2 files changed, 70 insertions, 13 deletions
diff --git a/runtime/lua/vim/lsp.lua b/runtime/lua/vim/lsp.lua
index 2e530ec17a..00839ec181 100644
--- a/runtime/lua/vim/lsp.lua
+++ b/runtime/lua/vim/lsp.lua
@@ -1112,9 +1112,9 @@ local text_document_did_change_handler
 do
   text_document_did_change_handler = function(_, bufnr, changedtick, firstline, lastline, new_lastline)
 
-    -- Don't do anything if there are no clients attached.
+    -- Detach (nvim_buf_attach) via returning True to on_lines if no clients are attached
     if tbl_isempty(all_buffer_active_clients[bufnr] or {}) then
-      return
+      return true
     end
     util.buf_versions[bufnr] = changedtick
     local compute_change_and_notify = changetracking.prepare(bufnr, firstline, lastline, new_lastline)
@@ -1156,6 +1156,12 @@ function lsp.buf_attach_client(bufnr, client_id)
     client_id = {client_id, 'n'};
   }
   bufnr = resolve_bufnr(bufnr)
+  if not vim.api.nvim_buf_is_loaded(bufnr) then
+    local _ = log.warn() and log.warn(
+      string.format("buf_attach_client called on unloaded buffer (id: %d): ", bufnr)
+    )
+    return false
+  end
   local buffer_client_ids = all_buffer_active_clients[bufnr]
   -- This is our first time attaching to this buffer.
   if not buffer_client_ids then
@@ -1214,6 +1220,50 @@ function lsp.buf_attach_client(bufnr, client_id)
   return true
 end
 
+--- Detaches client from the specified buffer.
+--- Note: While the server is notified that the text document (buffer)
+--- was closed, it is still able to send notifications should it ignore this notification.
+---
+---@param bufnr number Buffer handle, or 0 for current
+---@param client_id number Client id
+function lsp.buf_detach_client(bufnr, client_id)
+  validate {
+    bufnr     = {bufnr, 'n', true};
+    client_id = {client_id, 'n'};
+  }
+  bufnr = resolve_bufnr(bufnr)
+
+  local client = lsp.get_client_by_id(client_id)
+  if not client or not client.attached_buffers[bufnr] then
+    vim.notify(
+      string.format('Buffer (id: %d) is not attached to client (id: %d). Cannot detach.', client_id, bufnr)
+    )
+    return
+  end
+
+  changetracking.reset_buf(client, bufnr)
+
+  if client.resolved_capabilities.text_document_open_close then
+    local uri = vim.uri_from_bufnr(bufnr)
+    local params = { textDocument = { uri = uri; } }
+    client.notify('textDocument/didClose', params)
+  end
+
+  client.attached_buffers[bufnr] = nil
+  util.buf_versions[bufnr] = nil
+
+  all_buffer_active_clients[bufnr][client_id] = nil
+  if #vim.tbl_keys(all_buffer_active_clients[bufnr]) == 0 then
+    all_buffer_active_clients[bufnr] = nil
+  end
+
+  local namespace = vim.lsp.diagnostic.get_namespace(client_id)
+  vim.diagnostic.reset(namespace, bufnr)
+
+  vim.notify(string.format('Detached buffer (id: %d) from client (id: %d)', bufnr, client_id))
+
+end
+
 --- Checks if a buffer is attached for a particular client.
 ---
 ---@param bufnr (number) Buffer handle, or 0 for current
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