feat(lsp): add support for completionItem.command resolving

`command` was already resolved via a `completionItem/resolve` request
but only if `additionalTextEdits` were also present, and the
`resolveSupport` capability wasn't listed.

Closes https://github.com/neovim/neovim/issues/32406

runtime/lua/vim/lsp/completion.lua

@@ -610,13 +610,14 @@ local function on_complete_done()
       clear_word()
       if err then
         vim.notify_once(err.message, vim.log.levels.WARN)
-      elseif result and result.additionalTextEdits then
+      elseif result then
+        if result.additionalTextEdits then
           lsp.util.apply_text_edits(result.additionalTextEdits, bufnr, position_encoding)
+        end
         if result.command then
           completion_item.command = result.command
         end
       end
-
       apply_snippet_and_command()
     end, bufnr)
   else

runtime/lua/vim/lsp/protocol.lua

@@ -458,6 +458,7 @@ function protocol.make_client_capabilities()
           resolveSupport = {
             properties = {
               'additionalTextEdits',
+              'command',
             },
           },
           tagSupport = {

test/functional/plugin/lsp/completion_spec.lua

@@ -770,20 +770,25 @@ end)
 
 --- @param name string
 --- @param completion_result lsp.CompletionList
---- @param trigger_chars? string[]
+--- @param opts? {trigger_chars?: string[], resolve_result?: lsp.CompletionItem}
 --- @return integer
-local function create_server(name, completion_result, trigger_chars)
+local function create_server(name, completion_result, opts)
+  opts = opts or {}
   return exec_lua(function()
     local server = _G._create_server({
       capabilities = {
         completionProvider = {
-          triggerCharacters = trigger_chars or { '.' },
+          triggerCharacters = opts.trigger_chars or { '.' },
+          resolveProvider = opts.resolve_result ~= nil,
         },
       },
       handlers = {
         ['textDocument/completion'] = function(_, _, callback)
           callback(nil, completion_result)
         end,
+        ['completionItem/resolve'] = function(_, _, callback)
+          callback(nil, opts.resolve_result)
+        end,
       },
     })
 
@@ -794,7 +799,7 @@ local function create_server(name, completion_result, trigger_chars)
       cmd = server.cmd,
       on_attach = function(client, bufnr0)
         vim.lsp.completion.enable(true, client.id, bufnr0, {
-          autotrigger = trigger_chars ~= nil,
+          autotrigger = opts.trigger_chars ~= nil,
           convert = function(item)
             return { abbr = item.label:gsub('%b()', '') }
           end,
@@ -968,7 +973,7 @@ describe('vim.lsp.completion: protocol', function()
         },
       },
     }
-    create_server('dummy1', results1, { 'e' })
+    create_server('dummy1', results1, { trigger_chars = { 'e' } })
     local results2 = {
       isIncomplete = false,
       items = {
@@ -977,7 +982,7 @@ describe('vim.lsp.completion: protocol', function()
         },
       },
     }
-    create_server('dummy2', results2, { 'h' })
+    create_server('dummy2', results2, { trigger_chars = { 'h' } })
 
     feed('h')
     exec_lua(function()
@@ -1042,6 +1047,59 @@ describe('vim.lsp.completion: protocol', function()
     end)
   end)
 
+  it('resolves and executes commands', function()
+    local completion_list = {
+      isIncomplete = false,
+      items = {
+        {
+          label = 'hello',
+        },
+      },
+    }
+    local client_id = create_server('dummy', completion_list, {
+      resolve_result = {
+        label = 'hello',
+        command = {
+          arguments = { '1', '0' },
+          command = 'dummy',
+          title = '',
+        },
+      },
+    })
+    exec_lua(function()
+      _G.called = false
+      local client = assert(vim.lsp.get_client_by_id(client_id))
+      client.commands.dummy = function()
+        _G.called = true
+      end
+    end)
+
+    feed('ih')
+    trigger_at_pos({ 1, 1 })
+
+    local item = completion_list.items[1]
+    exec_lua(function()
+      vim.v.completed_item = {
+        user_data = {
+          nvim = {
+            lsp = {
+              client_id = client_id,
+              completion_item = item,
+            },
+          },
+        },
+      }
+    end)
+
+    feed('<C-x><C-o><C-y>')
+
+    assert_matches(function(matches)
+      eq(1, #matches)
+      eq('hello', matches[1].word)
+      eq(true, exec_lua('return _G.called'))
+    end)
+  end)
+
   it('enable(…,{convert=fn}) custom word/abbr format', function()
     create_server('dummy', {
       isIncomplete = false,