Add test for invalid schemars attributes

schemars/tests/ui.rs

@@ -0,0 +1,5 @@
+#[test]
+fn ui() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/ui/*.rs");
+}

schemars/tests/ui/invalid_attrs.rs

@@ -0,0 +1,11 @@
+use schemars::JsonSchema;
+
+#[derive(JsonSchema)]
+#[serde(default = 0, foo, deny_unknown_fields, deny_unknown_fields)]
+pub struct Struct1;
+
+#[derive(JsonSchema)]
+#[schemars(default = 0, foo, deny_unknown_fields, deny_unknown_fields)]
+pub struct Struct2;
+
+fn main() {}

schemars/tests/ui/invalid_attrs.stderr

@@ -0,0 +1,29 @@
+error: expected serde default attribute to be a string: `default = "..."`
+ --> $DIR/invalid_attrs.rs:4:19
+  |
+4 | #[serde(default = 0, foo, deny_unknown_fields, deny_unknown_fields)]
+  |                   ^
+
+error: duplicate serde attribute `deny_unknown_fields`
+ --> $DIR/invalid_attrs.rs:4:48
+  |
+4 | #[serde(default = 0, foo, deny_unknown_fields, deny_unknown_fields)]
+  |                                                ^^^^^^^^^^^^^^^^^^^
+
+error: expected serde default attribute to be a string: `default = "..."`
+ --> $DIR/invalid_attrs.rs:8:22
+  |
+8 | #[schemars(default = 0, foo, deny_unknown_fields, deny_unknown_fields)]
+  |                      ^
+
+error: duplicate serde attribute `deny_unknown_fields`
+ --> $DIR/invalid_attrs.rs:8:51
+  |
+8 | #[schemars(default = 0, foo, deny_unknown_fields, deny_unknown_fields)]
+  |                                                   ^^^^^^^^^^^^^^^^^^^
+
+error: unknown schemars container attribute `foo`
+ --> $DIR/invalid_attrs.rs:8:25
+  |
+8 | #[schemars(default = 0, foo, deny_unknown_fields, deny_unknown_fields)]
+  |                         ^^^