feat(lsp): add snippet API (#25301)

runtime/doc/lua.txt

@@ -3670,4 +3670,63 @@ totable({f}, {...})                                       *vim.iter.totable()*
     Return: ~
         (table)
 
+
+==============================================================================
+Lua module: vim.snippet                                          *vim.snippet*
+
+active()                                                    *snippet.active()*
+    Returns `true` if there's an active snippet in the current buffer.
+
+    Return: ~
+        (boolean)
+
+exit()                                                        *snippet.exit()*
+    Exits the current snippet.
+
+expand({input})                                             *snippet.expand()*
+    Expands the given snippet text. Refer to https://microsoft.github.io/language-server-protocol/specification/#snippet_syntax for the specification of valid input.
+
+    Tabstops are highlighted with hl-SnippetTabstop.
+
+    Parameters: ~
+      • {input}  (string)
+
+jump({direction})                                             *snippet.jump()*
+    Jumps within the active snippet in the given direction. If the jump isn't
+    possible, the function call does nothing.
+
+    You can use this function to navigate a snippet as follows: >lua
+        vim.keymap.set({ 'i', 's' }, '<Tab>', function()
+           if vim.snippet.jumpable(1) then
+             return '<cmd>lua vim.snippet.jump(1)<cr>'
+           else
+             return '<Tab>'
+           end
+         end, { expr = true })
+<
+
+    Parameters: ~
+      • {direction}  (vim.snippet.Direction) Navigation direction. -1 for
+                     previous, 1 for next.
+
+jumpable({direction})                                     *snippet.jumpable()*
+    Returns `true` if there is an active snippet which can be jumped in the
+    given direction. You can use this function to navigate a snippet as
+    follows: >lua
+        vim.keymap.set({ 'i', 's' }, '<Tab>', function()
+           if vim.snippet.jumpable(1) then
+             return '<cmd>lua vim.snippet.jump(1)<cr>'
+           else
+             return '<Tab>'
+           end
+         end, { expr = true })
+<
+
+    Parameters: ~
+      • {direction}  (vim.snippet.Direction) Navigation direction. -1 for
+                     previous, 1 for next.
+
+    Return: ~
+        (boolean)
+
  vim:tw=78:ts=8:sw=4:sts=4:et:ft=help:norl:

runtime/doc/news.txt

@@ -193,6 +193,8 @@ The following new APIs and features were added.
 
 • Added |:fclose| command.
 
+• Added |vim.snippet| for snippet expansion support.
+
 ==============================================================================
 CHANGED FEATURES                                                 *news-changed*
 

runtime/lua/vim/_editor.lua

@@ -38,6 +38,7 @@ for k, v in pairs({
   ui = true,
   health = true,
   secure = true,
+  snippet = true,
   _watch = true,
 }) do
   vim._submodules[k] = v

runtime/lua/vim/_meta.lua

@@ -19,6 +19,7 @@ vim.loader = require('vim.loader')
 vim.lsp = require('vim.lsp')
 vim.re = require('vim.re')
 vim.secure = require('vim.secure')
+vim.snippet = require('vim.snippet')
 vim.treesitter = require('vim.treesitter')
 vim.ui = require('vim.ui')
 vim.version = require('vim.version')

runtime/lua/vim/lsp/_snippet_grammar.lua

@@ -81,14 +81,40 @@ local Type = {
 M.NodeType = Type
 
 --- @class vim.snippet.Node<T>: { type: vim.snippet.Type, data: T }
---- @class vim.snippet.TabstopData: { tabstop: number }
+--- @class vim.snippet.TabstopData: { tabstop: integer }
 --- @class vim.snippet.TextData: { text: string }
---- @class vim.snippet.PlaceholderData: { tabstop: vim.snippet.TabstopData, value: vim.snippet.Node<any> }
---- @class vim.snippet.ChoiceData: { tabstop: vim.snippet.TabstopData, values: string[] }
+--- @class vim.snippet.PlaceholderData: { tabstop: integer, value: vim.snippet.Node<any> }
+--- @class vim.snippet.ChoiceData: { tabstop: integer, values: string[] }
 --- @class vim.snippet.VariableData: { name: string, default?: vim.snippet.Node<any>, regex?: string, format?: vim.snippet.Node<vim.snippet.FormatData|vim.snippet.TextData>[], options?: string }
 --- @class vim.snippet.FormatData: { capture: number, modifier?: string, if_text?: string, else_text?: string }
 --- @class vim.snippet.SnippetData: { children: vim.snippet.Node<any>[] }
 
+--- @type vim.snippet.Node<any>
+local Node = {}
+
+--- @return string
+--- @diagnostic disable-next-line: inject-field
+function Node:__tostring()
+  local node_text = {}
+  local type, data = self.type, self.data
+  if type == Type.Snippet then
+    --- @cast data vim.snippet.SnippetData
+    for _, child in ipairs(data.children) do
+      table.insert(node_text, tostring(child))
+    end
+  elseif type == Type.Choice then
+    --- @cast data vim.snippet.ChoiceData
+    table.insert(node_text, data.values[1])
+  elseif type == Type.Placeholder then
+    --- @cast data vim.snippet.PlaceholderData
+    table.insert(node_text, tostring(data.value))
+  elseif type == Type.Text then
+    --- @cast data vim.snippet.TextData
+    table.insert(node_text, data.text)
+  end
+  return table.concat(node_text)
+end
+
 --- Returns a function that constructs a snippet node of the given type.
 ---
 --- @generic T
@@ -96,7 +122,7 @@ M.NodeType = Type
 --- @return fun(data: T): vim.snippet.Node<T>
 local function node(type)
   return function(data)
-    return { type = type, data = data }
+    return setmetatable({ type = type, data = data }, Node)
   end
 end
 

runtime/lua/vim/lsp/protocol.lua

@@ -742,7 +742,6 @@ function protocol.make_client_capabilities()
           -- 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,

runtime/lua/vim/lsp/util.lua

@@ -616,35 +616,7 @@ function M.parse_snippet(input)
     return input
   end
 
-  --- @param node vim.snippet.Node<any>
-  --- @return string
-  local function node_to_string(node)
-    local insert_text = {}
-    if node.type == snippet.NodeType.Snippet then
-      for _, child in
-        ipairs((node.data --[[@as vim.snippet.SnippetData]]).children)
-      do
-        table.insert(insert_text, node_to_string(child))
-      end
-    elseif node.type == snippet.NodeType.Choice then
-      table.insert(insert_text, (node.data --[[@as vim.snippet.ChoiceData]]).values[1])
-    elseif node.type == snippet.NodeType.Placeholder then
-      table.insert(
-        insert_text,
-        node_to_string((node.data --[[@as vim.snippet.PlaceholderData]]).value)
-      )
-    elseif node.type == snippet.NodeType.Text then
-      table.insert(
-        insert_text,
-        node
-          .data --[[@as vim.snippet.TextData]]
-          .text
-      )
-    end
-    return table.concat(insert_text)
-  end
-
-  return node_to_string(parsed)
+  return tostring(parsed)
 end
 
 --- Sorts by CompletionItem.sortText.

runtime/lua/vim/snippet.lua

@@ -0,0 +1,535 @@
+local G = require('vim.lsp._snippet_grammar')
+local snippet_group = vim.api.nvim_create_augroup('vim/snippet', {})
+local snippet_ns = vim.api.nvim_create_namespace('vim/snippet')
+
+--- Returns the 0-based cursor position.
+---
+--- @return integer, integer
+local function cursor_pos()
+  local cursor = vim.api.nvim_win_get_cursor(0)
+  return cursor[1] - 1, cursor[2]
+end
+
+--- Resolves variables (like `$name` or `${name:default}`) as follows:
+--- - When a variable is unknown (i.e.: its name is not recognized in any of the cases below), return `nil`.
+--- - When a variable isn't set, return its default (if any) or an empty string.
+---
+--- Note that in some cases, the default is ignored since it's not clear how to distinguish an empty
+--- value from an unset value (e.g.: `TM_CURRENT_LINE`).
+---
+--- @param var string
+--- @param default string
+--- @return string?
+local function resolve_variable(var, default)
+  --- @param str string
+  --- @return string
+  local function expand_or_default(str)
+    local expansion = vim.fn.expand(str) --[[@as string]]
+    return expansion == '' and default or expansion
+  end
+
+  if var == 'TM_SELECTED_TEXT' then
+    -- Snippets are expanded in insert mode only, so there's no selection.
+    return default
+  elseif var == 'TM_CURRENT_LINE' then
+    return vim.api.nvim_get_current_line()
+  elseif var == 'TM_CURRENT_WORD' then
+    return expand_or_default('<cword>')
+  elseif var == 'TM_LINE_INDEX' then
+    return tostring(vim.fn.line('.') - 1)
+  elseif var == 'TM_LINE_NUMBER' then
+    return tostring(vim.fn.line('.'))
+  elseif var == 'TM_FILENAME' then
+    return expand_or_default('%:t')
+  elseif var == 'TM_FILENAME_BASE' then
+    -- Not using '%:t:r' since we want to remove all extensions.
+    local filename_base = expand_or_default('%:t'):gsub('%.[^%.]*$', '')
+    return filename_base
+  elseif var == 'TM_DIRECTORY' then
+    return expand_or_default('%:p:h:t')
+  elseif var == 'TM_FILEPATH' then
+    return expand_or_default('%:p')
+  end
+
+  -- Unknown variable.
+  return nil
+end
+
+--- Transforms the given text into an array of lines (so no line contains `\n`).
+---
+--- @param text string|string[]
+--- @return string[]
+local function text_to_lines(text)
+  text = type(text) == 'string' and { text } or text
+  --- @cast text string[]
+  return vim.split(table.concat(text), '\n', { plain = true })
+end
+
+--- Computes the 0-based position of a tabstop located at the end of `snippet` and spanning
+--- `placeholder` (if given).
+---
+--- @param snippet string[]
+--- @param placeholder string?
+--- @return Range4
+local function compute_tabstop_range(snippet, placeholder)
+  local cursor_row, cursor_col = cursor_pos()
+  local snippet_text = text_to_lines(snippet)
+  local placeholder_text = text_to_lines(placeholder or '')
+  local start_row = cursor_row + #snippet_text - 1
+  local start_col = #(snippet_text[#snippet_text] or '')
+
+  -- Add the cursor's column offset to the first line.
+  if start_row == cursor_row then
+    start_col = start_col + cursor_col
+  end
+
+  local end_row = start_row + #placeholder_text - 1
+  local end_col = (start_row == end_row and start_col or 0)
+    + #(placeholder_text[#placeholder_text] or '')
+
+  return { start_row, start_col, end_row, end_col }
+end
+
+--- @class vim.snippet.Tabstop
+--- @field extmark_id integer
+--- @field index integer
+--- @field bufnr integer
+local Tabstop = {}
+
+--- Creates a new tabstop.
+---
+--- @package
+--- @param index integer
+--- @param bufnr integer
+--- @param range Range4
+--- @return vim.snippet.Tabstop
+function Tabstop.new(index, bufnr, range)
+  local extmark_id = vim.api.nvim_buf_set_extmark(bufnr, snippet_ns, range[1], range[2], {
+    right_gravity = false,
+    end_right_gravity = true,
+    end_line = range[3],
+    end_col = range[4],
+    hl_group = 'SnippetTabstop',
+  })
+
+  local self = setmetatable(
+    { index = index, bufnr = bufnr, extmark_id = extmark_id },
+    { __index = Tabstop }
+  )
+
+  return self
+end
+
+--- Returns the tabstop's range.
+---
+--- @package
+--- @return Range4
+function Tabstop:get_range()
+  local mark =
+    vim.api.nvim_buf_get_extmark_by_id(self.bufnr, snippet_ns, self.extmark_id, { details = true })
+
+  --- @diagnostic disable-next-line: undefined-field
+  return { mark[1], mark[2], mark[3].end_row, mark[3].end_col }
+end
+
+--- Returns the text spanned by the tabstop.
+---
+--- @package
+--- @return string
+function Tabstop:get_text()
+  local range = self:get_range()
+  return table.concat(
+    vim.api.nvim_buf_get_text(self.bufnr, range[1], range[2], range[3], range[4], {}),
+    '\n'
+  )
+end
+
+--- Sets the tabstop's text.
+---
+--- @package
+--- @param text string
+function Tabstop:set_text(text)
+  local range = self:get_range()
+  vim.api.nvim_buf_set_text(self.bufnr, range[1], range[2], range[3], range[4], text_to_lines(text))
+end
+
+--- @class vim.snippet.Session
+--- @field bufnr integer
+--- @field tabstops table<integer, vim.snippet.Tabstop[]>
+--- @field current_tabstop vim.snippet.Tabstop
+local Session = {}
+
+--- Creates a new snippet session in the current buffer.
+---
+--- @package
+--- @return vim.snippet.Session
+function Session.new()
+  local bufnr = vim.api.nvim_get_current_buf()
+  local self = setmetatable({
+    bufnr = bufnr,
+    tabstops = {},
+    current_tabstop = Tabstop.new(0, bufnr, { 0, 0, 0, 0 }),
+  }, { __index = Session })
+
+  return self
+end
+
+--- Creates the session tabstops.
+---
+--- @package
+--- @param tabstop_ranges table<integer, Range4[]>
+function Session:set_tabstops(tabstop_ranges)
+  for index, ranges in pairs(tabstop_ranges) do
+    for _, range in ipairs(ranges) do
+      self.tabstops[index] = self.tabstops[index] or {}
+      table.insert(self.tabstops[index], Tabstop.new(index, self.bufnr, range))
+    end
+  end
+end
+
+--- Returns the destination tabstop index when jumping in the given direction.
+---
+--- @package
+--- @param direction vim.snippet.Direction
+--- @return integer?
+function Session:get_dest_index(direction)
+  local tabstop_indexes = vim.tbl_keys(self.tabstops) --- @type integer[]
+  table.sort(tabstop_indexes)
+  for i, index in ipairs(tabstop_indexes) do
+    if index == self.current_tabstop.index then
+      local dest_index = tabstop_indexes[i + direction] --- @type integer?
+      -- When jumping forwards, $0 is the last tabstop.
+      if not dest_index and direction == 1 then
+        dest_index = 0
+      end
+      -- When jumping backwards, make sure we don't think that $0 is the first tabstop.
+      if dest_index == 0 and direction == -1 then
+        dest_index = nil
+      end
+      return dest_index
+    end
+  end
+end
+
+--- @class vim.snippet.Snippet
+--- @field private _session? vim.snippet.Session
+local M = { session = nil }
+
+--- Select the given tabstop range.
+---
+--- @param tabstop vim.snippet.Tabstop
+local function select_tabstop(tabstop)
+  --- @param keys string
+  local function feedkeys(keys)
+    keys = vim.api.nvim_replace_termcodes(keys, true, false, true)
+    vim.api.nvim_feedkeys(keys, 'n', true)
+  end
+
+  --- NOTE: We don't use `vim.api.nvim_win_set_cursor` here because it causes the cursor to end
+  --- at the end of the selection instead of the start.
+  ---
+  --- @param row integer
+  --- @param col integer
+  local function move_cursor_to(row, col)
+    local line = vim.fn.getline(row) --[[ @as string ]]
+    col = math.max(vim.fn.strchars(line:sub(1, col)) - 1, 0)
+    feedkeys(string.format('%sG0%s', row, string.rep('<Right>', col)))
+  end
+
+  local range = tabstop:get_range()
+  local mode = vim.fn.mode()
+
+  -- Move the cursor to the start of the tabstop.
+  vim.api.nvim_win_set_cursor(0, { range[1] + 1, range[2] })
+
+  -- For empty and the final tabstop, start insert mode at the end of the range.
+  if tabstop.index == 0 or (range[1] == range[3] and range[2] == range[4]) then
+    if mode ~= 'i' then
+      if mode == 's' then
+        feedkeys('<Esc>')
+      end
+      vim.cmd.startinsert({ bang = range[4] >= #vim.api.nvim_get_current_line() })
+    end
+  else
+    -- Else, select the tabstop's text.
+    if mode ~= 'n' then
+      feedkeys('<Esc>')
+    end
+    move_cursor_to(range[1] + 1, range[2] + 1)
+    feedkeys('v')
+    move_cursor_to(range[3] + 1, range[4])
+    feedkeys('o<c-g>')
+  end
+end
+
+--- Sets up the necessary autocommands for snippet expansion.
+---
+--- @param bufnr integer
+local function setup_autocmds(bufnr)
+  vim.api.nvim_create_autocmd({ 'CursorMoved', 'CursorMovedI' }, {
+    group = snippet_group,
+    desc = 'Update snippet state when the cursor moves',
+    buffer = bufnr,
+    callback = function()
+      -- Just update the tabstop in insert and select modes.
+      if not vim.fn.mode():match('^[isS]') then
+        return
+      end
+
+      -- Update the current tabstop to be the one containing the cursor.
+      local cursor_row, cursor_col = cursor_pos()
+      for tabstop_index, tabstops in pairs(M._session.tabstops) do
+        for _, tabstop in ipairs(tabstops) do
+          local range = tabstop:get_range()
+          if
+            (cursor_row > range[1] or (cursor_row == range[1] and cursor_col >= range[2]))
+            and (cursor_row < range[3] or (cursor_row == range[3] and cursor_col <= range[4]))
+          then
+            M._session.current_tabstop = tabstop
+            if tabstop_index ~= 0 then
+              return
+            end
+          end
+        end
+      end
+
+      -- The cursor is either not on a tabstop or we reached the end, so exit the session.
+      M.exit()
+      return true
+    end,
+  })
+
+  vim.api.nvim_create_autocmd({ 'TextChanged', 'TextChangedI' }, {
+    group = snippet_group,
+    desc = 'Update active tabstops when buffer text changes',
+    buffer = bufnr,
+    callback = function()
+      if not M.active() then
+        return true
+      end
+
+      -- Sync the tabstops in the current group.
+      local current_tabstop = M._session.current_tabstop
+      local current_text = current_tabstop:get_text()
+      for _, tabstop in ipairs(M._session.tabstops[current_tabstop.index]) do
+        if tabstop.extmark_id ~= current_tabstop.extmark_id then
+          tabstop:set_text(current_text)
+        end
+      end
+    end,
+  })
+end
+
+--- Expands the given snippet text.
+--- Refer to https://microsoft.github.io/language-server-protocol/specification/#snippet_syntax
+--- for the specification of valid input.
+---
+--- Tabstops are highlighted with hl-SnippetTabstop.
+---
+--- @param input string
+function M.expand(input)
+  local snippet = G.parse(input)
+  local snippet_text = {}
+
+  M._session = Session.new()
+
+  -- Get the placeholders we should use for each tabstop index.
+  --- @type table<integer, string>
+  local placeholders = {}
+  for _, child in ipairs(snippet.data.children) do
+    local type, data = child.type, child.data
+    if type == G.NodeType.Placeholder then
+      --- @cast data vim.snippet.PlaceholderData
+      local tabstop, value = data.tabstop, tostring(data.value)
+      if placeholders[tabstop] and placeholders[tabstop] ~= value then
+        error('Snippet has multiple placeholders for tabstop $' .. tabstop)
+      end
+      placeholders[tabstop] = value
+    end
+  end
+
+  -- Keep track of tabstop nodes during expansion.
+  --- @type table<integer, Range4[]>
+  local tabstop_ranges = {}
+
+  --- @param index integer
+  --- @param placeholder string?
+  local function add_tabstop(index, placeholder)
+    tabstop_ranges[index] = tabstop_ranges[index] or {}
+    table.insert(tabstop_ranges[index], compute_tabstop_range(snippet_text, placeholder))
+  end
+
+  --- Appends the given text to the snippet, taking care of indentation.
+  ---
+  --- @param text string|string[]
+  local function append_to_snippet(text)
+    -- Get the base indentation based on the current line and the last line of the snippet.
+    local base_indent = vim.api.nvim_get_current_line():match('^%s*') or ''
+    if #snippet_text > 0 then
+      base_indent = base_indent .. (snippet_text[#snippet_text]:match('^%s*') or '') --- @type string
+    end
+
+    local lines = vim.iter.map(function(i, line)
+      -- Replace tabs by spaces.
+      if vim.o.expandtab then
+        line = line:gsub('\t', (' '):rep(vim.fn.shiftwidth())) --- @type string
+      end
+      -- Add the base indentation.
+      if i > 1 then
+        line = base_indent .. line
+      end
+      return line
+    end, ipairs(text_to_lines(text)))
+
+    table.insert(snippet_text, table.concat(lines, '\n'))
+  end
+
+  for _, child in ipairs(snippet.data.children) do
+    local type, data = child.type, child.data
+    if type == G.NodeType.Tabstop then
+      --- @cast data vim.snippet.TabstopData
+      local placeholder = placeholders[data.tabstop]
+      add_tabstop(data.tabstop, placeholder)
+      if placeholder then
+        append_to_snippet(placeholder)
+      end
+    elseif type == G.NodeType.Placeholder then
+      --- @cast data vim.snippet.PlaceholderData
+      local value = placeholders[data.tabstop]
+      add_tabstop(data.tabstop, value)
+      append_to_snippet(value)
+    elseif type == G.NodeType.Choice then
+      --- @cast data vim.snippet.ChoiceData
+      append_to_snippet(data.values[1])
+    elseif type == G.NodeType.Variable then
+      --- @cast data vim.snippet.VariableData
+      -- Try to get the variable's value.
+      local value = resolve_variable(data.name, data.default and tostring(data.default) or '')
+      if not value then
+        -- Unknown variable, make this a tabstop and use the variable name as a placeholder.
+        value = data.name
+        local tabstop_indexes = vim.tbl_keys(tabstop_ranges)
+        local index = math.max(unpack((#tabstop_indexes == 0 and { 0 }) or tabstop_indexes)) + 1
+        add_tabstop(index, value)
+      end
+      append_to_snippet(value)
+    elseif type == G.NodeType.Text then
+      --- @cast data vim.snippet.TextData
+      append_to_snippet(data.text)
+    end
+  end
+
+  -- $0, which defaults to the end of the snippet, defines the final cursor position.
+  -- Make sure the snippet has exactly one of these.
+  if vim.tbl_contains(vim.tbl_keys(tabstop_ranges), 0) then
+    assert(#tabstop_ranges[0] == 1, 'Snippet has multiple $0 tabstops')
+  else
+    add_tabstop(0)
+  end
+
+  -- Insert the snippet text.
+  local cursor_row, cursor_col = cursor_pos()
+  vim.api.nvim_buf_set_text(
+    M._session.bufnr,
+    cursor_row,
+    cursor_col,
+    cursor_row,
+    cursor_col,
+    text_to_lines(snippet_text)
+  )
+
+  -- Create the tabstops.
+  M._session:set_tabstops(tabstop_ranges)
+
+  -- Jump to the first tabstop.
+  M.jump(1)
+end
+
+--- @alias vim.snippet.Direction -1 | 1
+
+--- Returns `true` if there is an active snippet which can be jumped in the given direction.
+--- You can use this function to navigate a snippet as follows:
+---
+--- ```lua
+--- vim.keymap.set({ 'i', 's' }, '<Tab>', function()
+---    if vim.snippet.jumpable(1) then
+---      return '<cmd>lua vim.snippet.jump(1)<cr>'
+---    else
+---      return '<Tab>'
+---    end
+---  end, { expr = true })
+--- ```
+---
+--- @param direction (vim.snippet.Direction) Navigation direction. -1 for previous, 1 for next.
+--- @return boolean
+function M.jumpable(direction)
+  if not M.active() then
+    return false
+  end
+
+  return M._session:get_dest_index(direction) ~= nil
+end
+
+--- Jumps within the active snippet in the given direction.
+--- If the jump isn't possible, the function call does nothing.
+---
+--- You can use this function to navigate a snippet as follows:
+---
+--- ```lua
+--- vim.keymap.set({ 'i', 's' }, '<Tab>', function()
+---    if vim.snippet.jumpable(1) then
+---      return '<cmd>lua vim.snippet.jump(1)<cr>'
+---    else
+---      return '<Tab>'
+---    end
+---  end, { expr = true })
+--- ```
+---
+--- @param direction (vim.snippet.Direction) Navigation direction. -1 for previous, 1 for next.
+function M.jump(direction)
+  -- Get the tabstop index to jump to.
+  local dest_index = M._session and M._session:get_dest_index(direction)
+  if not dest_index then
+    return
+  end
+
+  -- Find the tabstop with the lowest range.
+  local tabstops = M._session.tabstops[dest_index]
+  local dest = tabstops[1]
+  for _, tabstop in ipairs(tabstops) do
+    local dest_range, range = dest:get_range(), tabstop:get_range()
+    if (range[1] < dest_range[1]) or (range[1] == dest_range[1] and range[2] < dest_range[2]) then
+      dest = tabstop
+    end
+  end
+
+  -- Clear the autocommands so that we can move the cursor freely while selecting the tabstop.
+  vim.api.nvim_clear_autocmds({ group = snippet_group, buffer = M._session.bufnr })
+
+  M._session.current_tabstop = dest
+  select_tabstop(dest)
+
+  -- Restore the autocommands.
+  setup_autocmds(M._session.bufnr)
+end
+
+--- Returns `true` if there's an active snippet in the current buffer.
+---
+--- @return boolean
+function M.active()
+  return M._session ~= nil and M._session.bufnr == vim.api.nvim_get_current_buf()
+end
+
+--- Exits the current snippet.
+function M.exit()
+  if not M.active() then
+    return
+  end
+
+  vim.api.nvim_clear_autocmds({ group = snippet_group, buffer = M._session.bufnr })
+  vim.api.nvim_buf_clear_namespace(M._session.bufnr, snippet_ns, 0, -1)
+
+  M._session = nil
+end
+
+return M

scripts/gen_vimdoc.py

@@ -165,6 +165,7 @@ CONFIG = {
             'secure.lua',
             'version.lua',
             'iter.lua',
+            'snippet.lua',
         ],
         'files': [
             'runtime/lua/vim/iter.lua',
@@ -181,6 +182,7 @@ CONFIG = {
             'runtime/lua/vim/secure.lua',
             'runtime/lua/vim/version.lua',
             'runtime/lua/vim/_inspector.lua',
+            'runtime/lua/vim/snippet.lua',
             'runtime/lua/vim/_meta/builtin.lua',
             'runtime/lua/vim/_meta/diff.lua',
             'runtime/lua/vim/_meta/mpack.lua',

src/nvim/highlight_group.c

@@ -235,6 +235,7 @@ static const char *highlight_init_both[] = {
   "default DiagnosticDeprecated cterm=strikethrough gui=strikethrough guisp=Red",
   "default link DiagnosticUnnecessary Comment",
   "default link LspInlayHint NonText",
+  "default link SnippetTabstop Visual",
 
   // Text
   "default link @text.literal Comment",

test/functional/lua/snippet_spec.lua

@@ -0,0 +1,157 @@
+local helpers = require('test.functional.helpers')(after_each)
+
+local clear = helpers.clear
+local eq = helpers.eq
+local exec_lua = helpers.exec_lua
+local feed = helpers.feed
+local matches = helpers.matches
+local pcall_err = helpers.pcall_err
+
+describe('vim.snippet', function()
+  before_each(function()
+    clear()
+
+    exec_lua([[
+      vim.keymap.set({ 'i', 's' }, '<Tab>', function() vim.snippet.jump(1) end, { buffer = true })
+      vim.keymap.set({ 'i', 's' }, '<S-Tab>', function() vim.snippet.jump(-1) end, { buffer = true })
+    ]])
+  end)
+  after_each(clear)
+
+  --- @param snippet string[]
+  --- @param expected string[]
+  --- @param settings? string
+  --- @param prefix? string
+  local function test_success(snippet, expected, settings, prefix)
+    if settings then
+      exec_lua(settings)
+    end
+    if prefix then
+      feed('i' .. prefix)
+    end
+    exec_lua('vim.snippet.expand(...)', table.concat(snippet, '\n'))
+    eq(expected, helpers.buf_lines(0))
+  end
+
+  --- @param snippet string
+  --- @param err string
+  local function test_fail(snippet, err)
+    matches(err, pcall_err(exec_lua, string.format('vim.snippet.expand("%s")', snippet)))
+  end
+
+  it('adds base indentation to inserted text', function()
+    test_success(
+      { 'function $1($2)', '  $0', 'end' },
+      { '  function ()', '    ', '  end' },
+      '',
+      '  '
+    )
+  end)
+
+  it('replaces tabs with spaces when expandtab is set', function()
+    test_success(
+      { 'function $1($2)', '\t$0', 'end' },
+      { 'function ()', '  ', 'end' },
+      [[
+      vim.o.expandtab = true
+      vim.o.shiftwidth = 2
+      ]]
+    )
+  end)
+
+  it('respects tabs when expandtab is not set', function()
+    test_success(
+      { 'function $1($2)', '\t$0', 'end' },
+      { 'function ()', '\t', 'end' },
+      'vim.o.expandtab = false'
+    )
+  end)
+
+  it('inserts known variable value', function()
+    test_success({ '; print($TM_CURRENT_LINE)' }, { 'foo; print(foo)' }, nil, 'foo')
+  end)
+
+  it('uses default when variable is not set', function()
+    test_success({ 'print(${TM_CURRENT_WORD:foo})' }, { 'print(foo)' })
+  end)
+
+  it('replaces unknown variables by placeholders', function()
+    test_success({ 'print($UNKNOWN)' }, { 'print(UNKNOWN)' })
+  end)
+
+  it('does not jump outside snippet range', function()
+    test_success({ 'function $1($2)', '  $0', 'end' }, { 'function ()', '  ', 'end' })
+    eq(false, exec_lua('return vim.snippet.jumpable(-1)'))
+    feed('<Tab><Tab>i')
+    eq(false, exec_lua('return vim.snippet.jumpable(1)'))
+  end)
+
+  it('navigates backwards', function()
+    test_success({ 'function $1($2) end' }, { 'function () end' })
+    feed('<Tab><S-Tab>foo')
+    eq({ 'function foo() end' }, helpers.buf_lines(0))
+  end)
+
+  it('visits all tabstops', function()
+    local function cursor()
+      return exec_lua('return vim.api.nvim_win_get_cursor(0)')
+    end
+
+    test_success({ 'function $1($2)', '  $0', 'end' }, { 'function ()', '  ', 'end' })
+    eq({ 1, 9 }, cursor())
+    feed('<Tab>')
+    eq({ 1, 10 }, cursor())
+    feed('<Tab>')
+    eq({ 2, 2 }, cursor())
+  end)
+
+  it('syncs text of tabstops with equal indexes', function()
+    test_success({ 'var double = ${1:x} + ${1:x}' }, { 'var double = x + x' })
+    feed('123')
+    eq({ 'var double = 123 + 123' }, helpers.buf_lines(0))
+  end)
+
+  it('cancels session with changes outside the snippet', function()
+    test_success({ 'print($1)' }, { 'print()' })
+    feed('<Esc>O-- A comment')
+    eq(false, exec_lua('return vim.snippet.active()'))
+    eq({ '-- A comment', 'print()' }, helpers.buf_lines(0))
+  end)
+
+  it('handles non-consecutive tabstops', function()
+    test_success({ 'class $1($3) {', '  $0', '}' }, { 'class () {', '  ', '}' })
+    feed('Foo') -- First tabstop
+    feed('<Tab><Tab>') -- Jump to $0
+    feed('// Inside') -- Insert text
+    eq({ 'class Foo() {', '  // Inside', '}' }, helpers.buf_lines(0))
+  end)
+
+  it('handles multiline placeholders', function()
+    test_success(
+      { 'public void foo() {', '  ${0:// TODO Auto-generated', '  throw;}', '}' },
+      { 'public void foo() {', '  // TODO Auto-generated', '  throw;', '}' }
+    )
+  end)
+
+  it('inserts placeholder in all tabstops when the first tabstop has the placeholder', function()
+    test_success(
+      { 'for (${1:int} ${2:x} = ${3:0}; $2 < ${4:N}; $2++) {', '  $0', '}' },
+      { 'for (int x = 0; x < N; x++) {', '  ', '}' }
+    )
+  end)
+
+  it('inserts placeholder in all tabstops when a later tabstop has the placeholder', function()
+    test_success(
+      { 'for (${1:int} $2 = ${3:0}; ${2:x} < ${4:N}; $2++) {', '  $0', '}' },
+      { 'for (int x = 0; x < N; x++) {', '  ', '}' }
+    )
+  end)
+
+  it('errors with multiple placeholders for the same index', function()
+    test_fail('class ${1:Foo} { void ${1:foo}() {} }', 'multiple placeholders for tabstop $1')
+  end)
+
+  it('errors with multiple $0 tabstops', function()
+    test_fail('function $1() { $0 }$0', 'multiple $0 tabstops')
+  end)
+end)