Auto merge of #3896 - RalfJung:ptr_offset_unsigned_overflow, r=RalfJung

ptr_offset_unsigned_overflow: extend test

Test that indeed, the signed version works before the unsigned version is UB.

tests/fail/intrinsics/ptr_offset_unsigned_overflow.rs

@@ -1,7 +1,8 @@
 fn main() {
  let x = &[0i32; 2];
  let x = x.as_ptr().wrapping_add(1);
- // If the `!0` is interpreted as `isize`, it is just `-1` and hence harmless.
- // However, this is unsigned arithmetic, so really this is `usize::MAX` and hence UB.
- unsafe { x.byte_add(!0).read() }; //~ERROR: does not fit in an `isize`
+ // If `usize::MAX` is interpreted as `isize`, it is just `-1` and hence harmless.
+ let _ = unsafe { x.byte_offset(usize::MAX as isize) };
+ // However, `byte_add` uses unsigned arithmetic, so really this is `usize::MAX` and hence UB.
+ let _ = unsafe { x.byte_add(usize::MAX) }; //~ERROR: does not fit in an `isize`
 }

tests/fail/intrinsics/ptr_offset_unsigned_overflow.stderr

@@ -1,8 +1,8 @@
 error: Undefined Behavior: overflowing pointer arithmetic: the total offset in bytes does not fit in an `isize`
  --> tests/fail/intrinsics/ptr_offset_unsigned_overflow.rs:LL:CC
  |
-LL | unsafe { x.byte_add(!0).read() };
- | ^^^^^^^^^^^^^^ overflowing pointer arithmetic: the total offset in bytes does not fit in an `isize`
+LL | let _ = unsafe { x.byte_add(usize::MAX) };
+ |  ^^^^^^^^^^^^^^^^^^^^^^ overflowing pointer arithmetic: the total offset in bytes does not fit in an `isize`
  |
  = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
  = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information