edit.c: CTRL-SPC: Insert previously-inserted text. #6090

Default Vim behavior of i_CTRL-<Space> is to insert the last-inserted
text and exit insert mode. :help i_CTRL-@

Before this commit that did not happen because insert_handle_key()
checks for NUL instead of checking for ' ' with a CTRL `mod_mask`.

I'm leaving the check for NUL despite the fact that at the moment that
key is never seen when using the terminal UI (not for C-Space, nor C-@).
This is because I assume it's still allowed for other front-ends to pass
NUL, but at the moment the terminal UI isn't.

src/nvim/edit.c

@@ -844,6 +844,11 @@ static int insert_handle_key(InsertState *s)
     return 0;  // exit insert mode
 
 
+  case ' ':
+    if (mod_mask != 4) {
+      goto normalchar;
+    }
+  // FALLTHROUGH
   case K_ZERO:        // Insert the previously inserted text.
   case NUL:
   case Ctrl_A:

test/functional/insert/last_inserted_spec.lua

@@ -0,0 +1,22 @@
+local helpers = require('test.functional.helpers')(after_each)
+local clear, feed, insert = helpers.clear, helpers.feed, helpers.insert
+local expect = helpers.expect
+
+clear()
+
+describe('insert-mode', function()
+  it('CTRL-@ inserts last-inserted text, leaves insert-mode', function()
+    insert('hello')
+    feed('i<C-@>x')
+    expect('hellhello')
+  end)
+  -- C-Space is the same as C-@
+  it('CTRL-SPC inserts last-inserted text, leaves insert-mode', function()
+    feed('i<C-Space>x')
+    expect('hellhellhello')
+  end)
+  it('CTRL-A inserts last inserted text', function()
+    feed('i<C-A>x')
+    expect('hellhellhellhelloxo')
+  end)
+end)