feat(api): implement nvim_{add,del}_user_command

Add support for adding and removing custom user commands with the Nvim
API.

test/functional/api/command_spec.lua

@@ -6,8 +6,14 @@ local command = helpers.command
 local curbufmeths = helpers.curbufmeths
 local eq = helpers.eq
 local meths = helpers.meths
+local bufmeths = helpers.bufmeths
+local matches = helpers.matches
 local source = helpers.source
 local pcall_err = helpers.pcall_err
+local exec_lua = helpers.exec_lua
+local assert_alive = helpers.assert_alive
+local feed = helpers.feed
+local funcs = helpers.funcs
 
 describe('nvim_get_commands', function()
   local cmd_dict  = { addr=NIL, bang=false, bar=false, complete=NIL, complete_arg=NIL, count=NIL, definition='echo "Hello World"', name='Hello', nargs='1', range=NIL, register=false, script_id=0, }
@@ -78,3 +84,118 @@ describe('nvim_get_commands', function()
     eq({Cmd2=cmd2, Cmd3=cmd3, Cmd4=cmd4, Finger=cmd1, TestCmd=cmd0}, meths.get_commands({builtin=false}))
   end)
 end)
+
+describe('nvim_add_user_command', function()
+  before_each(clear)
+
+  it('works with strings', function()
+    meths.add_user_command('SomeCommand', 'let g:command_fired = <args>', {nargs = 1})
+    meths.command('SomeCommand 42')
+    eq(42, meths.eval('g:command_fired'))
+  end)
+
+  it('works with Lua functions', function()
+    exec_lua [[
+      result = {}
+      vim.api.nvim_add_user_command('CommandWithLuaCallback', function(opts)
+        result = opts
+      end, {
+        nargs = "*",
+        bang = true,
+        count = 2,
+      })
+    ]]
+
+    eq({
+      args = "hello",
+      bang = false,
+      line1 = 1,
+      line2 = 1,
+      mods = "",
+      range = 0,
+      count = 2,
+      reg = "",
+    }, exec_lua [[
+      vim.api.nvim_command('CommandWithLuaCallback hello')
+      return result
+    ]])
+
+    eq({
+      args = "",
+      bang = true,
+      line1 = 10,
+      line2 = 10,
+      mods = "botright",
+      range = 1,
+      count = 10,
+      reg = "",
+    }, exec_lua [[
+      vim.api.nvim_command('botright 10CommandWithLuaCallback!')
+      return result
+    ]])
+
+    eq({
+      args = "",
+      bang = false,
+      line1 = 1,
+      line2 = 42,
+      mods = "",
+      range = 1,
+      count = 42,
+      reg = "",
+    }, exec_lua [[
+      vim.api.nvim_command('CommandWithLuaCallback 42')
+      return result
+    ]])
+  end)
+
+  it('can define buffer-local commands', function()
+    local bufnr = meths.create_buf(false, false)
+    bufmeths.add_user_command(bufnr, "Hello", "", {})
+    matches("Not an editor command: Hello", pcall_err(meths.command, "Hello"))
+    meths.set_current_buf(bufnr)
+    meths.command("Hello")
+    assert_alive()
+  end)
+
+  it('can use a Lua complete function', function()
+    exec_lua [[
+      vim.api.nvim_add_user_command('Test', '', {
+        nargs = "*",
+        complete = function(arg, cmdline, pos)
+          local options = {"aaa", "bbb", "ccc"}
+          local t = {}
+          for _, v in ipairs(options) do
+            if string.find(v, "^" .. arg) then
+              table.insert(t, v)
+            end
+          end
+          return t
+        end,
+      })
+    ]]
+
+    feed(':Test a<Tab>')
+    eq('Test aaa', funcs.getcmdline())
+    feed('<C-U>Test b<Tab>')
+    eq('Test bbb', funcs.getcmdline())
+  end)
+end)
+
+describe('nvim_del_user_command', function()
+  before_each(clear)
+
+  it('can delete global commands', function()
+    meths.add_user_command('Hello', 'echo "Hi"', {})
+    meths.command('Hello')
+    meths.del_user_command('Hello')
+    matches("Not an editor command: Hello", pcall_err(meths.command, "Hello"))
+  end)
+
+  it('can delete buffer-local commands', function()
+    bufmeths.add_user_command(0, 'Hello', 'echo "Hi"', {})
+    meths.command('Hello')
+    bufmeths.del_user_command(0, 'Hello')
+    matches("Not an editor command: Hello", pcall_err(meths.command, "Hello"))
+  end)
+end)