lua: simple snippet support in the completion items (#12118)

Old behavior is: foo(${placeholder: bar, ...)

with lots of random garbage you'd never want inserted.

New behavior is: foo(bar, baz) 

(which maybe is good, maybe is bad [depends on user], but definitely better than it was).

-----

* Implement rudimentary snippet parsing

Add support for parsing and discarding snippet tokens from the completion items.

Fixes #11982

* Enable snippet support

* Functional tests for snippet parsing

Add simplified real-world snippet text examples to the completion items
test

* Add a test for nested snippet tokens

* Remove TODO comment

* Return the unmodified item if the format is plain text

* Add a plain text completion item

1 files changed, 70 insertions, 2 deletions

diff --git a/runtime/lua/vim/lsp/util.lua b/runtime/lua/vim/lsp/util.lua
index b6eaae1fef..752d4ff439 100644
--- a/runtime/lua/vim/lsp/util.lua
+++ b/runtime/lua/vim/lsp/util.lua
@@ -199,6 +199,66 @@ function M.get_current_line_to_cursor()
   return line:sub(pos[2]+1)
 end
 
+local function parse_snippet_rec(input, inner)
+  local res = ""
+
+  local close, closeend = nil, nil
+  if inner then
+    close, closeend = input:find("}", 1, true)
+    while close ~= nil and input:sub(close-1,close-1) == "\\" do
+      close, closeend = input:find("}", closeend+1, true)
+    end
+  end
+
+  local didx = input:find('$',  1, true)
+  if didx == nil and close == nil then
+    return input, ""
+  elseif close ~=nil and (didx == nil or close < didx) then
+    -- No inner placeholders
+    return input:sub(0, close-1), input:sub(closeend+1)
+  end
+
+  res = res .. input:sub(0, didx-1)
+  input = input:sub(didx+1)
+
+  local tabstop, tabstopend = input:find('^%d+')
+  local placeholder, placeholderend = input:find('^{%d+:')
+  local choice, choiceend = input:find('^{%d+|')
+
+  if tabstop then
+    input = input:sub(tabstopend+1)
+  elseif choice then
+    input = input:sub(choiceend+1)
+    close, closeend = input:find("|}", 1, true)
+
+    res = res .. input:sub(0, close-1)
+    input = input:sub(closeend+1)
+  elseif placeholder then
+    -- TODO: add support for variables
+    input = input:sub(placeholderend+1)
+
+    -- placeholders and variables are recursive
+    while input ~= "" do
+      local r, tail = parse_snippet_rec(input, true)
+      r = r:gsub("\\}", "}")
+
+      res = res .. r
+      input = tail
+    end
+  else
+    res = res .. "$"
+  end
+
+  return res, input
+end
+
+-- Parse completion entries, consuming snippet tokens
+function M.parse_snippet(input)
+  local res, _ = parse_snippet_rec(input, false)
+
+  return res
+end
+
 -- Sort by CompletionItem.sortText
 -- https://microsoft.github.io/language-server-protocol/specifications/specification-current/#textDocument_completion
 local function sort_completion_items(items)
@@ -213,9 +273,17 @@ end
 -- https://microsoft.github.io/language-server-protocol/specifications/specification-current/#textDocument_completion
 local function get_completion_word(item)
   if item.textEdit ~= nil and item.textEdit.newText ~= nil then
-    return item.textEdit.newText
+    if protocol.InsertTextFormat[item.insertTextFormat] == "PlainText" then
+      return item.textEdit.newText
+    else
+      return M.parse_snippet(item.textEdit.newText)
+    end
   elseif item.insertText ~= nil then
-    return item.insertText
+    if protocol.InsertTextFormat[item.insertTextFormat] == "PlainText" then
+      return item.insertText
+    else
+      return M.parse_snippet(item.insertText)
+    end
   end
   return item.label
 end