miri: improve error when offset_from preconditions are violated

1 files changed, 19 insertions, 1 deletions

diff --git a/compiler/rustc_const_eval/src/interpret/intrinsics.rs b/compiler/rustc_const_eval/src/interpret/intrinsics.rs
index 9f5f2533e08..4ca317e3a1e 100644
--- a/compiler/rustc_const_eval/src/interpret/intrinsics.rs
+++ b/compiler/rustc_const_eval/src/interpret/intrinsics.rs
@@ -319,7 +319,25 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
 
                 // Check that the memory between them is dereferenceable at all, starting from the
                 // origin pointer: `dist` is `a - b`, so it is based on `b`.
-                self.check_ptr_access_signed(b, dist, CheckInAllocMsg::OffsetFromTest)?;
+                self.check_ptr_access_signed(b, dist, CheckInAllocMsg::OffsetFromTest)
+                    .map_err_kind(|_| {
+                        // This could mean they point to different allocations, or they point to the same allocation
+                        // but not the entire range between the pointers is in-bounds.
+                        if let Ok((a_alloc_id, ..)) = self.ptr_try_get_alloc_id(a, 0)
+                            && let Ok((b_alloc_id, ..)) = self.ptr_try_get_alloc_id(b, 0)
+                            && a_alloc_id == b_alloc_id
+                        {
+                            err_ub_custom!(
+                                fluent::const_eval_offset_from_out_of_bounds,
+                                name = intrinsic_name,
+                            )
+                        } else {
+                            err_ub_custom!(
+                                fluent::const_eval_offset_from_different_allocations,
+                                name = intrinsic_name,
+                            )
+                        }
+                    })?;
                 // Then check that this is also dereferenceable from `a`. This ensures that they are
                 // derived from the same allocation.
                 self.check_ptr_access_signed(