std: Deprecate RefCell::{try_borrow, try_borrow_mut}

The existence of these two functions is at odds with our current [error
conventions][conventions] which recommend that panicking and `Result`-like
variants should not be provided together.

[conventions]: https://github.com/rust-lang/rfcs/blob/master/text/0236-error-conventions.md#do-not-provide-both-result-and-fail-variants

This commit adds a new `borrow_state` function returning a `BorrowState` enum to
`RefCell` which serves as a replacemnt for the `try_borrow` and `try_borrow_mut`
functions.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/libcoretest/cell.rs b/src/libcoretest/cell.rs
index 5815dbc0acc..8939bd61fe4 100644
--- a/src/libcoretest/cell.rs
+++ b/src/libcoretest/cell.rs
@@ -60,6 +60,7 @@ fn no_mut_then_imm_borrow() {
     let x = RefCell::new(0);
     let _b1 = x.borrow_mut();
     assert!(x.try_borrow().is_none());
+    assert_eq!(x.borrow_state(), BorrowState::Writing);
 }
 
 #[test]
@@ -67,13 +68,16 @@ fn no_imm_then_borrow_mut() {
     let x = RefCell::new(0);
     let _b1 = x.borrow();
     assert!(x.try_borrow_mut().is_none());
+    assert_eq!(x.borrow_state(), BorrowState::Reading);
 }
 
 #[test]
 fn no_double_borrow_mut() {
     let x = RefCell::new(0);
+    assert_eq!(x.borrow_state(), BorrowState::Unused);
     let _b1 = x.borrow_mut();
     assert!(x.try_borrow_mut().is_none());
+    assert_eq!(x.borrow_state(), BorrowState::Writing);
 }
 
 #[test]