Revert "std: add auto traits to TAIT bound"

This reverts commit 29e3f8b.

Author: joboet

library/std/src/backtrace.rs

@@ -141,11 +141,6 @@ struct Capture {
     frames: Vec<BacktraceFrame>,
 }
 
-fn _assert_send_sync() {
-    fn _assert<T: Send + Sync>() {}
-    _assert::<Backtrace>();
-}
-
 /// A single frame of a backtrace.
 #[unstable(feature = "backtrace_frames", issue = "79676")]
 pub struct BacktraceFrame {
@@ -428,7 +423,7 @@ impl fmt::Display for Backtrace {
     }
 }
 
-type LazyResolve = impl (FnOnce() -> Capture) + Send + Sync + UnwindSafe;
+type LazyResolve = impl FnOnce() -> Capture + UnwindSafe;
 
 fn lazy_resolve(mut capture: Capture) -> LazyResolve {
     move || {

library/std/tests/backtrace.rs

@@ -0,0 +1,10 @@
+use std::backtrace::Backtrace;
+
+// Unfortunately, this cannot be a unit test because that causes problems
+// with type-alias-impl-trait (the assert counts as a defining use).
+#[test]
+fn assert_send_sync() {
+    fn assert<T: Send + Sync>() {}
+
+    assert::<Backtrace>();
+}