fix(lsp): fix `nil`-index behavior for UTF-8 in `_str_*index_enc` methods (#16731)

Previously, the `_str_utfindex_enc` and `_str_byteindex_enc` helper functions would return `nil` when `offset_encoding == "utf-8"` and `index == nil`. Clearly, this doesn't reflect the expected behavior of the functions they're wrapping which would return the length of the line in this case. This should fix behavior with servers that use UTF-8 `offset_encoding` when applying text edits, formatting a range, and doing range code actions (though this isn't tested currently).

1 files changed, 18 insertions, 11 deletions

diff --git a/runtime/lua/vim/lsp/util.lua b/runtime/lua/vim/lsp/util.lua
index 68a030d50b..5921eb5bf0 100644
--- a/runtime/lua/vim/lsp/util.lua
+++ b/runtime/lua/vim/lsp/util.lua
@@ -97,15 +97,17 @@ end
 ---@param encoding string utf-8|utf-16|utf-32|nil defaults to utf-16
 ---@return number `encoding` index of `index` in `line`
 function M._str_utfindex_enc(line, index, encoding)
-  if encoding ~= 'utf-8' then
-    local col32, col16 = vim.str_utfindex(line, index)
-    if encoding == 'utf-32' then
-      return col32
-    else
-      return col16
-    end
+  if not encoding then encoding = 'utf-16' end
+  if encoding == 'utf-8' then
+    if index then return index else return #line end
+  elseif encoding == 'utf-16' then
+    local _, col16 = vim.str_utfindex(line, index)
+    return col16
+  elseif encoding == 'utf-32' then
+    local col32, _ = vim.str_utfindex(line, index)
+    return col32
   else
-    return index
+    error("Invalid encoding: " .. vim.inspect(encoding))
   end
 end
 
@@ -117,10 +119,15 @@ end
 ---@param encoding string utf-8|utf-16|utf-32|nil defaults to utf-16
 ---@return number byte (utf-8) index of `encoding` index `index` in `line`
 function M._str_byteindex_enc(line, index, encoding)
-  if encoding ~= 'utf-8' then
-    return vim.str_byteindex(line, index, not encoding or encoding ~= 'utf-32')
+  if not encoding then encoding = 'utf-16' end
+  if encoding == 'utf-8' then
+    if index then return index else return #line end
+  elseif encoding == 'utf-16' then
+    return vim.str_byteindex(line, index, true)
+  elseif encoding == 'utf-32' then
+    return vim.str_byteindex(line, index)
   else
-    return index
+    error("Invalid encoding: " .. vim.inspect(encoding))
   end
 end