fix handling of alignment for dyn-sized places

Author: RalfJung

compiler/rustc_const_eval/src/interpret/place.rs

@@ -328,7 +328,8 @@ where
         };
 
         let mplace = MemPlace { ptr: ptr.to_pointer(self)?, meta };
-        // When deref'ing a pointer, the *static* alignment given by the type is what matters.
+        // `ref_to_mplace` is called on raw pointers even if they don't actually get dereferenced;
+        // we hence can't call `size_and_align_of` since that asserts more validity than we want.
         let align = layout.align.abi;
         Ok(MPlaceTy { mplace, layout, align })
     }
@@ -379,11 +380,12 @@ where
 
     /// Check if this mplace is dereferenceable and sufficiently aligned.
     pub fn check_mplace(&self, mplace: MPlaceTy<'tcx, M::Provenance>) -> InterpResult<'tcx> {
-        let (size, align) = self
+        let (size, _align) = self
             .size_and_align_of_mplace(&mplace)?
             .unwrap_or((mplace.layout.size, mplace.layout.align.abi));
-        assert!(mplace.align <= align, "dynamic alignment less strict than static one?");
-        let align = if M::enforce_alignment(self).should_check() { align } else { Align::ONE };
+        // Due to packed places, only `mplace.align` matters.
+        let align =
+            if M::enforce_alignment(self).should_check() { mplace.align } else { Align::ONE };
         self.check_ptr_access_align(mplace.ptr, size, align, CheckInAllocMsg::DerefTest)?;
         Ok(())
     }

src/tools/miri/tests/fail/unaligned_pointers/dyn_alignment.rs

@@ -1,5 +1,6 @@
-// should find the bug even without validation and stacked borrows, but gets masked by optimizations
-//@compile-flags: -Zmiri-disable-validation -Zmiri-disable-stacked-borrows -Zmir-opt-level=0 -Cdebug-assertions=no
+// should find the bug even without, but gets masked by optimizations
+//@compile-flags: -Zmiri-disable-stacked-borrows -Zmir-opt-level=0 -Cdebug-assertions=no
+//@normalize-stderr-test: "but found [0-9]+" -> "but found $$ALIGN"
 
 #[repr(align(256))]
 #[derive(Debug)]
@@ -19,6 +20,6 @@ fn main() {
             (&mut ptr as *mut _ as *mut *const u8).write(&buf as *const _ as *const u8);
         }
         // Re-borrow that. This should be UB.
-        let _ptr = &*ptr; //~ERROR: alignment 256 is required
+        let _ptr = &*ptr; //~ERROR: required 256 byte alignment
     }
 }

src/tools/miri/tests/fail/unaligned_pointers/dyn_alignment.stderr

@@ -1,8 +1,8 @@
-error: Undefined Behavior: accessing memory with alignment ALIGN, but alignment ALIGN is required
+error: Undefined Behavior: constructing invalid value: encountered an unaligned reference (required 256 byte alignment but found $ALIGN)
   --> $DIR/dyn_alignment.rs:LL:CC
    |
 LL |         let _ptr = &*ptr;
-   |                    ^^^^^ accessing memory with alignment ALIGN, but alignment ALIGN is required
+   |                    ^^^^^ constructing invalid value: encountered an unaligned reference (required 256 byte alignment but found $ALIGN)
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information