Enable math node rendering by default (without HTML builders)

Nowadays, math elements (inline and block level equations) are
integrated into reST spec by default.  But, in Sphinx, they are
not enabled by default.  For this reason, users have to enable
one of math extensions even if target builder supports math
elements directly.

This change starts to enable them by default.  As a first step,
this replaces math node and its structure by docutils based one.

tests/roots/test-ext-math-compat/conf.py

@@ -0,0 +1,14 @@
+# -*- coding: utf-8 -*-
+
+from sphinx.ext.mathbase import math
+
+master_doc = 'index'
+extensions = ['sphinx.ext.mathjax']
+
+
+def my_math_role(role, rawtext, text, lineno, inliner, options={}, content=[]):
+    return [math(latex='E = mc^2')], []
+
+
+def setup(app):
+    app.add_role('my_math', my_math_role)

tests/roots/test-ext-math-compat/index.rst

@@ -0,0 +1,25 @@
+Test Math
+=========
+
+inline
+------
+
+Inline: :math:`E=mc^2`
+Inline my math: :my_math:`:-)`
+
+block
+-----
+
+.. math:: a^2+b^2=c^2
+
+Second math
+
+.. math:: e^{i\pi}+1=0
+
+Multi math equations
+
+.. math::
+
+   S &= \pi r^2
+
+   V &= \frac{4}{3} \pi r^3

tests/test_ext_math.py

@@ -12,8 +12,12 @@
 import errno
 import re
 import subprocess
+import warnings
 
 import pytest
+from docutils import nodes
+
+from sphinx.testing.util import assert_node
 
 
 def has_binary(binary):
@@ -208,3 +212,21 @@ def test_imgmath_numfig_html(app, status, warning):
             'href="math.html#equation-foo">(1)</a> and '
             '<a class="reference internal" href="#equation-bar">(3)</a>.</p>')
     assert html in content
+
+
+@pytest.mark.sphinx('dummy', testroot='ext-math-compat')
+def test_math_compat(app, status, warning):
+    with warnings.catch_warnings(record=True):
+        app.builder.build_all()
+        doctree = app.env.get_and_resolve_doctree('index', app.builder)
+
+        assert_node(doctree,
+                    [nodes.document, nodes.section, (nodes.title,
+                                                     [nodes.section, (nodes.title,
+                                                                      nodes.paragraph)],
+                                                     nodes.section)])
+        assert_node(doctree[0][1][1],
+                    ('Inline: ',
+                     [nodes.math, "E=mc^2"],
+                     '\nInline my math: ',
+                     [nodes.math, "E = mc^2"]))