Expose myst_substitions to RST

While this doesn't fix the fact that .. replace:: does not work within
eval-rst, it does make it possible to perform substitutions within
eval-rst using the same substitutions available to Jinja substitutions.

Related: executablebooks#680

Signed-off-by: Chance Zibolski <chance.zibolski@gmail.com>

myst_parser/mdit_to_docutils/base.py

@@ -280,6 +280,17 @@ def _render_finalise(self) -> None:
                     substitution_node, f"wordcount-{key}"
                 )
 
+        for key, value in self.md_config.substitutions.items():
+            if value is None:
+                continue
+
+            substitution_node = nodes.substitution_definition(
+                str(value), nodes.Text(str(value))
+            )
+            substitution_node.source = self.document["source"]
+            substitution_node["names"].append(key)
+            self.document.note_substitution_def(substitution_node, key)
+
     def nested_render_text(
         self,
         text: str,

tests/test_sphinx/sourcedirs/substitutions/index.md

@@ -52,6 +52,12 @@ This will not process the substitution
 {{ text_with_nest }}
 ```
 
+Test substitutions are processed within eval-rst
+
+```{eval-rst}
+a |text| b
+```
+
 Using env and filters:
 
 {{ env.docname | upper }}

tests/test_sphinx/test_sphinx_builds/test_substitutions.html

@@ -62,6 +62,12 @@
 </pre>
     </div>
    </div>
+   <p>
+    Test substitutions are processed within eval-rst
+   </p>
+   <p>
+    a - text b
+   </p>
    <p>
     Using env and filters:
    </p>

tests/test_sphinx/test_sphinx_builds/test_substitutions.xml

@@ -47,6 +47,12 @@
         This will not process the substitution
     <literal_block language="python" xml:space="preserve">
         {{ text_with_nest }}
+    <paragraph>
+        Test substitutions are processed within eval-rst
+    <paragraph>
+        a 
+        - text
+         b
     <paragraph>
         Using env and filters:
     <paragraph>