cylc_lang: permit preprocessing in intercycle offsets

cylc/sphinx_ext/cylc_lang/__init__.py

@@ -30,12 +30,20 @@
 
    .. code-block:: cylc
 
+      # Jinja2
+
+      {% set final_cycle_point = final_cycle_point | default('') %}
+      {% set duration = duration | default('P1Y') %}
+
       [scheduling]
           initial cycle point = 2000
+          final cycle point = {{ final_cycle_point }}
           [[graph]]
               P1Y = """
                   @wall_clock => foo? => bar
                   (foo? & bar) => pub
+
+                  foo[-{{ duration }}+P1D] => foo
               """
 
 .. note::

cylc/sphinx_ext/cylc_lang/lexers.py

@@ -200,9 +200,35 @@ class CylcLexer(RegexLexer):
             include('integer-duration'),  # matches a subset of iso8601
             include('iso8601-duration'),
             (r'[\^\$]', INTERCYCLE_OFFSET_TOKEN),
+            (
+                # anything that contains Jinja2 syntax
+                r'(?=[^\]]*{{)',
+                INTERCYCLE_OFFSET_TOKEN,
+                'preproc-intercycle-offset',
+            ),
+            (
+                # anything that contains EmPy syntax
+                r'(?=[^\]]*@)',
+                INTERCYCLE_OFFSET_TOKEN,
+                'preproc-intercycle-offset',
+            ),
             (r'\]', Text, '#pop')
         ],
 
+        # Task inter-cycle offset with preprocessing: foo[-{{duration}}+P1D]
+        # Note: This is done in its own section so that we don't bypass the
+        # validation that has been implemented for explicit offsets
+        'preproc-intercycle-offset': [
+            # permit pre-processing
+            include('preproc'),
+            # interpret all other text as part of the offset
+            # (we can't perform validation when preprocessing is involved)
+            (r'[^\]]', INTERCYCLE_OFFSET_TOKEN),
+            # the first "]" (outside of preprocessing) marks the end of the
+            # offset
+            (r'(?=\])', Text, '#pop')
+        ],
+
         # generic Cylc cycle point:  2000
         'cycle-point': [
             # validating the cycle point as a regex [effectively] requires