[RFC] ":source" sources from current buffer if filename is omitted (#11444)

Fix https://github.com/neovim/neovim/issues/8722

test/functional/ex_cmds/source_spec.lua

@@ -0,0 +1,47 @@
+local helpers = require('test.functional.helpers')(after_each)
+local command = helpers.command
+local insert = helpers.insert
+local eq = helpers.eq
+local clear = helpers.clear
+local meths = helpers.meths
+local feed = helpers.feed
+local feed_command = helpers.feed_command
+
+describe(':source', function()
+  before_each(function()
+    clear()
+  end)
+
+  it('current buffer', function()
+    insert('let a = 2')
+    command('source')
+    eq('2', meths.exec('echo a', true))
+  end)
+
+  it('selection in current buffer', function()
+    insert(
+      'let a = 2\n'..
+      'let a = 3\n'..
+      'let a = 4\n')
+
+    -- Source the 2nd line only
+    feed('ggjV')
+    feed_command(':source')
+    eq('3', meths.exec('echo a', true))
+
+    -- Source from 2nd line to end of file
+    feed('ggjVG')
+    feed_command(':source')
+    eq('4', meths.exec('echo a', true))
+  end)
+
+  it('multiline heredoc command', function()
+    insert(
+      'lua << EOF\n'..
+      'y = 4\n'..
+      'EOF\n')
+
+    command('source')
+    eq('4', meths.exec('echo luaeval("y")', true))
+  end)
+end)