[clangd] Implement LSP 3.17 positionEncoding

[someoneinjd]

This PR adds support for the positionEncoding client capability introduced in LSP 3.17. Clangd can now negotiate the position encoding with the client during initialization.

Fix clangd/clangd#1746

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[llvmbot]

@llvm/pr-subscribers-clangd

@llvm/pr-subscribers-clang-tools-extra

Author: someoneinjd (someoneinjd)

Changes

This PR adds support for the positionEncoding client capability introduced in LSP 3.17. Clangd can now negotiate the position encoding with the client during initialization.

---

Full diff: https://github.com/llvm/llvm-project/pull/142903.diff

3 Files Affected:

• (modified) clang-tools-extra/clangd/ClangdLSPServer.cpp (+3)
• (modified) clang-tools-extra/clangd/Protocol.cpp (+6)
• (added) clang-tools-extra/clangd/test/positionencoding.test (+32)

diff --git a/clang-tools-extra/clangd/ClangdLSPServer.cpp b/clang-tools-extra/clangd/ClangdLSPServer.cpp
index 29321f7cd3fa2..e7b8cb5f6c79d 100644
--- a/clang-tools-extra/clangd/ClangdLSPServer.cpp
+++ b/clang-tools-extra/clangd/ClangdLSPServer.cpp
@@ -686,6 +686,9 @@ 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{
diff --git a/clang-tools-extra/clangd/Protocol.cpp b/clang-tools-extra/clangd/Protocol.cpp
index c9e8a175b5d76..bf733af18ad3a 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 *OffsetEncoding = General->get("positionEncodings")) {
+      R.offsetEncoding.emplace();
+      if (!fromJSON(*OffsetEncoding, *R.offsetEncoding,
+                    P.field("general").field("positionEncodings")))
+        return false;
+    }
   }
   if (auto *OffsetEncoding = O->get("offsetEncoding")) {
     R.offsetEncoding.emplace();
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"}

[snizovtsev]

@someoneinjd thanks you for work! I've tested PR on my local emacs/eglot setup and it seems to work fine.

initialize log

[jsonrpc] e[12:54:35.265] --> initialize[1] {"jsonrpc":"2.0","id":1,"method":"initialize","params":{"processId":112523,"clientInfo":{"name":"Eglot","version":"1.18"},"rootPath":"/home/sergio/src/tmux/","rootUri":"file:///home/sergio/src/tmux","initializationOptions":{},"capabilities":{"workspace":{"applyEdit":true,"executeCommand":{"dynamicRegistration":false},"workspaceEdit":{"documentChanges":true},"didChangeWatchedFiles":{"dynamicRegistration":true},"symbol":{"dynamicRegistration":false},"configuration":true,"workspaceFolders":true},"textDocument":{"synchronization":{"dynamicRegistration":false,"willSave":true,"willSaveWaitUntil":true,"didSave":true},"completion":{"dynamicRegistration":false,"completionItem":{"snippetSupport":true,"deprecatedSupport":true,"resolveSupport":{"properties":["documentation","details","additionalTextEdits"]},"tagSupport":{"valueSet":[1]},"insertReplaceSupport":true},"contextSupport":true},"hover":{"dynamicRegistration":false,"contentFormat":["markdown","plaintext"]},"signatureHelp":{"dynamicRegistration":false,"signatureInformation":{"parameterInformation":{"labelOffsetSupport":true},"documentationFormat":["markdown","plaintext"],"activeParameterSupport":true}},"references":{"dynamicRegistration":false},"definition":{"dynamicRegistration":false,"linkSupport":true},"declaration":{"dynamicRegistration":false,"linkSupport":true},"implementation":{"dynamicRegistration":false,"linkSupport":true},"typeDefinition":{"dynamicRegistration":false,"linkSupport":true},"documentSymbol":{"dynamicRegistration":false,"hierarchicalDocumentSymbolSupport":true,"symbolKind":{"valueSet":[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26]}},"documentHighlight":{"dynamicRegistration":false},"codeAction":{"dynamicRegistration":false,"resolveSupport":{"properties":["edit","command"]},"dataSupport":true,"codeActionLiteralSupport":{"codeActionKind":{"valueSet":["quickfix","refactor","refactor.extract","refactor.inline","refactor.rewrite","source","source.organizeImports"]}},"isPreferredSupport":true},"formatting":{"dynamicRegistration":false},"rangeFormatting":{"dynamicRegistration":false},"rename":{"dynamicRegistration":false},"inlayHint":{"dynamicRegistration":false},"callHierarchy":{"dynamicRegistration":false},"typeHierarchy":{"dynamicRegistration":false},"publishDiagnostics":{"relatedInformation":false,"codeDescriptionSupport":false,"tagSupport":{"valueSet":[1,2]}}},"window":{"showDocument":{"support":true},"showMessage":{"messageActionItem":{"additionalPropertiesSupport":true}},"workDoneProgress":true},"general":{"positionEncodings":["utf-32","utf-8","utf-16"]},"experimental":{}},"workspaceFolders":[{"uri":"file:///home/sergio/src/tmux","name":"~/src/tmux/"}]}}
[stderr]  I[12:54:35.267] clangd version 21.0.0git (https://github.com/llvm/llvm-project.git 5717ca8166a019e4c007099ce243d4f4bd88b599)
[stderr]  I[12:54:35.267] Features: linux
[stderr]  I[12:54:35.267] PID: 522825
[stderr]  I[12:54:35.267] Working directory: /home/sergio/src/tmux
[stderr]  I[12:54:35.267] argv[0]: /usr/local/bin/clangd
[stderr]  I[12:54:35.267] Starting LSP over stdin/stdout
[jsonrpc] e[12:54:35.267] <-- initialize[1] {"id":1,"jsonrpc":"2.0","result":{"capabilities":{"astProvider":true,"callHierarchyProvider":true,"clangdInlayHintsProvider":true,"codeActionProvider":{"codeActionKinds":["quickfix","refactor","info"]},"compilationDatabase":{"automaticReload":true},"completionProvider":{"resolveProvider":false,"triggerCharacters":[".","<",">",":","\"","/","*"]},"declarationProvider":true,"definitionProvider":true,"documentFormattingProvider":true,"documentHighlightProvider":true,"documentLinkProvider":{"resolveProvider":false},"documentOnTypeFormattingProvider":{"firstTriggerCharacter":"\n","moreTriggerCharacter":[]},"documentRangeFormattingProvider":{"rangesSupport":true},"documentSymbolProvider":true,"executeCommandProvider":{"commands":["clangd.applyFix","clangd.applyRename","clangd.applyTweak"]},"foldingRangeProvider":true,"hoverProvider":true,"implementationProvider":true,"inactiveRegionsProvider":true,"inlayHintProvider":true,"memoryUsageProvider":true,"positionEncoding":"utf-32","referencesProvider":true,"renameProvider":true,"selectionRangeProvider":true,"semanticTokensProvider":{"full":{"delta":true},"legend":{"tokenModifiers":["declaration","definition","deprecated","deduced","readonly","static","abstract","virtual","dependentName","defaultLibrary","usedAsMutableReference","usedAsMutablePointer","constructorOrDestructor","userDefined","functionScope","classScope","fileScope","globalScope"],"tokenTypes":["variable","variable","parameter","function","method","function","property","variable","class","interface","enum","enumMember","type","type","unknown","namespace","typeParameter","concept","type","macro","modifier","operator","bracket","label","comment"]},"range":false},"signatureHelpProvider":{"triggerCharacters":["(",")","{","}","<",">",","]},"standardTypeHierarchyProvider":true,"textDocumentSync":{"change":2,"openClose":true,"save":true},"typeDefinitionProvider":true,"typeHierarchyProvider":true,"workspaceSymbolProvider":true},"offsetEncoding":"utf-32","serverInfo":{"name":"clangd","version":"clangd version 21.0.0git (https://github.com/llvm/llvm-project.git 5717ca8166a019e4c007099ce243d4f4bd88b599) linux x86_64-unknown-linux-gnu"}}}

After clangd upgrade, linepos function is set to faster utf-32 variant instead of default utf-16:

Value in #<buffer tmux.c>
eglot-utf-32-linepos

(defvar eglot-current-linepos-function #'eglot-utf-16-linepos
...)
(defun eglot-utf-16-linepos (&optional lbp)
  "Calculate number of UTF-16 code units from position given by LBP.
LBP defaults to `eglot--bol'."
  (/ (- (length (encode-coding-region (or lbp (eglot--bol))
                                      ;; FIXME: How could `point' ever be
                                      ;; larger than `point-max' (sounds like
                                      ;; a bug in Emacs).
                                      ;; Fix github#860
                                      (min (point) (point-max)) 'utf-16 t))
        2)
     2))

(defun eglot-utf-32-linepos ()
  "Calculate number of Unicode codepoints from line beginning."
  (- (point) (eglot--bol)))

[someoneinjd]

Ping

[kadircet]

thanks!

[kadircet]

LMK if I should merge this for you

[someoneinjd]

LMK if I should merge this for you

Of course, please go ahead if there are no other suggestions or improvements.

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