Auto merge of #798 - RalfJung:format-ptr, r=RalfJung

test pointer string formatting

Author: bors

tests/run-pass/intptrcast.rs

@@ -1,9 +1,14 @@
 // compile-flags: -Zmiri-seed=0000000000000000
 
 fn main() {
+    // Some casting-to-int with arithmetic.
     let x = &42 as *const i32 as usize; 
     let y = x * 2;
     assert_eq!(y, x + x);
     let z = y as u8 as usize;
     assert_eq!(z, y % 256);
+
+    // Pointer string formatting! We can't check the output as it changes when libstd changes,
+    // but we can make sure Miri does not error.
+    format!("{:?}", &mut 13 as *mut _);
 }