From 4fec615ebf604cf7ef952770650d93ad5b7b784e Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Wed, 22 Aug 2018 12:02:09 +0200
Subject: [PATCH] fix error reporting in validation

---
 src/librustc_mir/interpret/validity.rs | 19 +++++++++++++------
 src/test/ui/union-ub-fat-ptr.stderr    |  2 +-
 2 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/src/librustc_mir/interpret/validity.rs b/src/librustc_mir/interpret/validity.rs
index 5eb32cd7eac7..8f0e81966054 100644
--- a/src/librustc_mir/interpret/validity.rs
+++ b/src/librustc_mir/interpret/validity.rs
@@ -5,7 +5,7 @@ use rustc::ty::layout::{self, Size, Primitive};
 use rustc::ty::{self, Ty};
 use rustc_data_structures::fx::FxHashSet;
 use rustc::mir::interpret::{
-    Scalar, AllocType, EvalResult, ScalarMaybeUndef,
+    Scalar, AllocType, EvalResult, ScalarMaybeUndef, EvalErrorKind
 };
 
 use super::{
@@ -285,15 +285,22 @@ impl<'a, 'mir, 'tcx, M: Machine<'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M> {
                     // This is a fat pointer.
                     let ptr = match self.ref_to_mplace(self.read_value(dest.into())?) {
                         Ok(ptr) => ptr,
-                        Err(ReadPointerAsBytes) =>
-                            return validation_failure!("fat pointer length is not a valid integer", path),
-                        Err(ReadBytesAsPointer) =>
-                            return validation_failure!("fat pointer vtable is not a valid pointer", path),
-                        Err(err) => return Err(err),
+                        Err(err) => match err.kind {
+                            EvalErrorKind::ReadPointerAsBytes =>
+                                return validation_failure!(
+                                    "fat pointer length is not a valid integer", path
+                                ),
+                            EvalErrorKind::ReadBytesAsPointer =>
+                                return validation_failure!(
+                                    "fat pointer vtable is not a valid pointer", path
+                                ),
+                            _ => return Err(err),
+                        }
                     };
                     let unpacked_ptr = self.unpack_unsized_mplace(ptr)?;
                     // for safe ptrs, recursively check it
                     if !dest.layout.ty.is_unsafe_ptr() {
+                        trace!("Recursing below fat ptr {:?} (unpacked: {:?})", ptr, unpacked_ptr);
                         if seen.insert(unpacked_ptr) {
                             todo.push((unpacked_ptr, path_clone_and_deref(path)));
                         }
diff --git a/src/test/ui/union-ub-fat-ptr.stderr b/src/test/ui/union-ub-fat-ptr.stderr
index 85a514ea99ea..cc22422304d6 100644
--- a/src/test/ui/union-ub-fat-ptr.stderr
+++ b/src/test/ui/union-ub-fat-ptr.stderr
@@ -58,7 +58,7 @@ error[E0080]: this constant likely exhibits undefined behavior
   --> $DIR/union-ub-fat-ptr.rs:104:1
    |
 LL | const F: &Trait = unsafe { DynTransmute { bad: BadDynRepr { ptr: &92, vtable: 3 } }.rust};
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered fat pointer length is not a valid integer
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered fat pointer vtable is not a valid pointer
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior