interpreter PointerArithmetic: use new Size helper methods

compiler/rustc_middle/src/mir/interpret/pointer.rs

@@ -3,7 +3,7 @@ use super::{AllocId, InterpResult};
 use rustc_macros::HashStable;
 use rustc_target::abi::{HasDataLayout, Size};
 
-use std::convert::TryFrom;
+use std::convert::{TryFrom, TryInto};
 use std::fmt;
 
 ////////////////////////////////////////////////////////////////////////////////
@@ -20,29 +20,27 @@ pub trait PointerArithmetic: HasDataLayout {
 
     #[inline]
     fn machine_usize_max(&self) -> u64 {
-        let max_usize_plus_1 = 1u128 << self.pointer_size().bits();
-        u64::try_from(max_usize_plus_1 - 1).unwrap()
+        self.pointer_size().unsigned_int_max().try_into().unwrap()
     }
 
     #[inline]
     fn machine_isize_min(&self) -> i64 {
-        let max_isize_plus_1 = 1i128 << (self.pointer_size().bits() - 1);
-        i64::try_from(-max_isize_plus_1).unwrap()
+        self.pointer_size().signed_int_min().try_into().unwrap()
     }
 
     #[inline]
     fn machine_isize_max(&self) -> i64 {
-        let max_isize_plus_1 = 1u128 << (self.pointer_size().bits() - 1);
-        i64::try_from(max_isize_plus_1 - 1).unwrap()
+        self.pointer_size().signed_int_max().try_into().unwrap()
     }
 
     #[inline]
     fn machine_usize_to_isize(&self, val: u64) -> i64 {
         let val = val as i64;
-        // Now clamp into the machine_isize range.
+        // Now wrap-around into the machine_isize range.
         if val > self.machine_isize_max() {
             // This can only happen the the ptr size is < 64, so we know max_usize_plus_1 fits into
             // i64.
+            debug_assert!(self.pointer_size().bits() < 64);
             let max_usize_plus_1 = 1u128 << self.pointer_size().bits();
             val - i64::try_from(max_usize_plus_1).unwrap()
         } else {