fix UI test, simplify error message

Author: joboet

library/core/src/fmt/mod.rs

@@ -459,6 +459,12 @@ impl<'a> Arguments<'a> {
     }
 }
 
+// Manually implementing these results in better error messages.
+#[stable(feature = "rust1", since = "1.0.0")]
+impl !Send for Arguments<'_> {}
+#[stable(feature = "rust1", since = "1.0.0")]
+impl !Sync for Arguments<'_> {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Debug for Arguments<'_> {
     fn fmt(&self, fmt: &mut Formatter<'_>) -> Result {

tests/ui/fmt/send-sync.stderr

@@ -1,45 +1,27 @@
-error[E0277]: `core::fmt::rt::Opaque` cannot be shared between threads safely
+error[E0277]: `Arguments<'_>` cannot be sent between threads safely
   --> $DIR/send-sync.rs:8:10
    |
 LL |     send(format_args!("{:?}", c));
-   |     ---- ^^^^^^^^^^^^^^^^^^^^^^^ `core::fmt::rt::Opaque` cannot be shared between threads safely
+   |     ---- ^^^^^^^^^^^^^^^^^^^^^^^ `Arguments<'_>` cannot be sent between threads safely
    |     |
    |     required by a bound introduced by this call
    |
-   = help: within `[core::fmt::rt::Argument<'_>]`, the trait `Sync` is not implemented for `core::fmt::rt::Opaque`, which is required by `Arguments<'_>: Send`
-   = note: required because it appears within the type `&core::fmt::rt::Opaque`
-note: required because it appears within the type `core::fmt::rt::ArgumentType<'_>`
-  --> $SRC_DIR/core/src/fmt/rt.rs:LL:COL
-note: required because it appears within the type `core::fmt::rt::Argument<'_>`
-  --> $SRC_DIR/core/src/fmt/rt.rs:LL:COL
-   = note: required because it appears within the type `[core::fmt::rt::Argument<'_>]`
-   = note: required for `&[core::fmt::rt::Argument<'_>]` to implement `Send`
-note: required because it appears within the type `Arguments<'_>`
-  --> $SRC_DIR/core/src/fmt/mod.rs:LL:COL
+   = help: the trait `Send` is not implemented for `Arguments<'_>`
 note: required by a bound in `send`
   --> $DIR/send-sync.rs:1:12
    |
 LL | fn send<T: Send>(_: T) {}
    |            ^^^^ required by this bound in `send`
 
-error[E0277]: `core::fmt::rt::Opaque` cannot be shared between threads safely
+error[E0277]: `Arguments<'_>` cannot be shared between threads safely
   --> $DIR/send-sync.rs:9:10
    |
 LL |     sync(format_args!("{:?}", c));
-   |     ---- ^^^^^^^^^^^^^^^^^^^^^^^ `core::fmt::rt::Opaque` cannot be shared between threads safely
+   |     ---- ^^^^^^^^^^^^^^^^^^^^^^^ `Arguments<'_>` cannot be shared between threads safely
    |     |
    |     required by a bound introduced by this call
    |
-   = help: within `Arguments<'_>`, the trait `Sync` is not implemented for `core::fmt::rt::Opaque`, which is required by `Arguments<'_>: Sync`
-   = note: required because it appears within the type `&core::fmt::rt::Opaque`
-note: required because it appears within the type `core::fmt::rt::ArgumentType<'_>`
-  --> $SRC_DIR/core/src/fmt/rt.rs:LL:COL
-note: required because it appears within the type `core::fmt::rt::Argument<'_>`
-  --> $SRC_DIR/core/src/fmt/rt.rs:LL:COL
-   = note: required because it appears within the type `[core::fmt::rt::Argument<'_>]`
-   = note: required because it appears within the type `&[core::fmt::rt::Argument<'_>]`
-note: required because it appears within the type `Arguments<'_>`
-  --> $SRC_DIR/core/src/fmt/mod.rs:LL:COL
+   = help: the trait `Sync` is not implemented for `Arguments<'_>`
 note: required by a bound in `sync`
   --> $DIR/send-sync.rs:2:12
    |