Add test for associated items in rustdoc JSON

src/test/rustdoc-json/assoc_items.rs

+#![no_std]
+
+// @has assoc_items.json
+
+pub struct Simple;
+
+impl Simple {
+    // @has - "$.index[*][?(@.name=='CONSTANT')].kind" \"assoc_const\"
+    pub const CONSTANT: usize = 0;
+}
+
+pub trait EasyToImpl {
+    // @has - "$.index[*][?(@.name=='ToDeclare')].kind" \"assoc_type\"
+    // @has - "$.index[*][?(@.name=='ToDeclare')].inner.default" null
+    type ToDeclare;
+    // @has - "$.index[*][?(@.name=='AN_ATTRIBUTE')].kind" \"assoc_const\"
+    // @has - "$.index[*][?(@.name=='AN_ATTRIBUTE')].inner.default" null
+    const AN_ATTRIBUTE: usize;
+}
+
+impl EasyToImpl for Simple {
+    // @has - "$.index[*][?(@.name=='ToDeclare')].inner.default.kind" \"primitive\"
+    // @has - "$.index[*][?(@.name=='ToDeclare')].inner.default.inner" \"usize\"
+    type ToDeclare = usize;
+    // @has - "$.index[*][?(@.name=='AN_ATTRIBUTE')].inner.type.kind" \"primitive\"
+    // @has - "$.index[*][?(@.name=='AN_ATTRIBUTE')].inner.type.inner" \"usize\"
+    // @has - "$.index[*][?(@.name=='AN_ATTRIBUTE')].inner.default" \"12\"
+    const AN_ATTRIBUTE: usize = 12;
+}