Add regression test for #86208

This test does not test the output as well as I would like, but I think
I am limited by htmldocck. I would really just like to strip all the
HTML tags from the output for the sake of the different `@has` checks,
but that doesn't seem to be currently possible.

src/test/rustdoc/macro_rules-matchers.rs

@@ -0,0 +1,42 @@
+// This is a regression test for issue #86208.
+// It is also a general test of macro_rules! display.
+
+#![crate_name = "foo"]
+
+// @has 'foo/macro.todo.html'
+// @has - '//span[@class="macro"]' 'macro_rules!'
+// @has - '//span[@class="ident"]' 'todo'
+// Note: count = 2 * ('=' + '>') + '+' = 2 * (1 + 1) + 1 = 5
+// @count - '//span[@class="op"]' 5
+
+// @has - '{ ()'
+// @has - '//span[@class="op"]' '='
+// @has - '//span[@class="op"]' '>'
+// @has - '{ ... };'
+
+// @has - '($('
+// @has - '//span[@class="macro-nonterminal"]' '$'
+// @has - '//span[@class="macro-nonterminal"]' 'arg'
+// @has - ':'
+// @has - '//span[@class="ident"]' 'tt'
+// @has - '//span[@class="op"]' '+'
+// @has - ')'
+pub use std::todo;
+
+mod mod1 {
+    // @has 'foo/macro.macro1.html'
+    // @has - 'macro_rules!'
+    // @has - 'macro1'
+    // @has - '{ ()'
+    // @has - '($('
+    // @has - 'arg'
+    // @has - 'expr'
+    // @has - ','
+    // @has - '+'
+    // @has - ')'
+    #[macro_export]
+    macro_rules! macro1 {
+        () => {};
+        ($($arg:expr),+) => { stringify!($($arg),+) };
+    }
+}