Improve interior mutability example for E0386.

diff --git a/src/librustc_borrowck/diagnostics.rs b/src/librustc_borrowck/diagnostics.rs
index 0d0456cfb42cc9332f4be8c797ebac698aa0cf4e..d7762896debd73716a763612dda2f8872a227765 100644 (file)
--- a/src/librustc_borrowck/diagnostics.rs
+++ b/src/librustc_borrowck/diagnostics.rs
@@ -184,7 +184,8 @@ fn main(){
 `RefCell`:
 
 ```
-let y: Cell<_> = Cell::new(1);
+let x: i64 = 1;
+let y: Box<Cell<_>> = Box::new(Cell::new(x));
 y.set(2);
 ```
 "##,