fix: UTF-16 strikes again

We need to report the jump target range in terms of a UTF-16 offset within the line, but to compute that we would have to load the target file, file-map it, and resolve that way each time the go-to-def handler runs. As an alternative, this stores the UTF-16 offsets in `DeclarationRange`.

src/Lean/DeclarationRange.lean

@@ -9,8 +9,15 @@ import Lean.AuxRecursor
 namespace Lean
 
 structure DeclarationRange where
-  pos    : Position
-  endPos : Position
+  pos          : Position
+  /-- A precomputed UTF-16 `character` field as in `Lean.Lsp.Position`. We need to store this
+  because LSP clients want us to report the range in terms of UTF-16, but converting a Unicode
+  codepoint stored in `Lean.Position` to UTF-16 requires loading and mapping the target source
+  file, which is IO-heavy. -/
+  charUtf16    : Nat
+  endPos       : Position
+  /-- See `charUtf16`. -/
+  endCharUtf16 : Nat
   deriving Inhabited, DecidableEq, Repr
 
 structure DeclarationRanges where

src/Lean/Elab/DeclarationRange.lean

@@ -5,14 +5,21 @@ Authors: Leonardo de Moura
 -/
 import Lean.DeclarationRange
 import Lean.Elab.Log
+import Lean.Data.Lsp.Utf16
 
 namespace Lean.Elab
 
 def getDeclarationRange [Monad m] [MonadFileMap m] (stx : Syntax) : m DeclarationRange := do
-  let pos    := stx.getPos.getD 0
-  let endPos := stx.getTailPos.getD pos
   let fileMap ← getFileMap
-  return { pos := fileMap.toPosition pos, endPos := fileMap.toPosition endPos }
+  let pos    := stx.getPos.getD 0
+  let endPos := stx.getTailPos.getD pos |> fileMap.toPosition
+  let pos    := pos |> fileMap.toPosition
+  return {
+    pos          := pos
+    charUtf16    := fileMap.leanPosToLspPos pos |>.character
+    endPos       := endPos
+    endCharUtf16 := fileMap.leanPosToLspPos endPos |>.character
+  }
 
 /--
   For most builtin declarations, the selection range is just its name, which is stored in the second position.

src/Lean/Server/FileWorker.lean

@@ -414,13 +414,14 @@ section RequestHandling
               let ranges? ← ci.runMetaM i.lctx <| findDeclarationRanges? n
 
               if let (some ranges, some mod) := (ranges?, mod?) then
-                let rangeToLspRange (r : DeclarationRange) : Lsp.Range :=
-                  ⟨text.leanPosToLspPos r.pos, text.leanPosToLspPos r.endPos⟩
+                let declRangeToLspRange (r : DeclarationRange) : Lsp.Range :=
+                  { start := ⟨r.pos.line - 1, r.charUtf16⟩
+                    «end» := ⟨r.endPos.line - 1, r.endCharUtf16⟩ }
                 let ll : LocationLink := {
                   originSelectionRange? := some ⟨text.utf8PosToLspPos i.pos?.get!, text.utf8PosToLspPos i.tailPos?.get!⟩
                   targetUri := mod
-                  targetRange := rangeToLspRange ranges.range
-                  targetSelectionRange := rangeToLspRange ranges.selectionRange
+                  targetRange := declRangeToLspRange ranges.range
+                  targetSelectionRange := declRangeToLspRange ranges.selectionRange
                 }
                 return #[ll]
         return #[]