange,
-    line_index: &Arc<LineIndex>,
+    line_index: &LineIndex,
     acc: &mut Vec<lsp_ext::CodeAction>,
 ) -> Result<()> {
     let diagnostics = snap.analysis.diagnostics(&snap.config.diagnostics(), frange.file_id)?;
@@ -1052,7 +1037,7 @@ pub(crate) fn handle_code_action_resolve(
     };
 
     let file_id = from_proto::file_id(&snap, &params.code_action_params.text_document.uri)?;
-    let line_index = snap.analysis.file_line_index(file_id)?;
+    let line_index = snap.file_line_index(file_id)?;
     let range = from_proto::text_range(&line_index, params.code_action_params.range);
     let frange = FileRange { file_id, range };
 
@@ -1131,7 +1116,7 @@ pub(crate) fn handle_document_highlight(
 ) -> Result<Option<Vec<DocumentHighlight>>> {
     let _p = profile::span("handle_document_highlight");
     let position = from_proto::file_position(&snap, params.text_document_position_params)?;
-    let line_index = snap.analysis.file_line_index(position.file_id)?;
+    let line_index = snap.file_line_index(position.file_id)?;
 
     let refs = match snap
         .analysis
@@ -1192,7 +1177,7 @@ pub(crate) fn publish_diagnostics(
     file_id: FileId,
 ) -> Result<Vec<Diagnostic>> {
     let _p = profile::span("publish_diagnostics");
-    let line_index = snap.analysis.file_line_index(file_id)?;
+    let line_index = snap.file_line_index(file_id)?;
 
     let diagnostics: Vec<Diagnostic> = snap
         .analysis
@@ -1226,7 +1211,7 @@ pub(crate) fn handle_inlay_hints(
 ) -> Result<Vec<InlayHint>> {
     let _p = profile::span("handle_inlay_hints");
     let file_id = from_proto::file_id(&snap, &params.text_document.uri)?;
-    let line_index = snap.analysis.file_line_index(file_id)?;
+    let line_index = snap.file_line_index(file_id)?;
     Ok(snap
         .analysis
         .inlay_hints(file_id, &snap.config.inlay_hints())?
@@ -1277,7 +1262,7 @@ pub(crate) fn handle_call_hierarchy_incoming(
 
     for call_item in call_items.into_iter() {
         let file_id = call_item.target.file_id;
-        let line_index = snap.analysis.file_line_index(file_id)?;
+        let line_index = snap.file_line_index(file_id)?;
         let item = to_proto::call_hierarchy_item(&snap, call_item.target)?;
         res.push(CallHierarchyIncomingCall {
             from: item,
@@ -1312,7 +1297,7 @@ pub(crate) fn handle_call_hierarchy_outgoing(
 
     for call_item in call_items.into_iter() {
         let file_id = call_item.target.file_id;
-        let line_index = snap.analysis.file_line_index(file_id)?;
+        let line_index = snap.file_line_index(file_id)?;
         let item = to_proto::call_hierarchy_item(&snap, call_item.target)?;
         res.push(CallHierarchyOutgoingCall {
             to: item,
@@ -1335,7 +1320,7 @@ pub(crate) fn handle_semantic_tokens_full(
 
     let file_id = from_proto::file_id(&snap, &params.text_document.uri)?;
     let text = snap.analysis.file_text(file_id)?;
-    let line_index = snap.analysis.file_line_index(file_id)?;
+    let line_index = snap.file_line_index(file_id)?;
 
     let highlights = snap.analysis.highlight(file_id)?;
     let semantic_tokens = to_proto::semantic_tokens(&text, &line_index, highlights);
@@ -1354,7 +1339,7 @@ pub(crate) fn handle_semantic_tokens_full_delta(
 
     let file_id = from_proto::file_id(&snap, &params.text_document.uri)?;
     let text = snap.analysis.file_text(file_id)?;
-    let line_index = snap.analysis.file_line_index(file_id)?;
+    let line_index = snap.file_line_index(file_id)?;
 
     let highlights = snap.analysis.highlight(file_id)?;
 
@@ -1384,7 +1369,7 @@ pub(crate) fn handle_semantic_tokens_range(
 
     let frange = from_proto::file_range(&snap, params.text_document, params.range)?;
     let text = snap.analysis.file_text(frange.file_id)?;
-    let line_index = snap.analysis.file_line_index(frange.file_id)?;
+    let line_index = snap.file_line_index(frange.file_id)?;
 
     let highlights = snap.analysis.highlight_range(frange)?;
     let semantic_tokens = to_proto::semantic_tokens(&text, &line_index, highlights);
@@ -1432,7 +1417,7 @@ fn show_impl_command_link(
     if snap.config.hover().implementations {
         if let Some(nav_data) = snap.analysis.goto_implementation(*position).unwrap_or(None) {
             let uri = to_proto::url(snap, position.file_id);
-            let line_index = snap.analysis.file_line_index(position.file_id).ok()?;
+            let line_index = snap.file_line_index(position.file_id).ok()?;
             let position = to_proto::position(&line_index, position.offset);
             let locations: Vec<_> = nav_data
                 .info
diff --git a/crates/rust-analyzer/src/lib.rs b/crates/rust-analyzer/src/lib.rs
index 2207b9a87..8b874239c 100644
--- a/crates/rust-analyzer/src/lib.rs
+++ b/crates/rust-analyzer/src/lib.rs
@@ -29,7 +29,7 @@ mod from_proto;
 mod semantic_tokens;
 mod markdown;
 mod diagnostics;
-mod line_endings;
+mod line_index;
 mod request_metrics;
 mod lsp_utils;
 mod thread_pool;
diff --git a/crates/rust-analyzer/src/line_endings.rs b/crates/rust-analyzer/src/line_index.rs
index bf0e255d9..c116414da 100644
--- a/crates/rust-analyzer/src/line_endings.rs
+++ b/crates/rust-analyzer/src/line_index.rs
@@ -1,7 +1,23 @@
+//! Enhances `ide::LineIndex` with additional info required to convert offsets
+//! into lsp positions.
+//!
 //! We maintain invariant that all internal strings use `\n` as line separator.
 //! This module does line ending conversion and detection (so that we can
 //! convert back to `\r\n` on the way out).
 
+use std::sync::Arc;
+
+pub enum OffsetEncoding {
+    Utf8,
+    Utf16,
+}
+
+pub(crate) struct LineIndex {
+    pub(crate) index: Arc<ide::LineIndex>,
+    pub(crate) endings: LineEndings,
+    pub(crate) encoding: OffsetEncoding,
+}
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub(crate) enum LineEndings {
     Unix,
diff --git a/crates/rust-analyzer/src/lsp_ext.rs b/crates/rust-analyzer/src/lsp_ext.rs
index a1ad855c3..0d2c8f7ff 100644
--- a/crates/rust-analyzer/src/lsp_ext.rs
+++ b/crates/rust-analyzer/src/lsp_ext.rs
@@ -385,3 +385,7 @@ pub(crate) enum CodeLensResolveData {
     Impls(lsp_types::request::GotoImplementationParams),
     References(lsp_types::TextDocumentPositionParams),
 }
+
+pub fn supports_utf8(caps: &lsp_types::ClientCapabilities) -> bool {
+    caps.offset_encoding.as_deref().unwrap_or_default().iter().any(|it| it == "utf-8")
+}
diff --git a/crates/rust-analyzer/src/lsp_utils.rs b/crates/rust-analyzer/src/lsp_utils.rs
index 2d06fe538..84f78b5b8 100644
--- a/crates/rust-analyzer/src/lsp_utils.rs
+++ b/crates/rust-analyzer/src/lsp_utils.rs
@@ -1,11 +1,14 @@
 //! Utilities for LSP-related boilerplate code.
-use std::{error::Error, ops::Range};
+use std::{error::Error, ops::Range, sync::Arc};
 
-use ide::LineIndex;
 use ide_db::base_db::Canceled;
 use lsp_server::Notification;
 
-use crate::{from_proto, global_state::GlobalState};
+use crate::{
+    from_proto,
+    global_state::GlobalState,
+    line_index::{LineEndings, LineIndex, OffsetEncoding},
+};
 
 pub(crate) fn is_canceled(e: &(dyn Error + 'static)) -> bool {
     e.downcast_ref::<Canceled>().is_some()
@@ -90,7 +93,13 @@ pub(crate) fn apply_document_changes(
     old_text: &mut String,
     content_changes: Vec<lsp_types::TextDocumentContentChangeEvent>,
 ) {
-    let mut line_index = LineIndex::new(old_text);
+    let mut line_index = LineIndex {
+        index: Arc::new(ide::LineIndex::new(old_text)),
+        // We don't care about line endings or offset encoding here.
+        endings: LineEndings::Unix,
+        encoding: OffsetEncoding::Utf16,
+    };
+
     // 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
@@ -115,7 +124,7 @@ pub(crate) fn apply_document_changes(
         match change.range {
             Some(range) => {
                 if !index_valid.covers(range.end.line) {
-                    line_index = LineIndex::new(&old_text);
+                    line_index.index = Arc::new(ide::LineIndex::new(&old_text));
                 }
                 index_valid = IndexValid::UpToLineExclusive(range.start.line);
                 let range = from_proto::text_range(&line_index, range);
diff --git a/crates/rust-analyzer/src/to_proto.rs b/crates/rust-analyzer/src/to_proto.rs
index b0ddb603d..70cb7fbab 100644
--- a/crates/rust-analyzer/src/to_proto.rs
+++ b/crates/rust-analyzer/src/to_proto.rs
@@ -7,22 +7,29 @@ use std::{
 use ide::{
     Annotation, AnnotationKind, Assist, AssistKind, CallInfo, CompletionItem, CompletionItemKind,
     Documentation, FileId, FileRange, FileSystemEdit, Fold, FoldKind, Highlight, HlMod, HlPunct,
-    HlRange, HlTag, Indel, InlayHint, InlayKind, InsertTextFormat, LineIndex, Markup,
-    NavigationTarget, ReferenceAccess, RenameError, Runnable, Severity, SourceChange, TextEdit,
-    TextRange, TextSize,
+    HlRange, HlTag, Indel, InlayHint, InlayKind, InsertTextFormat, Markup, NavigationTarget,
+    ReferenceAccess, RenameError, Runnable, Severity, SourceChange, TextEdit, TextRange, TextSize,
 };
 use ide_db::SymbolKind;
 use itertools::Itertools;
 use serde_json::to_value;
 
 use crate::{
-    cargo_target_spec::CargoTargetSpec, global_state::GlobalStateSnapshot,
-    line_endings::LineEndings, lsp_ext, semantic_tokens, Result,
+    cargo_target_spec::CargoTargetSpec,
+    global_state::GlobalStateSnapshot,
+    line_index::{LineEndings, LineIndex, OffsetEncoding},
+    lsp_ext, semantic_tokens, Result,
 };
 
 pub(crate) fn position(line_index: &LineIndex, offset: TextSize) -> lsp_types::Position {
-    let line_col = line_index.line_col(offset);
-    lsp_types::Position::new(line_col.line, line_col.col_utf16)
+    let line_col = line_index.index.line_col(offset);
+    match line_index.encoding {
+        OffsetEncoding::Utf8 => lsp_types::Position::new(line_col.line, line_col.col),
+        OffsetEncoding::Utf16 => {
+            let line_col = line_index.index.to_utf16(line_col);
+            lsp_types::Position::new(line_col.line, line_col.col)
+        }
+    }
 }
 
 pub(crate) fn range(line_index: &LineIndex, range: TextRange) -> lsp_types::Range {
@@ -122,13 +129,9 @@ pub(crate) fn completion_item_kind(
     }
 }
 
-pub(crate) fn text_edit(
-    line_index: &LineIndex,
-    line_endings: LineEndings,
-    indel: Indel,
-) -> lsp_types::TextEdit {
+pub(crate) fn text_edit(line_index: &LineIndex, indel: Indel) -> lsp_types::TextEdit {
     let range = range(line_index, indel.delete);
-    let new_text = match line_endings {
+    let new_text = match line_index.endings {
         LineEndings::Unix => indel.insert,
         LineEndings::Dos => indel.insert.replace('\n', "\r\n"),
     };
@@ -137,11 +140,10 @@ pub(crate) fn text_edit(
 
 pub(crate) fn snippet_text_edit(
     line_index: &LineIndex,
-    line_endings: LineEndings,
     is_snippet: bool,
     indel: Indel,
 ) -> lsp_ext::SnippetTextEdit {
-    let text_edit = text_edit(line_index, line_endings, indel);
+    let text_edit = text_edit(line_index, indel);
     let insert_text_format =
         if is_snippet { Some(lsp_types::InsertTextFormat::Snippet) } else { None };
     lsp_ext::SnippetTextEdit {
@@ -153,27 +155,24 @@ pub(crate) fn snippet_text_edit(
 
 pub(crate) fn text_edit_vec(
     line_index: &LineIndex,
-    line_endings: LineEndings,
     text_edit: TextEdit,
 ) -> Vec<lsp_types::TextEdit> {
-    text_edit.into_iter().map(|indel| self::text_edit(line_index, line_endings, indel)).collect()
+    text_edit.into_iter().map(|indel| self::text_edit(line_index, indel)).collect()
 }
 
 pub(crate) fn snippet_text_edit_vec(
     line_index: &LineIndex,
-    line_endings: LineEndings,
     is_snippet: bool,
     text_edit: TextEdit,
 ) -> Vec<lsp_ext::SnippetTextEdit> {
     text_edit
         .into_iter()
-        .map(|indel| self::snippet_text_edit(line_index, line_endings, is_snippet, indel))
+        .map(|indel| self::snippet_text_edit(line_index, is_snippet, indel))
         .collect()
 }
 
 pub(crate) fn completion_item(
     line_index: &LineIndex,
-    line_endings: LineEndings,
     completion_item: CompletionItem,
 ) -> Vec<lsp_types::CompletionItem> {
     fn set_score(res: &mut lsp_types::CompletionItem, label: &str) {
@@ -190,19 +189,19 @@ pub(crate) fn completion_item(
     for indel in completion_item.text_edit().iter() {
         if indel.delete.contains_range(source_range) {
             text_edit = Some(if indel.delete == source_range {
-                self::text_edit(line_index, line_endings, indel.clone())
+                self::text_edit(line_index, indel.clone())
             } else {
                 assert!(source_range.end() == indel.delete.end());
                 let range1 = TextRange::new(indel.delete.start(), source_range.start());
                 let range2 = source_range;
                 let indel1 = Indel::replace(range1, String::new());
                 let indel2 = Indel::replace(range2, indel.insert.clone());
-                additional_text_edits.push(self::text_edit(line_index, line_endings, indel1));
-                self::text_edit(line_index, line_endings, indel2)
+                additional_text_edits.push(self::text_edit(line_index, indel1));
+                self::text_edit(line_index, indel2)
             })
         } else {
             assert!(source_range.intersect(indel.delete).is_none());
-            let text_edit = self::text_edit(line_index, line_endings, indel.clone());
+            let text_edit = self::text_edit(line_index, indel.clone());
             additional_text_edits.push(text_edit);
         }
     }
@@ -358,7 +357,7 @@ pub(crate) fn semantic_tokens(
         let token_index = semantic_tokens::type_index(type_);
         let modifier_bitset = mods.0;
 
-        for mut text_range in line_index.lines(highlight_range.range) {
+        for mut text_range in line_index.index.lines(highlight_range.range) {
             if text[text_range].ends_with('\n') {
                 text_range =
                     TextRange::new(text_range.start(), text_range.end() - TextSize::of('\n'));
@@ -565,7 +564,7 @@ pub(crate) fn location(
     frange: FileRange,
 ) -> Result<lsp_types::Location> {
     let url = url(snap, frange.file_id);
-    let line_index = snap.analysis.file_line_index(frange.file_id)?;
+    let line_index = snap.file_line_index(frange.file_id)?;
     let range = range(&line_index, frange.range);
     let loc = lsp_types::Location::new(url, range);
     Ok(loc)
@@ -577,7 +576,7 @@ pub(crate) fn location_from_nav(
     nav: NavigationTarget,
 ) -> Result<lsp_types::Location> {
     let url = url(snap, nav.file_id);
-    let line_index = snap.analysis.file_line_index(nav.file_id)?;
+    let line_index = snap.file_line_index(nav.file_id)?;
     let range = range(&line_index, nav.full_range);
     let loc = lsp_types::Location::new(url, range);
     Ok(loc)
@@ -590,7 +589,7 @@ pub(crate) fn location_link(
 ) -> Result<lsp_types::LocationLink> {
     let origin_selection_range = match src {
         Some(src) => {
-            let line_index = snap.analysis.file_line_index(src.file_id)?;
+            let line_index = snap.file_line_index(src.file_id)?;
             let range = range(&line_index, src.range);
             Some(range)
         }
@@ -610,7 +609,7 @@ fn location_info(
     snap: &GlobalStateSnapshot,
     target: NavigationTarget,
 ) -> Result<(lsp_types::Url, lsp_types::Range, lsp_types::Range)> {
-    let line_index = snap.analysis.file_line_index(target.file_id)?;
+    let line_index = snap.file_line_index(target.file_id)?;
 
     let target_uri = url(snap, target.file_id);
     let target_range = range(&line_index, target.full_range);
@@ -648,12 +647,8 @@ pub(crate) fn snippet_text_document_edit(
     edit: TextEdit,
 ) -> Result<lsp_ext::SnippetTextDocumentEdit> {
     let text_document = optional_versioned_text_document_identifier(snap, file_id);
-    let line_index = snap.analysis.file_line_index(file_id)?;
-    let line_endings = snap.file_line_endings(file_id);
-    let edits = edit
-        .into_iter()
-        .map(|it| snippet_text_edit(&line_index, line_endings, is_snippet, it))
-        .collect();
+    let line_index = snap.file_line_index(file_id)?;
+    let edits = edit.into_iter().map(|it| snippet_text_edit(&line_index, is_snippet, it)).collect();
     Ok(lsp_ext::SnippetTextDocumentEdit { text_document, edits })
 }
 
@@ -674,9 +669,8 @@ pub(crate) fn snippet_text_document_ops(
             if !initial_contents.is_empty() {
                 let text_document =
                     lsp_types::OptionalVersionedTextDocumentIdentifier { uri, version: None };
-                let range = range(&LineIndex::new(""), TextRange::empty(TextSize::from(0)));
                 let text_edit = lsp_ext::SnippetTextEdit {
-                    range,
+                    range: lsp_types::Range::default(),
                     new_text: initial_contents,
                     insert_text_format: Some(lsp_types::InsertTextFormat::PlainText),
                 };
@@ -867,7 +861,7 @@ pub(crate) fn code_lens(
 ) -> Result<lsp_types::CodeLens> {
     match annotation.kind {
         AnnotationKind::Runnable { debug, runnable: run } => {
-            let line_index = snap.analysis.file_line_index(run.nav.file_id)?;
+            let line_index = snap.file_line_index(run.nav.file_id)?;
             let annotation_range = range(&line_index, annotation.range);
 
             let action = run.action();
@@ -883,7 +877,7 @@ pub(crate) fn code_lens(
             Ok(lsp_types::CodeLens { range: annotation_range, command: Some(command), data: None })
         }
         AnnotationKind::HasImpls { position: file_position, data } => {
-            let line_index = snap.analysis.file_line_index(file_position.file_id)?;
+            let line_index = snap.file_line_index(file_position.file_id)?;
             let annotation_range = range(&line_index, annotation.range);
             let url = url(snap, file_position.file_id);
 
@@ -926,7 +920,7 @@ pub(crate) fn code_lens(
             })
         }
         AnnotationKind::HasReferences { position: file_position, data } => {
-            let line_index = snap.analysis.file_line_index(file_position.file_id)?;
+            let line_index = snap.file_line_index(file_position.file_id)?;
             let annotation_range = range(&line_index, annotation.range);
             let url = url(snap, file_position.file_id);
 
@@ -1060,6 +1054,8 @@ pub(crate) fn rename_error(err: RenameError) -> crate::LspError {
 
 #[cfg(test)]
 mod tests {
+    use std::sync::Arc;
+
     use hir::PrefixKind;
     use ide::Analysis;
     use ide_db::helpers::{insert_use::InsertUseConfig, SnippetCap};
@@ -1077,7 +1073,11 @@ mod tests {
         }"#;
 
         let (offset, text) = test_utils::extract_offset(fixture);
-        let line_index = LineIndex::new(&text);
+        let line_index = LineIndex {
+            index: Arc::new(ide::LineIndex::new(&text)),
+            endings: LineEndings::Unix,
+            encoding: OffsetEncoding::Utf16,
+        };
         let (analysis, file_id) = Analysis::from_single_file(text);
         let completions: Vec<(String, Option<String>)> = analysis
             .completions(
@@ -1095,7 +1095,7 @@ mod tests {
             .unwrap()
             .into_iter()
             .filter(|c| c.label().ends_with("arg"))
-            .map(|c| completion_item(&line_index, LineEndings::Unix, c))
+            .map(|c| completion_item(&line_index, c))
             .flat_map(|comps| comps.into_iter().map(|c| (c.label, c.sort_text)))
             .collect();
         expect_test::expect![[r#"
@@ -1133,7 +1133,11 @@ fn main() {
         let folds = analysis.folding_ranges(file_id).unwrap();
         assert_eq!(folds.len(), 4);
 
-        let line_index = LineIndex::new(&text);
+        let line_index = LineIndex {
+            index: Arc::new(ide::LineIndex::new(&text)),
+            endings: LineEndings::Unix,
+            encoding: OffsetEncoding::Utf16,
+        };
         let converted: Vec<lsp_types::FoldingRange> =
             folds.into_iter().map(|it| folding_range(&text, &line_index, true, it)).collect();
 
diff --git a/docs/dev/lsp-extensions.md b/docs/dev/lsp-extensions.md
index a4e108724..164c8482e 100644
--- a/docs/dev/lsp-extensions.md
+++ b/docs/dev/lsp-extensions.md
@@ -1,5 +1,5 @@
 <!---
-lsp_ext.rs hash: 34aec6bfeaeb97a
+lsp_ext.rs hash: d279d971d4f62cd7
 
 If you need to change the above hash to make the test pass, please check if you
 need to adjust this doc as well and ping this  issue:
@@ -19,6 +19,12 @@ Requests, which are likely to always remain specific to `rust-analyzer` are unde
 
 If you want to be notified about the changes to this document, subscribe to [#4604](https://github.com/rust-analyzer/rust-analyzer/issues/4604).
 
+## UTF-8 offsets
+
+rust-analyzer supports clangd's extension for opting into UTF-8 as the coordinate space for offsets (by default, LSP uses UTF-16 offsets).
+
+https://clangd.llvm.org/extensions.html#utf-8-offsets
+
 ## `initializationOptions`
 
 For `initializationOptions`, `rust-analyzer` expects `"rust-analyzer"` section of the configuration.