Add debug-refcell feature to libcore

See https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Attaching.20backtraces.20to.20RefCell/near/226273614
for some background discussion

This PR adds a new off-by-default feature `debug-refcell` to libcore.
When enabled, this feature stores additional debugging information in
`RefCell`. This information is included in the panic message when
`borrow()` or `borrow_mut()` panics, to make it easier to track down the
source of the issue.

Currently, we store the caller location for the earliest active borrow.
This has a number of advantages:
* There is only a constant amount of overhead per `RefCell`
* We don't need any heap memory, so it can easily be implemented in core
* Since we are storing the *earliest* active borrow, we don't need any
  extra logic in the `Drop` implementation for `Ref` and `RefMut`

Limitations:
* We only store the caller location, not a full `Backtrace`. Until
  we get support for `Backtrace` in libcore, this is the best tha we can
do.
* The captured location is only displayed when `borrow()` or
  `borrow_mut()` panics. If a crate calls `try_borrow().unwrap()`
  or `try_borrow_mut().unwrap()`, this extra information will be lost.

To make testing easier, I've enabled the `debug-refcell` feature by
default. I'm not sure how to write a test for this feature - we would
need to rebuild core from the test framework, and create a separate
sysroot.

Since this feature will be off-by-default, users will need to use
`xargo` or `cargo -Z build-std` to enable this feature. For users using
a prebuilt standard library, this feature will be disabled with zero
overhead.

I've created a simple test program:

```rust
use std::cell::RefCell;

fn main() {
    let _ = std::panic::catch_unwind(|| {
        let val = RefCell::new(true);
        let _first = val.borrow();
        let _second = val.borrow();
        let _third = val.borrow_mut();
    });

    let _ = std::panic::catch_unwind(|| {
        let val  = RefCell::new(true);
        let first = val.borrow_mut();
        drop(first);

        let _second = val.borrow_mut();

        let _thid = val.borrow();
    });
}
```

which produces the following output:

```
thread 'main' panicked at 'already borrowed: BorrowMutError { location: Location { file: "refcell_test.rs", line: 6, col: 26 } }', refcell_test.rs:8:26
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'main' panicked at 'already mutably borrowed: BorrowError { location: Location { file: "refcell_test.rs", line: 16, col: 27 } }', refcell_test.rs:18:25
```

library/core/Cargo.toml

@@ -25,3 +25,6 @@ rand = "0.7"
 [features]
 # Make panics and failed asserts immediately abort without formatting any message
 panic_immediate_abort = []
+# Make `RefCell` store additional debugging information, which is printed out when
+# a borrow error occurs
+debug_refcell = []

library/core/src/cell.rs

@@ -575,19 +575,32 @@ impl<T> Cell<[T]> {
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct RefCell<T: ?Sized> {
     borrow: Cell<BorrowFlag>,
+    // Stores the location of the earliest currently active borrow.
+    // This gets updated whenver we go from having zero borrows
+    // to having a single borrow. When a borrow occurs, this gets included
+    // in the generated `BorroeError/`BorrowMutError`
+    #[cfg(feature = "debug_refcell")]
+    borrowed_at: Cell<Option<&'static crate::panic::Location<'static>>>,
     value: UnsafeCell<T>,
 }
 
 /// An error returned by [`RefCell::try_borrow`].
 #[stable(feature = "try_borrow", since = "1.13.0")]
 pub struct BorrowError {
     _private: (),
+    #[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 {
-        f.debug_struct("BorrowError").finish()
+        let mut builder = f.debug_struct("BorrowError");
+
+        #[cfg(feature = "debug_refcell")]
+        builder.field("location", self.location);
+
+        builder.finish()
     }
 }
 
@@ -602,12 +615,19 @@ impl Display for BorrowError {
 #[stable(feature = "try_borrow", since = "1.13.0")]
 pub struct BorrowMutError {
     _private: (),
+    #[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 {
-        f.debug_struct("BorrowMutError").finish()
+        let mut builder = f.debug_struct("BorrowMutError");
+
+        #[cfg(feature = "debug_refcell")]
+        builder.field("location", self.location);
+
+        builder.finish()
     }
 }
 
@@ -658,7 +678,12 @@ impl<T> RefCell<T> {
     #[rustc_const_stable(feature = "const_refcell_new", since = "1.24.0")]
     #[inline]
     pub const fn new(value: T) -> RefCell<T> {
-        RefCell { value: UnsafeCell::new(value), borrow: Cell::new(UNUSED) }
+        RefCell {
+            value: UnsafeCell::new(value),
+            borrow: Cell::new(UNUSED),
+            #[cfg(feature = "debug_refcell")]
+            borrowed_at: Cell::new(None),
+        }
     }
 
     /// Consumes the `RefCell`, returning the wrapped value.
@@ -823,12 +848,29 @@ impl<T: ?Sized> RefCell<T> {
     /// ```
     #[stable(feature = "try_borrow", since = "1.13.0")]
     #[inline]
+    #[cfg_attr(feature = "debug_refcell", track_caller)]
     pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> {
         match BorrowRef::new(&self.borrow) {
-            // SAFETY: `BorrowRef` ensures that there is only immutable access
-            // to the value while borrowed.
-            Some(b) => Ok(Ref { value: unsafe { &*self.value.get() }, borrow: b }),
-            None => Err(BorrowError { _private: () }),
+            Some(b) => {
+                #[cfg(feature = "debug_refcell")]
+                {
+                    // `borrowed_at` is always the *first* active borrow
+                    if b.borrow.get() == 1 {
+                        self.borrowed_at.set(Some(crate::panic::Location::caller()));
+                    }
+                }
+
+                // SAFETY: `BorrowRef` ensures that there is only immutable access
+                // to the value while borrowed.
+                Ok(Ref { value: unsafe { &*self.value.get() }, borrow: b })
+            }
+            None => Err(BorrowError {
+                _private: (),
+                // If a borrow occured, then we must already have an outstanding borrow,
+                // so `borrowed_at` will be `Some`
+                #[cfg(feature = "debug_refcell")]
+                location: self.borrowed_at.get().unwrap(),
+            }),
         }
     }
 
@@ -896,11 +938,25 @@ impl<T: ?Sized> RefCell<T> {
     /// ```
     #[stable(feature = "try_borrow", since = "1.13.0")]
     #[inline]
+    #[cfg_attr(feature = "debug_refcell", track_caller)]
     pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
         match BorrowRefMut::new(&self.borrow) {
-            // SAFETY: `BorrowRef` guarantees unique access.
-            Some(b) => Ok(RefMut { value: unsafe { &mut *self.value.get() }, borrow: b }),
-            None => Err(BorrowMutError { _private: () }),
+            Some(b) => {
+                #[cfg(feature = "debug_refcell")]
+                {
+                    self.borrowed_at.set(Some(crate::panic::Location::caller()));
+                }
+
+                // SAFETY: `BorrowRef` guarantees unique access.
+                Ok(RefMut { value: unsafe { &mut *self.value.get() }, borrow: b })
+            }
+            None => Err(BorrowMutError {
+                _private: (),
+                // If a borrow occured, then we must already have an outstanding borrow,
+                // so `borrowed_at` will be `Some`
+                #[cfg(feature = "debug_refcell")]
+                location: self.borrowed_at.get().unwrap(),
+            }),
         }
     }
 
@@ -1016,7 +1072,13 @@ impl<T: ?Sized> RefCell<T> {
             // and is thus guaranteed to be valid for the lifetime of `self`.
             Ok(unsafe { &*self.value.get() })
         } else {
-            Err(BorrowError { _private: () })
+            Err(BorrowError {
+                _private: (),
+                // If a borrow occured, then we must already have an outstanding borrow,
+                // so `borrowed_at` will be `Some`
+                #[cfg(feature = "debug_refcell")]
+                location: self.borrowed_at.get().unwrap(),
+            })
         }
     }
 }