Add expected/provided byte alignments to validation error message

src/librustc_mir/interpret/validity.rs

@@ -357,8 +357,10 @@ impl<'rt, 'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>>
                         match err.kind {
                             EvalErrorKind::InvalidNullPointerUsage =>
                                 return validation_failure!("NULL reference", self.path),
-                            EvalErrorKind::AlignmentCheckFailed { .. } =>
-                                return validation_failure!("unaligned reference", self.path),
+                            EvalErrorKind::AlignmentCheckFailed { required, has } =>
+                                return validation_failure!(format!("unaligned reference \
+                                    (required {} byte alignment but found {})",
+                                    required.bytes(), has.bytes()), self.path),
                             _ =>
                                 return validation_failure!(
                                     "dangling (out-of-bounds) reference (might be NULL at \

src/test/ui/consts/const-eval/ub-ref.rs

@@ -1,10 +1,12 @@
+// ignore-tidy-linelength
 #![feature(const_transmute)]
 #![allow(const_err)] // make sure we cannot allow away the errors tested here
 
 use std::mem;
 
 const UNALIGNED: &u16 = unsafe { mem::transmute(&[0u8; 4]) };
 //~^ ERROR it is undefined behavior to use this value
+//~^^ type validation failed: encountered unaligned reference (required 2 byte alignment but found 1)
 
 const NULL: &u16 = unsafe { mem::transmute(0usize) };
 //~^ ERROR it is undefined behavior to use this value

src/test/ui/consts/const-eval/ub-ref.stderr

@@ -1,37 +1,37 @@
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-ref.rs:6:1
+  --> $DIR/ub-ref.rs:7:1
    |
 LL | const UNALIGNED: &u16 = unsafe { mem::transmute(&[0u8; 4]) };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered unaligned reference
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered unaligned reference (required 2 byte alignment but found 1)
    |
    = 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
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-ref.rs:9:1
+  --> $DIR/ub-ref.rs:11:1
    |
 LL | const NULL: &u16 = unsafe { mem::transmute(0usize) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0, but expected something greater or equal to 1
    |
    = 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
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-ref.rs:12:1
+  --> $DIR/ub-ref.rs:14:1
    |
 LL | const REF_AS_USIZE: usize = unsafe { mem::transmute(&0) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a pointer, but expected initialized plain (non-pointer) bytes
    |
    = 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
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-ref.rs:15:1
+  --> $DIR/ub-ref.rs:17:1
    |
 LL | const REF_AS_USIZE_SLICE: &[usize] = &[unsafe { mem::transmute(&0) }];
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a pointer at .<deref>, but expected plain (non-pointer) bytes
    |
    = 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
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-ref.rs:18:1
+  --> $DIR/ub-ref.rs:20:1
    |
 LL | const USIZE_AS_REF: &'static u8 = unsafe { mem::transmute(1337usize) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered integer pointer in non-ZST reference