Minor tweaks to refcell logging

- Fix the logging when debug_refcell is enabled so the location is output as a string rather than a sequence of bytes
- Avoid logging an empty struct when debug_refcell is disabled
- Make the panic error message consistent with the compiler borrow-checking error messages

Author: nealsid

library/core/src/cell.rs

@@ -743,7 +743,7 @@ impl Debug for BorrowError {
         let mut builder = f.debug_struct("BorrowError");
 
         #[cfg(feature = "debug_refcell")]
-        builder.field("location", self.location);
+        builder.field("location", &format_args!("{}", self.location));
 
         builder.finish()
     }
@@ -770,7 +770,7 @@ impl Debug for BorrowMutError {
         let mut builder = f.debug_struct("BorrowMutError");
 
         #[cfg(feature = "debug_refcell")]
-        builder.field("location", self.location);
+        builder.field("location", &format_args!("{}", self.location));
 
         builder.finish()
     }
@@ -787,16 +787,24 @@ impl Display for BorrowMutError {
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
 #[track_caller]
 #[cold]
-fn panic_already_borrowed(err: BorrowMutError) -> ! {
-    panic!("already borrowed: {:?}", err)
+fn panic_already_borrowed(_err: BorrowMutError) -> ! {
+    if cfg!(feature = "debug_refcell") {
+        panic!("cannot borrow as mutable because it is also borrowed as immutable here {:?}", _err);
+    } else {
+        panic!("cannot borrow as mutable because it is also borrowed as immutable");
+    }
 }
 
 // This ensures the panicking code is outlined from `borrow` for `RefCell`.
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
 #[track_caller]
 #[cold]
-fn panic_already_mutably_borrowed(err: BorrowError) -> ! {
-    panic!("already mutably borrowed: {:?}", err)
+fn panic_already_mutably_borrowed(_err: BorrowError) -> ! {
+    if cfg!(feature = "debug_refcell") {
+        panic!("cannot borrow as immutable because it is also borrowed as mutable here {:?}", _err);
+    } else {
+        panic!("cannot borrow as immutable because it is also borrowed as mutable");
+    }
 }
 
 // Positive values represent the number of `Ref` active. Negative values