patch 8.2.2529: Vim9: Not possible to use legacy and Vim9 script in one file

Problem:    Vim9: Not possible to use legacy and Vim9 script in one file.
Solution:   Vim9: allow for "if false" before :vim9script. (closes #7851)

runtime/doc/vim9.txt

@@ -1051,9 +1051,9 @@ that you don't do that.
 Namespace ~
 							*vim9-namespace*
 To recognize a file that can be imported the `vim9script` statement must
-appear as the first statement in the file.  It tells Vim to interpret the
-script in its own namespace, instead of the global namespace.  If a file
-starts with: >
+appear as the first statement in the file (see |vim9-mix| for an exception).
+It tells Vim to interpret the script in its own namespace, instead of the
+global namespace.  If a file starts with: >
 	vim9script
 	var myvar = 'yes'
 Then "myvar" will only exist in this file.  While without `vim9script` it would
@@ -1073,6 +1073,27 @@ Vim default value, like with: >
 One of the effects is that |line-continuation| is always enabled.
 The original value of 'cpoptions' is restored at the end of the script.
 
+							*vim9-mix*
+There is one way to use both legacy and Vim9 syntax in one script file: >
+	" comments may go here
+	if !has('vim9script')
+	   " legacy script commands go here
+	   finish
+	endif
+	vim9script
+	# Vim9 script commands go here
+This allows for writing a script that takes advantage of the Vim9 script
+syntax if possible, but will also work on an Vim version without it.
+
+This can only work in two ways:
+1. The "if" statement evaluates to false, the commands up to `endif` are
+   skipped and `vim9script` is then the first command actually executed.
+2. The "if" statement evaluates to true, the commands up to `endif` are
+   executed and `finish` bails out before reaching `vim9script`.
+
+TODO: The "vim9script" feature does not exist yet, it will only be added once
+the Vim9 script syntax has been fully implemented.
+
 
 Export ~
 							*:export* *:exp*

src/ex_docmd.c

@@ -2595,8 +2595,12 @@ do_one_cmd(
 
 #ifdef FEAT_EVAL
     // Set flag that any command was executed, used by ex_vim9script().
+    // Not if this was a command that wasn't executed or :endif.
     if (getline_equal(ea.getline, ea.cookie, getsourceline)
-						    && current_sctx.sc_sid > 0)
+	    && current_sctx.sc_sid > 0
+	    && ea.cmdidx != CMD_endif
+	    && (cstack->cs_idx < 0
+		    || (cstack->cs_flags[cstack->cs_idx] & CSF_ACTIVE)))
 	SCRIPT_ITEM(current_sctx.sc_sid)->sn_state = SN_STATE_HAD_COMMAND;
 
     /*

src/testdir/test_vim9_script.vim

@@ -1262,6 +1262,27 @@ def Test_use_import_in_mapping()
   nunmap <F3>
 enddef
 
+def Test_vim9script_mix()
+  var lines =<< trim END
+    if has(g:feature)
+      " legacy script
+      let g:legacy = 1
+      finish
+    endif
+    vim9script
+    g:legacy = 0
+  END
+  g:feature = 'eval'
+  g:legacy = -1
+  CheckScriptSuccess(lines)
+  assert_equal(1, g:legacy)
+
+  g:feature = 'noteval'
+  g:legacy = -1
+  CheckScriptSuccess(lines)
+  assert_equal(0, g:legacy)
+enddef
+
 def Test_vim9script_fails()
   CheckScriptFailure(['scriptversion 2', 'vim9script'], 'E1039:')
   CheckScriptFailure(['vim9script', 'scriptversion 2'], 'E1040:')

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2529,
 /**/
     2528,
 /**/