Add ui test of sync bound

tests/ui/non-sync.rs

@@ -0,0 +1,21 @@
+use std::rc::Rc;
+use std::thread;
+
+struct Thing(Rc<i32>);
+
+inventory::collect!(Thing);
+
+fn clone_all() {
+    for thing in inventory::iter::<Thing> {
+        let _ = Rc::clone(&thing.0);
+    }
+}
+
+fn main() {
+    // It would be bad if this were allowed. These threads would race on the
+    // nonatomic reference counts.
+    let thread1 = thread::spawn(clone_all);
+    let thread2 = thread::spawn(clone_all);
+    thread1.join().unwrap();
+    thread2.join().unwrap();
+}

tests/ui/non-sync.stderr

@@ -0,0 +1,18 @@
+error[E0277]: `Rc<i32>` cannot be shared between threads safely
+   --> tests/ui/non-sync.rs:6:1
+    |
+6   | inventory::collect!(Thing);
+    | ^^^^^^^^^^^^^^^^^^^^^^^^^^ `Rc<i32>` cannot be shared between threads safely
+    |
+    = help: within `Thing`, the trait `Sync` is not implemented for `Rc<i32>`
+note: required because it appears within the type `Thing`
+   --> tests/ui/non-sync.rs:4:8
+    |
+4   | struct Thing(Rc<i32>);
+    |        ^^^^^
+note: required by a bound in `Collect`
+   --> src/lib.rs
+    |
+    | pub trait Collect: Sync + Sized + 'static {
+    |                    ^^^^ required by this bound in `Collect`
+    = note: this error originates in the macro `inventory::collect` (in Nightly builds, run with -Z macro-backtrace for more info)