Add backslash line continuation to productionlist

Fixes sphinx-doc/sphinx#1027

CHANGES

@@ -44,6 +44,7 @@ Features added
   :py:meth:`.Sphinx.connect()`
 * #3077: Implement the scoping for :rst:dir:`productionlist` as indicated
   in the documentation.
+* #1027: Support backslash line continuation in :rst:dir:`productionlist`.
 
 Bugs fixed
 ----------

sphinx/domains/std.py

@@ -439,9 +439,16 @@ class ProductionList(SphinxDirective):
     def run(self) -> List[Node]:
         domain = cast(StandardDomain, self.env.get_domain('std'))
         node = addnodes.productionlist()  # type: Element
+        # The backslash handling is from ObjectDescription.get_signatures
+        nl_escape_re = re.compile(r'\\\n')
+        strip_backslash_re = re.compile(r'\\(.)')
+        lines = nl_escape_re.sub('', self.arguments[0]).split('\n')
+        # remove backslashes to support (dummy) escapes; helps Vim highlighting
+        lines = [strip_backslash_re.sub(r'\1', line.strip()) for line in lines]
+
         productionGroup = ""
         i = 0
-        for rule in self.arguments[0].split('\n'):
+        for rule in lines:
             if i == 0 and ':' not in rule:
                 productionGroup = rule.strip()
                 continue

tests/roots/test-productionlist/LineContinuation.rst

@@ -0,0 +1,6 @@
+LineContinuation
+================
+
+.. productionlist:: lineContinuation
+	A: B C D \
+	   E F G

tests/roots/test-productionlist/index.rst

@@ -6,6 +6,7 @@
 	Dup1
 	Dup2
 	firstLineRule
+	LineContinuation
 
 - A: :token:`A`
 - B: :token:`B`

tests/test_domain_std.py

@@ -370,3 +370,6 @@ def test_productionlist(app, status, warning):
         ('FirstLine', 'firstLineRule.html#grammar-token2-_firstline', 'FirstLine'),
         ('SecondLine', 'firstLineRule.html#grammar-token2-_secondline', 'SecondLine'),
     ]
+
+    text = (app.outdir / 'LineContinuation.html').text()
+    assert "A</strong> ::=  B C D    E F G" in text