Adjust const eval code to reflect offset_from docs

src/librustc_mir/interpret/intrinsics.rs

@@ -249,14 +249,9 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 let usize_layout = self.layout_of(self.tcx.types.usize)?;
                 let a_offset = ImmTy::from_uint(a.offset.bytes(), usize_layout);
                 let b_offset = ImmTy::from_uint(b.offset.bytes(), usize_layout);
-                let (val, overflowed, _) = self.overflowing_binary_op(
+                let (val, _overflowed, _) = self.overflowing_binary_op(
                     BinOp::Sub, a_offset, b_offset,
                 )?;
-                if overflowed {
-                    throw_ub_format!(
-                        "second argument to `ptr_offset_from` must be smaller than first",
-                    );
-                }
                 let pointee_layout = self.layout_of(substs.type_at(0))?;
                 let isize_layout = self.layout_of(self.tcx.types.isize)?;
                 let val = ImmTy::from_scalar(val, isize_layout);

src/test/ui/consts/offset_from.rs

@@ -33,7 +33,15 @@ pub const OFFSET_2: usize = {
     offset as usize
 };
 
+pub const OVERFLOW: isize = {
+    let uninit = std::mem::MaybeUninit::<Struct2>::uninit();
+    let base_ptr: *const Struct2 = &uninit as *const _ as *const Struct2;
+    let field_ptr = unsafe { &(*base_ptr).field as *const u8 };
+    unsafe { (base_ptr as *const u8).offset_from(field_ptr) }
+};
+
 fn main() {
     assert_eq!(OFFSET, 0);
     assert_eq!(OFFSET_2, 1);
+    assert_eq!(OVERFLOW, -1);
 }

src/test/ui/consts/offset_from_ub.rs

@@ -32,13 +32,4 @@ pub const NOT_MULTIPLE_OF_SIZE: usize = {
     offset as usize
 };
 
-pub const OVERFLOW: usize = {
-    //~^ NOTE
-    let uninit = std::mem::MaybeUninit::<Struct>::uninit();
-    let base_ptr: *const Struct = &uninit as *const _ as *const Struct;
-    let field_ptr = unsafe { &(*base_ptr).field as *const u8 };
-    let offset = unsafe { (base_ptr as *const u8).offset_from(field_ptr) };
-    offset as usize
-};
-
 fn main() {}

src/test/ui/consts/offset_from_ub.stderr

@@ -57,25 +57,5 @@ LL | |     offset as usize
 LL | | };
    | |__-
 
-error: any use of this value will cause an error
+error: aborting due to 3 previous errors
-  --> $SRC_DIR/libcore/ptr/mod.rs:LL:COL
-   |
-LL |           intrinsics::ptr_offset_from(self, origin)
-   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |           |
-   |           second argument to `ptr_offset_from` must be smaller than first
-   |           inside call to `std::ptr::<impl *const u8>::offset_from` at $DIR/offset_from_ub.rs:40:27
-   | 
-  ::: $DIR/offset_from_ub.rs:35:1
-   |
-LL | / pub const OVERFLOW: usize = {
-LL | |
-LL | |     let uninit = std::mem::MaybeUninit::<Struct>::uninit();
-LL | |     let base_ptr: *const Struct = &uninit as *const _ as *const Struct;
-...  |
-LL | |     offset as usize
-LL | | };
-   | |__-
-
-error: aborting due to 4 previous errors