fix ICE on Miri/CTFE copy of half a pointer

compiler/rustc_const_eval/src/interpret/memory.rs

@@ -1057,20 +1057,19 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
             Some(dest_ptr) => dest_ptr,
         };
 
+        // This checks relocation edges on the src, which needs to happen before
+        // `prepare_relocation_copy`.
+        let src_bytes = src_alloc
+            .get_bytes_with_uninit_and_ptr(&tcx, src_range)
+            .map_err(|e| e.to_interp_error(src_alloc_id))?
+            .as_ptr(); // raw ptr, so we can also get a ptr to the destination allocation
         // first copy the relocations to a temporary buffer, because
         // `get_bytes_mut` will clear the relocations, which is correct,
         // since we don't want to keep any relocations at the target.
-        // (`get_bytes_with_uninit_and_ptr` below checks that there are no
-        // relocations overlapping the edges; those would not be handled correctly).
         let relocations =
             src_alloc.prepare_relocation_copy(self, src_range, dest_offset, num_copies);
         // Prepare a copy of the initialization mask.
         let compressed = src_alloc.compress_uninit_range(src_range);
-        // This checks relocation edges on the src.
-        let src_bytes = src_alloc
-            .get_bytes_with_uninit_and_ptr(&tcx, src_range)
-            .map_err(|e| e.to_interp_error(src_alloc_id))?
-            .as_ptr(); // raw ptr, so we can also get a ptr to the destination allocation
 
         // Destination alloc preparations and access hooks.
         let (dest_alloc, extra) = self.get_raw_mut(dest_alloc_id)?;

src/test/ui/consts/issue-miri-1910.rs

@@ -0,0 +1,12 @@
+// error-pattern unable to turn pointer into raw bytes
+#![feature(const_ptr_read)]
+#![feature(const_ptr_offset)]
+
+const C: () = unsafe {
+    let foo = Some(&42 as *const i32);
+    let one_and_a_half_pointers = std::mem::size_of::<*const i32>()/2*3;
+    (&foo as *const _ as *const u8).add(one_and_a_half_pointers).read();
+};
+
+fn main() {
+}

src/test/ui/consts/issue-miri-1910.stderr

@@ -0,0 +1,26 @@
+error: any use of this value will cause an error
+  --> $SRC_DIR/core/src/ptr/mod.rs:LL:COL
+   |
+LL |           copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |           |
+   |           unable to turn pointer into raw bytes
+   |           inside `std::ptr::read::<u8>` at $SRC_DIR/core/src/ptr/mod.rs:LL:COL
+   |           inside `ptr::const_ptr::<impl *const u8>::read` at $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
+   |           inside `C` at $DIR/issue-miri-1910.rs:8:5
+   |
+  ::: $DIR/issue-miri-1910.rs:5:1
+   |
+LL | / const C: () = unsafe {
+LL | |     let foo = Some(&42 as *const i32);
+LL | |     let one_and_a_half_pointers = std::mem::size_of::<*const i32>()/2*3;
+LL | |     (&foo as *const _ as *const u8).add(one_and_a_half_pointers).read();
+LL | | };
+   | |__-
+   |
+   = note: `#[deny(const_err)]` on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+
+error: aborting due to previous error
+