TB: protector end semantics never causes immediate UB

Author: JoJoDeveloping

src/tools/miri/src/borrow_tracker/tree_borrows/diagnostics.rs

@@ -28,8 +28,11 @@ impl fmt::Display for AccessCause {
             Self::Explicit(kind) => write!(f, "{kind}"),
             Self::Reborrow => write!(f, "reborrow"),
             Self::Dealloc => write!(f, "deallocation"),
-            Self::FnExit(AccessKind::Read) => write!(f, "protector release read"),
-            Self::FnExit(AccessKind::Write) => write!(f, "protector release write"),
+            // This is dead code, since the protector release access itself can never
+            // cause UB (while the protector is active, if some other access invalidates
+            // further use of the protected tag, that is immediate UB).
+            // Describing the cause of UB is the only time this function is called.
+            Self::FnExit(_) => unreachable!("protector accesses can never be the source of UB"),
         }
     }
 }