Add #[schemars(bound = ...)] attribute

Based on https://github.com/GREsau/schemars/pull/162

Co-authored-by: teozkr <teo@nullable.se>

schemars/tests/bound.rs

@@ -0,0 +1,29 @@
+mod util;
+use std::marker::PhantomData;
+
+use schemars::JsonSchema;
+use util::*;
+
+struct MyIterator;
+
+impl Iterator for MyIterator {
+    type Item = String;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        unimplemented!()
+    }
+}
+
+// The default trait bounds would require T to implement JsonSchema,
+// which MyIterator does not.
+#[derive(JsonSchema)]
+#[schemars(bound = "T::Item: JsonSchema", rename = "MyContainer")]
+pub struct MyContainer<T> where T: Iterator {
+    pub associated: T::Item,
+    pub generic: PhantomData<T>,
+}
+
+#[test]
+fn manual_bound_set() -> TestResult {
+    test_default_generated_schema::<MyContainer<MyIterator>>("bound")
+}

schemars/tests/expected/bound.json

@@ -0,0 +1,17 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "MyContainer",
+  "type": "object",
+  "required": [
+    "associated",
+    "generic"
+  ],
+  "properties": {
+    "associated": {
+      "type": "string"
+    },
+    "generic": {
+      "type": "null"
+    }
+  }
+}