docs: --implicit-check-not option

Author: stanislaw

docs/06-options.rst

@@ -1,8 +1,8 @@
 Options
 =======
 
-strict-whitespace
------------------
+--strict-whitespace
+-------------------
 
 When the ``--strict-whitespace``  option is not provided, FileCheck ignores
 differences between spaces and tabs. Additionally multiple spaces are ignored
@@ -27,8 +27,8 @@ will pass on any of the following inputs:
 
 Adding ``--strict-whitespace`` disables this behavior.
 
-match-full-lines
-----------------
+--match-full-lines
+------------------
 
 When the ``--match-full-lines``  option is not provided, FileCheck does not
 match full lines.
@@ -76,8 +76,8 @@ Notice absence of spaces between ``CHECK:`` and the lines.
     $ echo $?
     0
 
-check-prefix
-------------
+--check-prefix
+--------------
 
 The ``--check-prefix`` option allows changing a default match keyword `CHECK`
 to an arbitrary keyword. This is useful when you want to test different behavior
@@ -92,3 +92,27 @@ in the same file:
 
 One usual case is testing of how a program behaves when it is run with or
 without a specific option.
+
+--implicit-check-not
+--------------------
+
+The ``--implicit-check-not`` option adds implicit `CHECK-NOT` check that works
+on every input line.
+
+FileCheck.py follows LLVM FileCheck in the following implementation details:
+
+- The implicit checks are substring-matched i.e. their are ``in`` checks, not
+``==`` checks.
+- The implicit checks are case sensitive, so ``error`` check will not match
+``ERROR`` in the input.
+- The implicit check has lower priority than the positive `CHECK*` checks,
+but it has higher priority than negative `CHECK-NOT` checks.
+- To provide multiple implicit checks, duplicate the argument
+``--implicit-check-not <your check>`` multiple times.
+
+Example
+~~~~~~~
+
+A very useful application of this option is to add implicit
+``--implicit-check-not error --implicit-check-not warning`` checks to make sure
+that the input never has lines that contain ``error`` or ``warning`` in them.