feat(lsp): completion side effects

runtime/doc/lsp.txt

@@ -1604,6 +1604,32 @@ save({lenses}, {bufnr}, {client_id})                 *vim.lsp.codelens.save()*
       • {client_id}  (`integer`)
 
 
+==============================================================================
+Lua module: vim.lsp.completion                                *lsp-completion*
+
+*vim.lsp.completion.BufferOpts*
+
+    Fields: ~
+      • {autotrigger}?  (`boolean`) Whether to trigger completion
+                        automatically. Default: false
+
+
+                                                 *vim.lsp.completion.enable()*
+enable({enable}, {client_id}, {bufnr}, {opts})
+    Enables or disables completions from the given language client in the
+    given buffer.
+
+    Parameters: ~
+      • {enable}     (`boolean`) True to enable, false to disable
+      • {client_id}  (`integer`) Client ID
+      • {bufnr}      (`integer`) Buffer handle, or 0 for the current buffer
+      • {opts}       (`vim.lsp.completion.BufferOpts?`) See
+                     |vim.lsp.completion.BufferOpts|.
+
+trigger()                                       *vim.lsp.completion.trigger()*
+    Trigger LSP completion in the current buffer.
+
+
 ==============================================================================
 Lua module: vim.lsp.inlay_hint                                *lsp-inlay_hint*
 

runtime/doc/news.txt

@@ -95,7 +95,8 @@ EVENTS
 
 LSP
 
-• TODO
+• Completion side effects (including snippet expansion, execution of commands
+  and application of additional text edits) is now built-in.
 
 LUA
 

runtime/lua/vim/lsp.lua

@@ -3,7 +3,6 @@ local validate = vim.validate
 
 local lsp = vim._defer_require('vim.lsp', {
   _changetracking = ..., --- @module 'vim.lsp._changetracking'
-  _completion = ..., --- @module 'vim.lsp._completion'
   _dynamic = ..., --- @module 'vim.lsp._dynamic'
   _snippet_grammar = ..., --- @module 'vim.lsp._snippet_grammar'
   _tagfunc = ..., --- @module 'vim.lsp._tagfunc'
@@ -11,6 +10,7 @@ local lsp = vim._defer_require('vim.lsp', {
   buf = ..., --- @module 'vim.lsp.buf'
   client = ..., --- @module 'vim.lsp.client'
   codelens = ..., --- @module 'vim.lsp.codelens'
+  completion = ..., --- @module 'vim.lsp.completion'
   diagnostic = ..., --- @module 'vim.lsp.diagnostic'
   handlers = ..., --- @module 'vim.lsp.handlers'
   inlay_hint = ..., --- @module 'vim.lsp.inlay_hint'
@@ -1002,8 +1002,7 @@ end
 --- - findstart=0: column where the completion starts, or -2 or -3
 --- - findstart=1: list of matches (actually just calls |complete()|)
 function lsp.omnifunc(findstart, base)
-  log.debug('omnifunc.findstart', { findstart = findstart, base = base })
+  return vim.lsp.completion._omnifunc(findstart, base)
-  return vim.lsp._completion.omnifunc(findstart, base)
 end
 
 --- @class vim.lsp.formatexpr.Opts

runtime/lua/vim/lsp/_completion.lua

@@ -1,276 +0,0 @@
-local M = {}
-local api = vim.api
-local lsp = vim.lsp
-local protocol = lsp.protocol
-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)
-  local ok, parsed = pcall(function()
-    return vim.lsp._snippet_grammar.parse(input)
-  end)
-  return ok and tostring(parsed) or input
-end
-
---- Returns text that should be inserted when selecting completion item. The
---- precedence is as follows: textEdit.newText > insertText > label
----
---- See https://microsoft.github.io/language-server-protocol/specifications/specification-current/#textDocument_completion
----
----@param item lsp.CompletionItem
----@return string
-local function get_completion_word(item)
-  if item.textEdit ~= nil and item.textEdit.newText ~= nil and item.textEdit.newText ~= '' then
-    if item.insertTextFormat == protocol.InsertTextFormat.PlainText then
-      return item.textEdit.newText
-    else
-      return parse_snippet(item.textEdit.newText)
-    end
-  elseif item.insertText ~= nil and item.insertText ~= '' then
-    if item.insertTextFormat == protocol.InsertTextFormat.PlainText then
-      return item.insertText
-    else
-      return parse_snippet(item.insertText)
-    end
-  end
-  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
-end
-
---- Turns the result of a `textDocument/completion` request into vim-compatible
---- |complete-items|.
----
----@param result vim.lsp.CompletionResult Result of `textDocument/completion`
----@param prefix string prefix to filter the completion items
----@return table[]
----@see complete-items
-function M._lsp_to_complete_items(result, prefix)
-  local items = get_items(result)
-  if vim.tbl_isempty(items) then
-    return {}
-  end
-
-  local function matches_prefix(item)
-    return vim.startswith(get_completion_word(item), prefix)
-  end
-
-  items = vim.tbl_filter(matches_prefix, items) --[[@as lsp.CompletionItem[]|]]
-  table.sort(items, function(a, b)
-    return (a.sortText or a.label) < (b.sortText or b.label)
-  end)
-
-  local matches = {}
-  for _, item in ipairs(items) do
-    local info = ''
-    local documentation = item.documentation
-    if documentation then
-      if type(documentation) == 'string' and documentation ~= '' then
-        info = documentation
-      elseif type(documentation) == 'table' and type(documentation.value) == 'string' then
-        info = documentation.value
-      else
-        vim.notify(
-          ('invalid documentation value %s'):format(vim.inspect(documentation)),
-          vim.log.levels.WARN
-        )
-      end
-    end
-    local word = get_completion_word(item)
-    table.insert(matches, {
-      word = word,
-      abbr = item.label,
-      kind = protocol.CompletionItemKind[item.kind] or 'Unknown',
-      menu = item.detail or '',
-      info = #info > 0 and info or nil,
-      icase = 1,
-      dup = 1,
-      empty = 1,
-      user_data = {
-        nvim = {
-          lsp = {
-            completion_item = item,
-          },
-        },
-      },
-    })
-  end
-  return matches
-end
-
----@param lnum integer 0-indexed
----@param items lsp.CompletionItem[]
-local function adjust_start_col(lnum, line, items, encoding)
-  local min_start_char = nil
-  for _, item in pairs(items) do
-    if item.textEdit and item.textEdit.range.start.line == lnum then
-      if min_start_char and min_start_char ~= item.textEdit.range.start.character then
-        return nil
-      end
-      min_start_char = item.textEdit.range.start.character
-    end
-  end
-  if min_start_char then
-    return vim.lsp.util._str_byteindex_enc(line, min_start_char, encoding)
-  else
-    return nil
-  end
-end
-
----@private
----@param line string line content
----@param lnum integer 0-indexed line number
----@param client_start_boundary integer 0-indexed word boundary
----@param server_start_boundary? integer 0-indexed word boundary, based on textEdit.range.start.character
----@param result vim.lsp.CompletionResult
----@param encoding string
----@return table[] matches
----@return integer? server_start_boundary
-function M._convert_results(
-  line,
-  lnum,
-  cursor_col,
-  client_start_boundary,
-  server_start_boundary,
-  result,
-  encoding
-)
-  -- Completion response items may be relative to a position different than `client_start_boundary`.
-  -- Concrete example, with lua-language-server:
-  --
-  -- require('plenary.asy|
-  --         ▲       ▲   ▲
-  --         │       │   └── cursor_pos:                     20
-  --         │       └────── client_start_boundary:          17
-  --         └────────────── textEdit.range.start.character: 9
-  --                                 .newText = 'plenary.async'
-  --                  ^^^
-  --                  prefix (We'd remove everything not starting with `asy`,
-  --                  so we'd eliminate the `plenary.async` result
-  --
-  -- `adjust_start_col` is used to prefer the language server boundary.
-  --
-  local candidates = get_items(result)
-  local curstartbyte = adjust_start_col(lnum, line, candidates, encoding)
-  if server_start_boundary == nil then
-    server_start_boundary = curstartbyte
-  elseif curstartbyte ~= nil and curstartbyte ~= server_start_boundary then
-    server_start_boundary = client_start_boundary
-  end
-  local prefix = line:sub((server_start_boundary or client_start_boundary) + 1, cursor_col)
-  local matches = M._lsp_to_complete_items(result, prefix)
-  return matches, server_start_boundary
-end
-
----@param findstart integer 0 or 1, decides behavior
----@param base integer findstart=0, text to match against
----@return integer|table Decided by {findstart}:
---- - findstart=0: column where the completion starts, or -2 or -3
---- - findstart=1: list of matches (actually just calls |complete()|)
-function M.omnifunc(findstart, base)
-  assert(base) -- silence luals
-  local bufnr = api.nvim_get_current_buf()
-  local clients = lsp.get_clients({ bufnr = bufnr, method = ms.textDocument_completion })
-  local remaining = #clients
-  if remaining == 0 then
-    return findstart == 1 and -1 or {}
-  end
-
-  local win = api.nvim_get_current_win()
-  local cursor = api.nvim_win_get_cursor(win)
-  local lnum = cursor[1] - 1
-  local cursor_col = cursor[2]
-  local line = api.nvim_get_current_line()
-  local line_to_cursor = line:sub(1, cursor_col)
-  local client_start_boundary = vim.fn.match(line_to_cursor, '\\k*$') --[[@as integer]]
-  local server_start_boundary = nil
-  local items = {}
-
-  local function on_done()
-    local mode = api.nvim_get_mode()['mode']
-    if mode == 'i' or mode == 'ic' then
-      vim.fn.complete((server_start_boundary or client_start_boundary) + 1, items)
-    end
-  end
-
-  local util = vim.lsp.util
-  for _, client in ipairs(clients) do
-    local params = util.make_position_params(win, client.offset_encoding)
-    client.request(ms.textDocument_completion, params, function(err, result)
-      if err then
-        vim.lsp.log.warn(err.message)
-      end
-      if result and vim.fn.mode() == 'i' then
-        local matches
-        matches, server_start_boundary = M._convert_results(
-          line,
-          lnum,
-          cursor_col,
-          client_start_boundary,
-          server_start_boundary,
-          result,
-          client.offset_encoding
-        )
-        vim.list_extend(items, matches)
-      end
-      remaining = remaining - 1
-      if remaining == 0 then
-        vim.schedule(on_done)
-      end
-    end, bufnr)
-  end
-
-  -- Return -2 to signal that we should continue completion so that we can
-  -- async complete.
-  return -2
-end
-
-return M

runtime/lua/vim/lsp/client.lua

@@ -869,7 +869,8 @@ end
 --- @param command lsp.Command
 --- @param context? {bufnr: integer}
 --- @param handler? lsp.Handler only called if a server command
-function Client:_exec_cmd(command, context, handler)
+--- @param on_unsupported? function handler invoked when the command is not supported by the client.
+function Client:_exec_cmd(command, context, handler, on_unsupported)
   context = vim.deepcopy(context or {}, true) --[[@as lsp.HandlerContext]]
   context.bufnr = context.bufnr or api.nvim_get_current_buf()
   context.client_id = self.id
@@ -883,14 +884,18 @@ function Client:_exec_cmd(command, context, handler)
   local command_provider = self.server_capabilities.executeCommandProvider
   local commands = type(command_provider) == 'table' and command_provider.commands or {}
   if not vim.list_contains(commands, cmdname) then
-    vim.notify_once(
+    if on_unsupported then
-      string.format(
+      on_unsupported()
-        'Language server `%s` does not support command `%s`. This command may require a client extension.',
+    else
-        self.name,
+      vim.notify_once(
-        cmdname
+        string.format(
-      ),
+          'Language server `%s` does not support command `%s`. This command may require a client extension.',
-      vim.log.levels.WARN
+          self.name,
-    )
+          cmdname
+        ),
+        vim.log.levels.WARN
+      )
+    end
     return
   end
   -- Not using command directly to exclude extra properties,

runtime/lua/vim/lsp/completion.lua

@@ -0,0 +1,734 @@
+local M = {}
+
+local api = vim.api
+local lsp = vim.lsp
+local protocol = lsp.protocol
+local ms = protocol.Methods
+
+local rtt_ms = 50
+local ns_to_ms = 0.000001
+
+--- @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).
+--- @nodoc
+--- @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
+
+--- @nodoc
+--- @class vim.lsp.completion.BufHandle
+--- @field clients table<integer, vim.lsp.Client>
+--- @field triggers table<string, vim.lsp.Client[]>
+
+--- @type table<integer, vim.lsp.completion.BufHandle>
+local buf_handles = {}
+
+--- @nodoc
+--- @class vim.lsp.completion.Context
+local Context = {
+  cursor = nil, --- @type { [1]: integer, [2]: integer }?
+  last_request_time = nil, --- @type integer?
+  pending_requests = {}, --- @type function[]
+  isIncomplete = false,
+}
+
+--- @nodoc
+function Context:cancel_pending()
+  for _, cancel in ipairs(self.pending_requests) do
+    cancel()
+  end
+
+  self.pending_requests = {}
+end
+
+--- @nodoc
+function Context:reset()
+  -- Note that the cursor isn't reset here, it needs to survive a `CompleteDone` event.
+  self.isIncomplete = false
+  self.last_request_time = nil
+  self:cancel_pending()
+end
+
+--- @type uv.uv_timer_t?
+local completion_timer = nil
+
+--- @return uv.uv_timer_t
+local function new_timer()
+  return assert(vim.uv.new_timer())
+end
+
+local function reset_timer()
+  if completion_timer then
+    completion_timer:stop()
+    completion_timer:close()
+  end
+
+  completion_timer = nil
+end
+
+--- @param window integer
+--- @param warmup integer
+--- @return fun(sample: number): number
+local function exp_avg(window, warmup)
+  local count = 0
+  local sum = 0
+  local value = 0
+
+  return function(sample)
+    if count < warmup then
+      count = count + 1
+      sum = sum + sample
+      value = sum / count
+    else
+      local factor = 2.0 / (window + 1)
+      value = value * (1 - factor) + sample * factor
+    end
+    return value
+  end
+end
+local compute_new_average = exp_avg(10, 10)
+
+--- @return number
+local function next_debounce()
+  if not Context.last_request_time then
+    return rtt_ms
+  end
+
+  local ms_since_request = (vim.uv.hrtime() - Context.last_request_time) * ns_to_ms
+  return math.max((ms_since_request - rtt_ms) * -1, 0)
+end
+
+--- @param input string Unparsed snippet
+--- @return string # Parsed snippet if successful, else returns its input
+local function parse_snippet(input)
+  local ok, parsed = pcall(function()
+    return lsp._snippet_grammar.parse(input)
+  end)
+  return ok and tostring(parsed) or input
+end
+
+--- @param item lsp.CompletionItem
+--- @param suffix? string
+local function apply_snippet(item, suffix)
+  if item.textEdit then
+    vim.snippet.expand(item.textEdit.newText .. suffix)
+  elseif item.insertText then
+    vim.snippet.expand(item.insertText .. suffix)
+  end
+end
+
+--- Returns text that should be inserted when a selecting completion item. The
+--- precedence is as follows: textEdit.newText > insertText > label
+---
+--- See https://microsoft.github.io/language-server-protocol/specifications/specification-current/#textDocument_completion
+---
+--- @param item lsp.CompletionItem
+--- @return string
+local function get_completion_word(item)
+  if item.textEdit ~= nil and item.textEdit.newText ~= nil and item.textEdit.newText ~= '' then
+    if item.insertTextFormat == protocol.InsertTextFormat.PlainText then
+      return item.textEdit.newText
+    else
+      return parse_snippet(item.textEdit.newText)
+    end
+  elseif item.insertText ~= nil and item.insertText ~= '' then
+    if item.insertTextFormat == protocol.InsertTextFormat.PlainText then
+      return item.insertText
+    else
+      return parse_snippet(item.insertText)
+    end
+  end
+  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
+    -- When we have a list, apply the defaults and return an array of items.
+    for _, item in ipairs(result.items) do
+      ---@diagnostic disable-next-line: param-type-mismatch
+      apply_defaults(item, result.itemDefaults)
+    end
+    return result.items
+  else
+    -- Else just return the items as they are.
+    return result
+  end
+end
+
+--- Turns the result of a `textDocument/completion` request into vim-compatible
+--- |complete-items|.
+---
+--- @private
+--- @param result vim.lsp.CompletionResult Result of `textDocument/completion`
+--- @param prefix string prefix to filter the completion items
+--- @param client_id integer? Client ID
+--- @return table[]
+--- @see complete-items
+function M._lsp_to_complete_items(result, prefix, client_id)
+  local items = get_items(result)
+  if vim.tbl_isempty(items) then
+    return {}
+  end
+
+  local function matches_prefix(item)
+    return vim.startswith(get_completion_word(item), prefix)
+  end
+
+  items = vim.tbl_filter(matches_prefix, items) --[[@as lsp.CompletionItem[]|]]
+  table.sort(items, function(a, b)
+    return (a.sortText or a.label) < (b.sortText or b.label)
+  end)
+
+  local matches = {}
+  for _, item in ipairs(items) do
+    local info = ''
+    local documentation = item.documentation
+    if documentation then
+      if type(documentation) == 'string' and documentation ~= '' then
+        info = documentation
+      elseif type(documentation) == 'table' and type(documentation.value) == 'string' then
+        info = documentation.value
+      else
+        vim.notify(
+          ('invalid documentation value %s'):format(vim.inspect(documentation)),
+          vim.log.levels.WARN
+        )
+      end
+    end
+    local word = get_completion_word(item)
+    table.insert(matches, {
+      word = word,
+      abbr = item.label,
+      kind = protocol.CompletionItemKind[item.kind] or 'Unknown',
+      menu = item.detail or '',
+      info = #info > 0 and info or '',
+      icase = 1,
+      dup = 1,
+      empty = 1,
+      user_data = {
+        nvim = {
+          lsp = {
+            completion_item = item,
+            client_id = client_id,
+          },
+        },
+      },
+    })
+  end
+  return matches
+end
+
+--- @param lnum integer 0-indexed
+--- @param line string
+--- @param items lsp.CompletionItem[]
+--- @param encoding string
+--- @return integer?
+local function adjust_start_col(lnum, line, items, encoding)
+  local min_start_char = nil
+  for _, item in pairs(items) do
+    if item.textEdit and item.textEdit.range.start.line == lnum then
+      if min_start_char and min_start_char ~= item.textEdit.range.start.character then
+        return nil
+      end
+      min_start_char = item.textEdit.range.start.character
+    end
+  end
+  if min_start_char then
+    return lsp.util._str_byteindex_enc(line, min_start_char, encoding)
+  else
+    return nil
+  end
+end
+
+--- @private
+--- @param line string line content
+--- @param lnum integer 0-indexed line number
+--- @param cursor_col integer
+--- @param client_id integer client ID
+--- @param client_start_boundary integer 0-indexed word boundary
+--- @param server_start_boundary? integer 0-indexed word boundary, based on textEdit.range.start.character
+--- @param result vim.lsp.CompletionResult
+--- @param encoding string
+--- @return table[] matches
+--- @return integer? server_start_boundary
+function M._convert_results(
+  line,
+  lnum,
+  cursor_col,
+  client_id,
+  client_start_boundary,
+  server_start_boundary,
+  result,
+  encoding
+)
+  -- Completion response items may be relative to a position different than `client_start_boundary`.
+  -- Concrete example, with lua-language-server:
+  --
+  -- require('plenary.asy|
+  --         ▲       ▲   ▲
+  --         │       │   └── cursor_pos:                     20
+  --         │       └────── client_start_boundary:          17
+  --         └────────────── textEdit.range.start.character: 9
+  --                                 .newText = 'plenary.async'
+  --                  ^^^
+  --                  prefix (We'd remove everything not starting with `asy`,
+  --                  so we'd eliminate the `plenary.async` result
+  --
+  -- `adjust_start_col` is used to prefer the language server boundary.
+  --
+  local candidates = get_items(result)
+  local curstartbyte = adjust_start_col(lnum, line, candidates, encoding)
+  if server_start_boundary == nil then
+    server_start_boundary = curstartbyte
+  elseif curstartbyte ~= nil and curstartbyte ~= server_start_boundary then
+    server_start_boundary = client_start_boundary
+  end
+  local prefix = line:sub((server_start_boundary or client_start_boundary) + 1, cursor_col)
+  local matches = M._lsp_to_complete_items(result, prefix, client_id)
+  return matches, server_start_boundary
+end
+
+--- Implements 'omnifunc' compatible LSP completion.
+---
+--- @see |complete-functions|
+--- @see |complete-items|
+--- @see |CompleteDone|
+---
+--- @param findstart integer 0 or 1, decides behavior
+--- @param base integer findstart=0, text to match against
+---
+--- @return integer|table Decided by {findstart}:
+--- - findstart=0: column where the completion starts, or -2 or -3
+--- - findstart=1: list of matches (actually just calls |complete()|)
+function M._omnifunc(findstart, base)
+  vim.lsp.log.debug('omnifunc.findstart', { findstart = findstart, base = base })
+  assert(base) -- silence luals
+  local bufnr = api.nvim_get_current_buf()
+  local clients = lsp.get_clients({ bufnr = bufnr, method = ms.textDocument_completion })
+  local remaining = #clients
+  if remaining == 0 then
+    return findstart == 1 and -1 or {}
+  end
+
+  local win = api.nvim_get_current_win()
+  local cursor = api.nvim_win_get_cursor(win)
+  local lnum = cursor[1] - 1
+  local cursor_col = cursor[2]
+  local line = api.nvim_get_current_line()
+  local line_to_cursor = line:sub(1, cursor_col)
+  local client_start_boundary = vim.fn.match(line_to_cursor, '\\k*$') --[[@as integer]]
+  local server_start_boundary = nil
+  local items = {}
+
+  local function on_done()
+    local mode = api.nvim_get_mode()['mode']
+    if mode == 'i' or mode == 'ic' then
+      vim.fn.complete((server_start_boundary or client_start_boundary) + 1, items)
+    end
+  end
+
+  local util = vim.lsp.util
+  for _, client in ipairs(clients) do
+    local params = util.make_position_params(win, client.offset_encoding)
+    client.request(ms.textDocument_completion, params, function(err, result)
+      if err then
+        lsp.log.warn(err.message)
+      end
+      if result and vim.fn.mode() == 'i' then
+        local matches
+        matches, server_start_boundary = M._convert_results(
+          line,
+          lnum,
+          cursor_col,
+          client.id,
+          client_start_boundary,
+          server_start_boundary,
+          result,
+          client.offset_encoding
+        )
+        vim.list_extend(items, matches)
+      end
+      remaining = remaining - 1
+      if remaining == 0 then
+        vim.schedule(on_done)
+      end
+    end, bufnr)
+  end
+
+  -- Return -2 to signal that we should continue completion so that we can
+  -- async complete.
+  return -2
+end
+
+--- @param clients table<integer, vim.lsp.Client>
+--- @param bufnr integer
+--- @param win integer
+--- @param callback fun(responses: table<integer, { err: lsp.ResponseError, result: vim.lsp.CompletionResult }>)
+--- @return function # Cancellation function
+local function request(clients, bufnr, win, callback)
+  local responses = {} --- @type table<integer, { err: lsp.ResponseError, result: any }>
+  local request_ids = {} --- @type table<integer, integer>
+  local remaining_requests = vim.tbl_count(clients)
+
+  for client_id, client in pairs(clients) do
+    local params = lsp.util.make_position_params(win, client.offset_encoding)
+    local ok, request_id = client.request(ms.textDocument_completion, params, function(err, result)
+      responses[client_id] = { err = err, result = result }
+      remaining_requests = remaining_requests - 1
+      if remaining_requests == 0 then
+        callback(responses)
+      end
+    end, bufnr)
+
+    if ok then
+      request_ids[client_id] = request_id
+    end
+  end
+
+  return function()
+    for client_id, request_id in pairs(request_ids) do
+      local client = lsp.get_client_by_id(client_id)
+      if client then
+        client.cancel_request(request_id)
+      end
+    end
+  end
+end
+
+--- @param handle vim.lsp.completion.BufHandle
+local function on_insert_char_pre(handle)
+  if tonumber(vim.fn.pumvisible()) == 1 then
+    if Context.isIncomplete then
+      reset_timer()
+
+      local debounce_ms = next_debounce()
+      if debounce_ms == 0 then
+        vim.schedule(M.trigger)
+      else
+        completion_timer = new_timer()
+        completion_timer:start(debounce_ms, 0, vim.schedule_wrap(M.trigger))
+      end
+    end
+
+    return
+  end
+
+  local char = api.nvim_get_vvar('char')
+  if not completion_timer and handle.triggers[char] then
+    completion_timer = assert(vim.uv.new_timer())
+    completion_timer:start(25, 0, function()
+      reset_timer()
+      vim.schedule(M.trigger)
+    end)
+  end
+end
+
+local function on_insert_leave()
+  reset_timer()
+  Context.cursor = nil
+  Context:reset()
+end
+
+local function on_complete_done()
+  local completed_item = api.nvim_get_vvar('completed_item')
+  if not completed_item or not completed_item.user_data or not completed_item.user_data.nvim then
+    Context:reset()
+    return
+  end
+
+  local cursor_row, cursor_col = unpack(api.nvim_win_get_cursor(0)) --- @type integer, integer
+  cursor_row = cursor_row - 1
+  local completion_item = completed_item.user_data.nvim.lsp.completion_item --- @type lsp.CompletionItem
+  local client_id = completed_item.user_data.nvim.lsp.client_id --- @type integer
+  if not completion_item or not client_id then
+    Context:reset()
+    return
+  end
+
+  local bufnr = api.nvim_get_current_buf()
+  local expand_snippet = completion_item.insertTextFormat == protocol.InsertTextFormat.Snippet
+    and (completion_item.textEdit ~= nil or completion_item.insertText ~= nil)
+
+  Context:reset()
+
+  local client = lsp.get_client_by_id(client_id)
+  if not client then
+    return
+  end
+
+  local offset_encoding = client.offset_encoding or 'utf-16'
+  local resolve_provider = (client.server_capabilities.completionProvider or {}).resolveProvider
+
+  local function clear_word()
+    if not expand_snippet then
+      return nil
+    end
+
+    -- Remove the already inserted word.
+    local start_char = cursor_col - #completed_item.word
+    local line = api.nvim_buf_get_lines(bufnr, cursor_row, cursor_row + 1, true)[1]
+    api.nvim_buf_set_text(bufnr, cursor_row, start_char, cursor_row, #line, { '' })
+    return line:sub(cursor_col + 1)
+  end
+
+  --- @param suffix? string
+  local function apply_snippet_and_command(suffix)
+    if expand_snippet then
+      apply_snippet(completion_item, suffix)
+    end
+
+    local command = completion_item.command
+    if command then
+      client:_exec_cmd(command, { bufnr = bufnr }, nil, function()
+        vim.lsp.log.warn(
+          string.format(
+            'Language server `%s` does not support command `%s`. This command may require a client extension.',
+            client.name,
+            command.command
+          )
+        )
+      end)
+    end
+  end
+
+  if completion_item.additionalTextEdits and next(completion_item.additionalTextEdits) then
+    local suffix = clear_word()
+    lsp.util.apply_text_edits(completion_item.additionalTextEdits, bufnr, offset_encoding)
+    apply_snippet_and_command(suffix)
+  elseif resolve_provider and type(completion_item) == 'table' then
+    local changedtick = vim.b[bufnr].changedtick
+
+    --- @param result lsp.CompletionItem
+    client.request(ms.completionItem_resolve, completion_item, function(err, result)
+      if changedtick ~= vim.b[bufnr].changedtick then
+        return
+      end
+
+      local suffix = clear_word()
+      if err then
+        vim.notify_once(err.message, vim.log.levels.WARN)
+      elseif result and result.additionalTextEdits then
+        lsp.util.apply_text_edits(result.additionalTextEdits, bufnr, offset_encoding)
+        if result.command then
+          completion_item.command = result.command
+        end
+      end
+
+      apply_snippet_and_command(suffix)
+    end, bufnr)
+  else
+    local suffix = clear_word()
+    apply_snippet_and_command(suffix)
+  end
+end
+
+--- @class vim.lsp.completion.BufferOpts
+--- @field autotrigger? boolean Whether to trigger completion automatically. Default: false
+
+--- @param client_id integer
+---@param bufnr integer
+---@param opts vim.lsp.completion.BufferOpts
+local function enable_completions(client_id, bufnr, opts)
+  if not buf_handles[bufnr] then
+    buf_handles[bufnr] = { clients = {}, triggers = {} }
+
+    -- Attach to buffer events.
+    api.nvim_buf_attach(bufnr, false, {
+      on_detach = function(_, buf)
+        buf_handles[buf] = nil
+      end,
+      on_reload = function(_, buf)
+        M.enable(true, client_id, buf, opts)
+      end,
+    })
+
+    -- Set up autocommands.
+    local group =
+      api.nvim_create_augroup(string.format('vim/lsp/completion-%d', bufnr), { clear = true })
+    api.nvim_create_autocmd('CompleteDone', {
+      group = group,
+      buffer = bufnr,
+      callback = function()
+        local reason = api.nvim_get_vvar('event').reason --- @type string
+        if reason == 'accept' then
+          on_complete_done()
+        end
+      end,
+    })
+    if opts.autotrigger then
+      api.nvim_create_autocmd('InsertCharPre', {
+        group = group,
+        buffer = bufnr,
+        callback = function()
+          on_insert_char_pre(buf_handles[bufnr])
+        end,
+      })
+      api.nvim_create_autocmd('InsertLeave', {
+        group = group,
+        buffer = bufnr,
+        callback = on_insert_leave,
+      })
+    end
+  end
+
+  if not buf_handles[bufnr].clients[client_id] then
+    local client = lsp.get_client_by_id(client_id)
+    assert(client, 'invalid client ID')
+
+    -- Add the new client to the buffer's clients.
+    buf_handles[bufnr].clients[client_id] = client
+
+    -- Add the new client to the clients that should be triggered by its trigger characters.
+    --- @type string[]
+    local triggers = vim.tbl_get(
+      client.server_capabilities,
+      'completionProvider',
+      'triggerCharacters'
+    ) or {}
+    for _, char in ipairs(triggers) do
+      local clients_for_trigger = buf_handles[bufnr].triggers[char]
+      if not clients_for_trigger then
+        clients_for_trigger = {}
+        buf_handles[bufnr].triggers[char] = clients_for_trigger
+      end
+      local client_exists = vim.iter(clients_for_trigger):any(function(c)
+        return c.id == client_id
+      end)
+      if not client_exists then
+        table.insert(clients_for_trigger, client)
+      end
+    end
+  end
+end
+
+--- @param client_id integer
+--- @param bufnr integer
+local function disable_completions(client_id, bufnr)
+  local handle = buf_handles[bufnr]
+  if not handle then
+    return
+  end
+
+  handle.clients[client_id] = nil
+  if not next(handle.clients) then
+    buf_handles[bufnr] = nil
+    api.nvim_del_augroup_by_name(string.format('vim/lsp/completion-%d', bufnr))
+  else
+    for char, clients in pairs(handle.triggers) do
+      --- @param c vim.lsp.Client
+      handle.triggers[char] = vim.tbl_filter(function(c)
+        return c.id ~= client_id
+      end, clients)
+    end
+  end
+end
+
+--- Enables or disables completions from the given language client in the given buffer.
+---
+--- @param enable boolean True to enable, false to disable
+--- @param client_id integer Client ID
+--- @param bufnr integer Buffer handle, or 0 for the current buffer
+--- @param opts? vim.lsp.completion.BufferOpts
+function M.enable(enable, client_id, bufnr, opts)
+  bufnr = (bufnr == 0 and api.nvim_get_current_buf()) or bufnr
+
+  if enable then
+    enable_completions(client_id, bufnr, opts or {})
+  else
+    disable_completions(client_id, bufnr)
+  end
+end
+
+--- Trigger LSP completion in the current buffer.
+function M.trigger()
+  reset_timer()
+  Context:cancel_pending()
+
+  local win = api.nvim_get_current_win()
+  local bufnr = api.nvim_get_current_buf()
+  local cursor_row, cursor_col = unpack(api.nvim_win_get_cursor(win)) --- @type integer, integer
+  local line = api.nvim_get_current_line()
+  local line_to_cursor = line:sub(1, cursor_col)
+  local clients = (buf_handles[bufnr] or {}).clients or {}
+  local word_boundary = vim.fn.match(line_to_cursor, '\\k*$')
+  local start_time = vim.uv.hrtime()
+  Context.last_request_time = start_time
+
+  local cancel_request = request(clients, bufnr, win, function(responses)
+    local end_time = vim.uv.hrtime()
+    rtt_ms = compute_new_average((end_time - start_time) * ns_to_ms)
+
+    Context.pending_requests = {}
+    Context.isIncomplete = false
+
+    local row_changed = api.nvim_win_get_cursor(win)[1] ~= cursor_row
+    local mode = api.nvim_get_mode().mode
+    if row_changed or not (mode == 'i' or mode == 'ic') then
+      return
+    end
+
+    local matches = {}
+    local server_start_boundary --- @type integer?
+    for client_id, response in pairs(responses) do
+      if response.err then
+        vim.notify_once(response.err.message, vim.log.levels.warn)
+      end
+
+      local result = response.result
+      if result then
+        Context.isIncomplete = Context.isIncomplete or result.isIncomplete
+        local client = lsp.get_client_by_id(client_id)
+        local encoding = client and client.offset_encoding or 'utf-16'
+        local client_matches
+        client_matches, server_start_boundary = M._convert_results(
+          line,
+          cursor_row - 1,
+          cursor_col,
+          client_id,
+          word_boundary,
+          nil,
+          result,
+          encoding
+        )
+        vim.list_extend(matches, client_matches)
+      end
+    end
+    local start_col = (server_start_boundary or word_boundary) + 1
+    vim.fn.complete(start_col, matches)
+  end)
+
+  table.insert(Context.pending_requests, cancel_request)
+end
+
+return M

runtime/lua/vim/lsp/handlers.lua

@@ -3,7 +3,7 @@ local protocol = require('vim.lsp.protocol')
 local ms = protocol.Methods
 local util = require('vim.lsp.util')
 local api = vim.api
-local completion = require('vim.lsp._completion')
+local completion = require('vim.lsp.completion')
 
 --- @type table<string,lsp.Handler>
 local M = {}

runtime/lua/vim/lsp/protocol.lua

@@ -738,14 +738,16 @@ function protocol.make_client_capabilities()
       completion = {
         dynamicRegistration = false,
         completionItem = {
-          -- Until we can actually expand snippet, move cursor and allow for true snippet experience,
+          snippetSupport = true,
-          -- this should be disabled out of the box.
-          -- However, users can turn this back on if they have a snippet plugin.
-          snippetSupport = false,
           commitCharactersSupport = false,
           preselectSupport = false,
           deprecatedSupport = false,
           documentationFormat = { constants.MarkupKind.Markdown, constants.MarkupKind.PlainText },
+          resolveSupport = {
+            properties = {
+              'additionalTextEdits',
+            },
+          },
         },
         completionItemKind = {
           valueSet = get_value_set(constants.CompletionItemKind),

scripts/gen_vimdoc.lua

@@ -273,6 +273,7 @@ local config = {
       'buf.lua',
       'diagnostic.lua',
       'codelens.lua',
+      'completion.lua',
       'inlay_hint.lua',
       'tagfunc.lua',
       'semantic_tokens.lua',

test/functional/plugin/lsp/completion_spec.lua

@@ -21,7 +21,7 @@ local function complete(line, candidates, lnum)
     local line, cursor_col, lnum, result = ...
     local line_to_cursor = line:sub(1, cursor_col)
     local client_start_boundary = vim.fn.match(line_to_cursor, '\\k*$')
-    local items, server_start_boundary = require("vim.lsp._completion")._convert_results(
+    local items, server_start_boundary = require("vim.lsp.completion")._convert_results(
       line,
       lnum,
       cursor_col,
@@ -42,7 +42,7 @@ local function complete(line, candidates, lnum)
   )
 end
 
-describe('vim.lsp._completion', function()
+describe('vim.lsp.completion', function()
   before_each(n.clear)
 
   -- https://microsoft.github.io/language-server-protocol/specifications/specification-current/#textDocument_completion
@@ -159,6 +159,7 @@ describe('vim.lsp._completion', function()
     end, result.items)
     eq(expected, result)
   end)
+
   it('uses correct start boundary', function()
     local completion_list = {
       isIncomplete = false,