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

3 files changed, 83 insertions, 5 deletions

diff --git a/runtime/lua/vim/lsp/protocol.lua b/runtime/lua/vim/lsp/protocol.lua
index 877d11411b..7d5f8f5ef1 100644
--- a/runtime/lua/vim/lsp/protocol.lua
+++ b/runtime/lua/vim/lsp/protocol.lua
@@ -633,8 +633,7 @@ function protocol.make_client_capabilities()
         dynamicRegistration = false;
         completionItem = {
 
-          -- TODO(tjdevries): Is it possible to implement this in plain lua?
-          snippetSupport = false;
+          snippetSupport = true;
           commitCharactersSupport = false;
           preselectSupport = false;
           deprecatedSupport = false;
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
diff --git a/test/functional/plugin/lsp_spec.lua b/test/functional/plugin/lsp_spec.lua
index e39511d0ea..f1478c782d 100644
--- a/test/functional/plugin/lsp_spec.lua
+++ b/test/functional/plugin/lsp_spec.lua
@@ -973,7 +973,14 @@ describe('LSP', function()
         { label='foocar', insertText='foobar', textEdit={} },
         -- resolves into textEdit.newText
         { label='foocar', insertText='foodar', textEdit={newText='foobar'} },
-        { label='foocar', textEdit={newText='foobar'} }
+        { label='foocar', textEdit={newText='foobar'} },
+        -- real-world snippet text
+        { label='foocar', insertText='foodar', textEdit={newText='foobar(${1:place holder}, ${2:more ...holder{\\}})'} },
+        { label='foocar', insertText='foodar(${1:var1} typ1, ${2:var2} *typ2) {$0\\}', textEdit={} },
+        -- nested snippet tokens
+        { label='foocar', insertText='foodar(${1:var1 ${2|typ2,typ3|} ${3:tail}}) {$0\\}', textEdit={} },
+        -- plain text
+        { label='foocar', insertText='foodar(${1:var1})', insertTextFormat=1, textEdit={} },
       }
       local completion_list_items = {items=completion_list}
       local expected = {
@@ -983,6 +990,10 @@ describe('LSP', function()
         { abbr = 'foocar', dup = 1, empty = 1, icase = 1, info = ' ', kind = 'Unknown', menu = '', word = 'foobar', user_data = { nvim = { lsp = { completion_item = { label='foocar', insertText='foobar', textEdit={} } } } } },
         { abbr = 'foocar', dup = 1, empty = 1, icase = 1, info = ' ', kind = 'Unknown', menu = '', word = 'foobar', user_data = { nvim = { lsp = { completion_item = { label='foocar', insertText='foodar', textEdit={newText='foobar'} } } } } },
         { abbr = 'foocar', dup = 1, empty = 1, icase = 1, info = ' ', kind = 'Unknown', menu = '', word = 'foobar', user_data = { nvim = { lsp = { completion_item = { label='foocar', textEdit={newText='foobar'} } } } } },
+        { abbr = 'foocar', dup = 1, empty = 1, icase = 1, info = ' ', kind = 'Unknown', menu = '', word = 'foobar(place holder, more ...holder{})', user_data = { nvim = { lsp = { completion_item = { label='foocar', insertText='foodar', textEdit={newText='foobar(${1:place holder}, ${2:more ...holder{\\}})'} } } } } },
+        { abbr = 'foocar', dup = 1, empty = 1, icase = 1, info = ' ', kind = 'Unknown', menu = '', word = 'foodar(var1 typ1, var2 *typ2) {}', user_data = { nvim = { lsp = { completion_item = { label='foocar', insertText='foodar(${1:var1} typ1, ${2:var2} *typ2) {$0\\}', textEdit={} } } } } },
+        { abbr = 'foocar', dup = 1, empty = 1, icase = 1, info = ' ', kind = 'Unknown', menu = '', word = 'foodar(var1 typ2,typ3 tail) {}', user_data = { nvim = { lsp = { completion_item = { label='foocar', insertText='foodar(${1:var1 ${2|typ2,typ3|} ${3:tail}}) {$0\\}', textEdit={} } } } } },
+        { abbr = 'foocar', dup = 1, empty = 1, icase = 1, info = ' ', kind = 'Unknown', menu = '', word = 'foodar(${1:var1})', user_data = { nvim = { lsp = { completion_item = { label='foocar', insertText='foodar(${1:var1})', insertTextFormat=1, textEdit={} } } } } },
       }
 
       eq(expected, exec_lua([[return vim.lsp.util.text_document_completion_list_to_complete_items(...)]], completion_list, prefix))