feat(lsp): support completion itemDefaults

runtime/doc/news.txt

@@ -125,6 +125,15 @@ The following changes may require adaptations in user config or plugins.
 • Returning any truthy value from a callback passed to |nvim_create_autocmd()|
   (rather than just `true`) will delete the autocommand.
 
+• |vim.lsp.util.extract_completion_items()| will no longer return reliable
+  results, since it does not apply `itemDefaults` when its input is a
+  `CompletionList`.
+  Moreover, since support for LSP `completionList.itemDefaults` was added,
+  some third party plugins might be negatively impacted in case the language
+  servers support the feature but the plugin does not.
+  If necessary, the respective capability can be
+  removed when calling |vim.lsp.protocol.make_client_capabilities()|.
+
 ==============================================================================
 BREAKING CHANGES IN HEAD                                    *news-breaking-dev*
 
@@ -213,6 +222,11 @@ The following new APIs and features were added.
     the original LSP `Location` or `LocationLink`.
   • Added support for connecting to servers using named pipes (Windows) or
     unix domain sockets (Unix) via |vim.lsp.rpc.domain_socket_connect()|.
+  • Added support for `completionList.itemDefaults`, reducing overhead when
+    computing completion items where properties often share the same value
+    (e.g. `commitCharacters`). Note that this might affect plugins and
+    language servers that don't support the feature, and in such cases the
+    respective capability can be unset.
 
 • Treesitter
   • Bundled parsers and queries (highlight, folds) for Markdown, Python, and

runtime/lua/vim/lsp/_completion.lua

@@ -6,6 +6,14 @@ local ms = protocol.Methods
 
 --- @alias vim.lsp.CompletionResult lsp.CompletionList | lsp.CompletionItem[]
 
+-- TODO(mariasolos): Remove this declaration once we figure out a better way to handle
+-- literal/anonymous types (see https://github.com/neovim/neovim/pull/27542/files#r1495259331).
+--- @class lsp.ItemDefaults
+--- @field editRange lsp.Range | { insert: lsp.Range, replace: lsp.Range } | nil
+--- @field insertTextFormat lsp.InsertTextFormat?
+--- @field insertTextMode lsp.InsertTextMode?
+--- @field data any
+
 ---@param input string unparsed snippet
 ---@return string parsed snippet
 local function parse_snippet(input)
@@ -39,13 +47,43 @@ local function get_completion_word(item)
   return item.label
 end
 
+--- Applies the given defaults to the completion item, modifying it in place.
+---
+--- @param item lsp.CompletionItem
+--- @param defaults lsp.ItemDefaults?
+local function apply_defaults(item, defaults)
+  if not defaults then
+    return
+  end
+
+  item.insertTextFormat = item.insertTextFormat or defaults.insertTextFormat
+  item.insertTextMode = item.insertTextMode or defaults.insertTextMode
+  item.data = item.data or defaults.data
+  if defaults.editRange then
+    local textEdit = item.textEdit or {}
+    item.textEdit = textEdit
+    textEdit.newText = textEdit.newText or item.textEditText or item.insertText
+    if defaults.editRange.start then
+      textEdit.range = textEdit.range or defaults.editRange
+    elseif defaults.editRange.insert then
+      textEdit.insert = defaults.editRange.insert
+      textEdit.replace = defaults.editRange.replace
+    end
+  end
+end
+
 ---@param result vim.lsp.CompletionResult
 ---@return lsp.CompletionItem[]
 local function get_items(result)
   if result.items then
+    for _, item in ipairs(result.items) do
+      ---@diagnostic disable-next-line: param-type-mismatch
+      apply_defaults(item, result.itemDefaults)
+    end
     return result.items
+  else
+    return result
   end
-  return result
 end
 
 --- Turns the result of a `textDocument/completion` request into vim-compatible

runtime/lua/vim/lsp/protocol.lua

@@ -764,6 +764,14 @@ function protocol.make_client_capabilities()
             return res
           end)(),
         },
+        completionList = {
+          itemDefaults = {
+            'editRange',
+            'insertTextFormat',
+            'insertTextMode',
+            'data',
+          },
+        },
 
         -- TODO(tjdevries): Implement this
         contextSupport = false,

runtime/lua/vim/lsp/util.lua

@@ -551,6 +551,10 @@ end
 --- Can be used to extract the completion items from a
 --- `textDocument/completion` request, which may return one of
 --- `CompletionItem[]`, `CompletionList` or null.
+---
+--- Note that this method doesn't apply `itemDefaults` to `CompletionList`s, and hence the returned
+--- results might be incorrect.
+---
 ---@deprecated
 ---@param result table The result of a `textDocument/completion` request
 ---@return lsp.CompletionItem[] List of completion items