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/Cargo.toml

@@ -16,6 +16,7 @@ test = false
 bench = false
 
 [features]
+default = ["debug_refcell"]
 # Make panics and failed asserts immediately abort without formatting any message
 panic_immediate_abort = []
 # Choose algorithms that are optimized for binary size instead of runtime performance

library/core/src/cell.rs

@@ -767,12 +767,13 @@ pub struct BorrowMutError {
 #[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");
+        write!(f, "{}", self.location)
+        // let mut builder = f.debug_struct("BorrowMutError");
 
-        #[cfg(feature = "debug_refcell")]
-        builder.field("location", self.location);
+        // #[cfg(feature = "debug_refcell")]
+        // builder.field("location", self.location);
 
-        builder.finish()
+        // builder.finish()
     }
 }
 
@@ -788,15 +789,23 @@ impl Display for BorrowMutError {
 #[track_caller]
 #[cold]
 fn panic_already_borrowed(err: BorrowMutError) -> ! {
-    panic!("already borrowed: {:?}", err)
+    if cfg!(feature = "debug_refcell") {
+        panic!("already borrowed here: {:?}", err);
+    } else {
+        panic!("already borrowed");
+    }
 }
 
 // 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)
+    if cfg!(feature = "debug_refcell") {
+        panic!("already mutably borrowed here: {:?}", err);
+    } else {
+        panic!("already mutably borrowed");
+    }
 }
 
 // Positive values represent the number of `Ref` active. Negative values