feat(lua): allow `:=expr` as a shorter version of `:lua =expr`

existing behavior of
  :=
and
  :[range]=

are unchanged. `|` is still allowed with this usage.

However,
   :=p
and similar are changed in a way which could be construed as a breaking
change. Allowing |ex-flags| for := in the first place was a mistake as
any form of := DOES NOT MOVE THE CURSOR. So it would print one line number
and then print a completely different line contents after that.

2 files changed, 15 insertions, 5 deletions

diff --git a/test/functional/lua/commands_spec.lua b/test/functional/lua/commands_spec.lua
index 943095c51e..5bb9e0281b 100644
--- a/test/functional/lua/commands_spec.lua
+++ b/test/functional/lua/commands_spec.lua
@@ -146,6 +146,7 @@ describe(':lua command', function()
   it('prints result of =expr', function()
     exec_lua("x = 5")
     eq("5", exec_capture(':lua =x'))
+    eq("5", exec_capture('=x'))
     exec_lua("function x() return 'hello' end")
     eq('hello', exec_capture(':lua = x()'))
     exec_lua("x = {a = 1, b = 2}")
@@ -165,7 +166,7 @@ describe(':lua command', function()
       false
       nil
       Error message]],
-    exec_capture(':lua  =x(false)'))
+    exec_capture('=x(false)'))
   end)
 end)
 
diff --git a/test/old/testdir/test_ex_equal.vim b/test/old/testdir/test_ex_equal.vim
index 05ad276836..32e23704eb 100644
--- a/test/old/testdir/test_ex_equal.vim
+++ b/test/old/testdir/test_ex_equal.vim
@@ -7,6 +7,19 @@ func Test_ex_equal()
   let a = execute('=')
   call assert_equal("\n2", a)
 
+  let a = execute('.=')
+  call assert_equal("\n1", a)
+
+  call assert_fails('3=', 'E16:')
+  bwipe!
+endfunc
+
+func Test_ex_equal_arg()
+  throw 'skipped: Nvim evaluates lua with := [arg]'
+
+  new
+  call setline(1, ["foo\tbar", "bar\tfoo"])
+
   let a = execute('=#')
   call assert_equal("\n2\n  1 foo     bar", a)
 
@@ -22,10 +35,6 @@ func Test_ex_equal()
   let a = execute('=p#')
   call assert_equal("\n2\n  1 foo     bar", a)
 
-  let a = execute('.=')
-  call assert_equal("\n1", a)
-
-  call assert_fails('3=', 'E16:')
   call assert_fails('=x', 'E488:')
 
   bwipe!