[clang-tools-extra] [clangd] Implement LSP 3.17 positionEncoding (PR #142903)

[via cfe-commits]

https://github.com/someoneinjd updated https://github.com/llvm/llvm-project/pull/142903

>From 398978803ba789efc5eb472e245acbf9a7c65540 Mon Sep 17 00:00:00 2001
From: someoneinjd <someoneinjd at outlook.com>
Date: Thu, 5 Jun 2025 13:51:40 +0800
Subject: [PATCH] [clangd] Implement LSP 3.17 positionEncoding

---
 clang-tools-extra/clangd/ClangdLSPServer.cpp  | 34 +++++++++++++++----
 clang-tools-extra/clangd/Protocol.cpp         |  6 ++++
 clang-tools-extra/clangd/Protocol.h           |  6 ++++
 .../clangd/test/positionencoding.test         | 32 +++++++++++++++++
 4 files changed, 72 insertions(+), 6 deletions(-)
 create mode 100644 clang-tools-extra/clangd/test/positionencoding.test

diff --git a/clang-tools-extra/clangd/ClangdLSPServer.cpp b/clang-tools-extra/clangd/ClangdLSPServer.cpp
index 29321f7cd3fa2..c2ffa07f08d9f 100644
--- a/clang-tools-extra/clangd/ClangdLSPServer.cpp
+++ b/clang-tools-extra/clangd/ClangdLSPServer.cpp
@@ -494,13 +494,22 @@ static std::vector<llvm::StringRef> semanticTokenModifiers() {
 void ClangdLSPServer::onInitialize(const InitializeParams &Params,
                                    Callback<llvm::json::Value> Reply) {
   // Determine character encoding first as it affects constructed ClangdServer.
-  if (Params.capabilities.offsetEncoding && !Opts.Encoding) {
-    Opts.Encoding = OffsetEncoding::UTF16; // fallback
-    for (OffsetEncoding Supported : *Params.capabilities.offsetEncoding)
-      if (Supported != OffsetEncoding::UnsupportedEncoding) {
-        Opts.Encoding = Supported;
-        break;
+  if (!Opts.Encoding) {
+    if (Params.capabilities.PositionEncodings) {
+      for (OffsetEncoding Supported : *Params.capabilities.PositionEncodings) {
+        if (Supported != OffsetEncoding::UnsupportedEncoding) {
+          Opts.Encoding = Supported;
+          break;
+        }
+      }
+    } else if (Params.capabilities.offsetEncoding) {
+      for (OffsetEncoding Supported : *Params.capabilities.offsetEncoding) {
+        if (Supported != OffsetEncoding::UnsupportedEncoding) {
+          Opts.Encoding = Supported;
+          break;
+        }
       }
+    }
   }
 
   if (Params.capabilities.TheiaSemanticHighlighting &&
@@ -686,6 +695,9 @@ void ClangdLSPServer::onInitialize(const InitializeParams &Params,
   ServerCaps["executeCommandProvider"] =
       llvm::json::Object{{"commands", Commands}};
 
+  if (Opts.Encoding)
+    ServerCaps["positionEncoding"] = *Opts.Encoding;
+
   llvm::json::Object Result{
       {{"serverInfo",
         llvm::json::Object{
@@ -697,6 +709,16 @@ void ClangdLSPServer::onInitialize(const InitializeParams &Params,
     Result["offsetEncoding"] = *Opts.Encoding;
   Reply(std::move(Result));
 
+  if (Params.capabilities.offsetEncoding) {
+    ShowMessageParams Msg;
+    Msg.message =
+        "offsetEncoding is deprecated in favor of general.positionEncodings. "
+        "Migrate to standard positionEncodings request";
+    Msg.type = MessageType::Warning;
+    elog(Msg.message.c_str());
+    ShowMessage(Msg);
+  }
+
   // Apply settings after we're fully initialized.
   // This can start background indexing and in turn trigger LSP notifications.
   applyConfiguration(Params.initializationOptions.ConfigSettings);
diff --git a/clang-tools-extra/clangd/Protocol.cpp b/clang-tools-extra/clangd/Protocol.cpp
index c9e8a175b5d76..ab274acfaa0a1 100644
--- a/clang-tools-extra/clangd/Protocol.cpp
+++ b/clang-tools-extra/clangd/Protocol.cpp
@@ -497,6 +497,12 @@ bool fromJSON(const llvm::json::Value &Params, ClientCapabilities &R,
       if (auto Cancel = StaleRequestSupport->getBoolean("cancel"))
         R.CancelsStaleRequests = *Cancel;
     }
+    if (auto *PositionEncodings = General->get("positionEncodings")) {
+      R.PositionEncodings.emplace();
+      if (!fromJSON(*PositionEncodings, *R.PositionEncodings,
+                    P.field("general").field("positionEncodings")))
+        return false;
+    }
   }
   if (auto *OffsetEncoding = O->get("offsetEncoding")) {
     R.offsetEncoding.emplace();
diff --git a/clang-tools-extra/clangd/Protocol.h b/clang-tools-extra/clangd/Protocol.h
index 8a7809d6677ee..2bf97fefa5721 100644
--- a/clang-tools-extra/clangd/Protocol.h
+++ b/clang-tools-extra/clangd/Protocol.h
@@ -529,8 +529,14 @@ struct ClientCapabilities {
   bool TheiaSemanticHighlighting = false;
 
   /// Supported encodings for LSP character offsets. (clangd extension).
+  ///
+  /// @deprecated in favour of PositionEncodings.
   std::optional<std::vector<OffsetEncoding>> offsetEncoding;
 
+  /// Supported encodings for LSP character offsets. (introduced by LSP 3.17).
+  /// general.positionEncodings
+  std::optional<std::vector<OffsetEncoding>> PositionEncodings;
+
   /// The content format that should be used for Hover requests.
   /// textDocument.hover.contentEncoding
   MarkupKind HoverContentFormat = MarkupKind::PlainText;
diff --git a/clang-tools-extra/clangd/test/positionencoding.test b/clang-tools-extra/clangd/test/positionencoding.test
new file mode 100644
index 0000000000000..eea7a1a596e9a
--- /dev/null
+++ b/clang-tools-extra/clangd/test/positionencoding.test
@@ -0,0 +1,32 @@
+# RUN: clangd -lit-test < %s | FileCheck -strict-whitespace %s
+# This test verifies that we can negotiate UTF-8 offsets via the positionEncodings capability introduced in LSP 3.17.
+{"jsonrpc":"2.0","id":0,"method":"initialize","params":{"processId":123,"rootPath":"clangd","capabilities":{"general":{"positionEncodings":["utf-8","utf-16"]}},"trace":"off"}}
+# CHECK: "positionEncoding": "utf-8"
+---
+{"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"uri":"test:///main.cpp","languageId":"cpp","version":1,"text":"/*ö*/int x;\nint y=x;"}}}
+---
+{"jsonrpc":"2.0","id":1,"method":"textDocument/definition","params":{"textDocument":{"uri":"test:///main.cpp"},"position":{"line":1,"character":6}}}
+# /*ö*/int x;
+# 01234567890
+# x is character (and utf-16) range [9,10) but byte range [10,11).
+#      CHECK:  "id": 1,
+# CHECK-NEXT:  "jsonrpc": "2.0",
+# CHECK-NEXT:  "result": [
+# CHECK-NEXT:    {
+# CHECK-NEXT:      "range": {
+# CHECK-NEXT:        "end": {
+# CHECK-NEXT:          "character": 11,
+# CHECK-NEXT:          "line": 0
+# CHECK-NEXT:        },
+# CHECK-NEXT:        "start": {
+# CHECK-NEXT:          "character": 10,
+# CHECK-NEXT:          "line": 0
+# CHECK-NEXT:        }
+# CHECK-NEXT:      },
+# CHECK-NEXT:      "uri": "file://{{.*}}/main.cpp"
+# CHECK-NEXT:    }
+# CHECK-NEXT:  ]
+---
+{"jsonrpc":"2.0","id":10000,"method":"shutdown"}
+---
+{"jsonrpc":"2.0","method":"exit"}