Have BorrowError & BorrowMutError derive Debug rather than implement Debug themselves

The implementation does not print anything differently from what the
derived implementation does, so we don't need it.

Author: nealsid

library/core/src/cell.rs

@@ -732,21 +732,10 @@ pub struct RefCell<T: ?Sized> {
 /// An error returned by [`RefCell::try_borrow`].
 #[stable(feature = "try_borrow", since = "1.13.0")]
 #[non_exhaustive]
+#[derive(Debug)]
 pub struct BorrowError {
     #[cfg(feature = "debug_refcell")]
-    location: &'static crate::panic::Location<'static>,
-}
-
-#[stable(feature = "try_borrow", since = "1.13.0")]
-impl Debug for BorrowError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let mut builder = f.debug_struct("BorrowError");
-
-        #[cfg(feature = "debug_refcell")]
-        builder.field("location", self.location);
-
-        builder.finish()
-    }
+    _location: &'static crate::panic::Location<'static>,
 }
 
 #[stable(feature = "try_borrow", since = "1.13.0")]
@@ -759,21 +748,10 @@ impl Display for BorrowError {
 /// An error returned by [`RefCell::try_borrow_mut`].
 #[stable(feature = "try_borrow", since = "1.13.0")]
 #[non_exhaustive]
+#[derive(Debug)]
 pub struct BorrowMutError {
     #[cfg(feature = "debug_refcell")]
-    location: &'static crate::panic::Location<'static>,
-}
-
-#[stable(feature = "try_borrow", since = "1.13.0")]
-impl Debug for BorrowMutError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let mut builder = f.debug_struct("BorrowMutError");
-
-        #[cfg(feature = "debug_refcell")]
-        builder.field("location", self.location);
-
-        builder.finish()
-    }
+    _location: &'static crate::panic::Location<'static>,
 }
 
 #[stable(feature = "try_borrow", since = "1.13.0")]
@@ -1036,7 +1014,7 @@ impl<T: ?Sized> RefCell<T> {
                 // If a borrow occurred, then we must already have an outstanding borrow,
                 // so `borrowed_at` will be `Some`
                 #[cfg(feature = "debug_refcell")]
-                location: self.borrowed_at.get().unwrap(),
+                _location: self.borrowed_at.get().unwrap(),
             }),
         }
     }
@@ -1125,7 +1103,7 @@ impl<T: ?Sized> RefCell<T> {
                 // If a borrow occurred, then we must already have an outstanding borrow,
                 // so `borrowed_at` will be `Some`
                 #[cfg(feature = "debug_refcell")]
-                location: self.borrowed_at.get().unwrap(),
+                _location: self.borrowed_at.get().unwrap(),
             }),
         }
     }
@@ -1255,7 +1233,7 @@ impl<T: ?Sized> RefCell<T> {
                 // If a borrow occurred, then we must already have an outstanding borrow,
                 // so `borrowed_at` will be `Some`
                 #[cfg(feature = "debug_refcell")]
-                location: self.borrowed_at.get().unwrap(),
+                _location: self.borrowed_at.get().unwrap(),
             })
         }
     }