Add information to Display implementation for BorrowError/BorrowMutError

This change adds the location field of BorrowError/BorrowMutError to
the the output when it is present and also rewords the error message.
The change also uses the Display trait for outputting the error
message instead of Debug.

Author: nealsid

library/core/Cargo.toml

@@ -16,7 +16,6 @@ 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

@@ -12,7 +12,7 @@
 //!
 //! Shareable mutable containers exist to permit mutability in a controlled manner, even in the
 //! presence of aliasing. [`Cell<T>`], [`RefCell<T>`], and [`OnceCell<T>`] allow doing this in
-//! a single-threaded way—they do not implement [`Sync`]. (If you need to do aliasing and
+//! a single-threaded way-they do not implement [`Sync`]. (If you need to do aliasing and
 //! mutation among multiple threads, [`Mutex<T>`], [`RwLock<T>`], [`OnceLock<T>`] or [`atomic`]
 //! types are the correct data structures to do so).
 //!
@@ -741,7 +741,17 @@ pub struct BorrowError {
 #[stable(feature = "try_borrow", since = "1.13.0")]
 impl Display for BorrowError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        Display::fmt("already mutably borrowed", f)
+        #[cfg(feature = "debug_refcell")]
+        let res = write!(
+            f,
+            "RefCell already mutably borrowed, a previous borrow was at this location: {}",
+            self._location
+        );
+
+        #[cfg(not(feature = "debug_refcell"))]
+        let res = Display::fmt("RefCell already mutably borrowed", f);
+
+        res
     }
 }
 
@@ -757,7 +767,17 @@ pub struct BorrowMutError {
 #[stable(feature = "try_borrow", since = "1.13.0")]
 impl Display for BorrowMutError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        Display::fmt("already borrowed", f)
+        #[cfg(feature = "debug_refcell")]
+        let res = write!(
+            f,
+            "RefCell already borrowed, a previous borrow was at this location: {}",
+            self._location
+        );
+
+        #[cfg(not(feature = "debug_refcell"))]
+        let res = Display::fmt("RefCell already borrowed", f);
+
+        res
     }
 }
 
@@ -766,23 +786,15 @@ impl Display for BorrowMutError {
 #[track_caller]
 #[cold]
 fn panic_already_borrowed(err: BorrowMutError) -> ! {
-    if cfg!(feature = "debug_refcell") {
-        panic!("already borrowed here: {:?}", err);
-    } else {
-        panic!("already borrowed");
-    }
+    panic!("{}", err)
 }
 
 // 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) -> ! {
-    if cfg!(feature = "debug_refcell") {
-        panic!("already mutably borrowed here: {:?}", err);
-    } else {
-        panic!("already mutably borrowed");
-    }
+    panic!("{}", err)
 }
 
 // Positive values represent the number of `Ref` active. Negative values