Alignment of const_field results must be of the field's type

We're taking the value of the field, any future accesses to the constant will not know about this, so we need to adjust the allocation's alignment.

Author: oli-obk

src/librustc_mir/const_eval.rs

@@ -100,15 +100,19 @@ pub fn mplace_to_const<'tcx>(
     ecx: &CompileTimeEvalContext<'_, '_, 'tcx>,
     mplace: MPlaceTy<'tcx>,
 ) -> EvalResult<'tcx, &'tcx ty::Const<'tcx>> {
-    let MemPlace { ptr, align, meta } = *mplace;
     // extract alloc-offset pair
-    assert!(meta.is_none());
-    let ptr = ptr.to_ptr()?;
+    assert!(mplace.meta.is_none());
+    let ptr = mplace.ptr.to_ptr()?;
     let alloc = ecx.memory.get(ptr.alloc_id)?;
-    assert!(alloc.align.abi() >= align.abi());
+    assert!(alloc.align.abi() >= mplace.align.abi());
     assert!(alloc.bytes.len() as u64 - ptr.offset.bytes() >= mplace.layout.size.bytes());
+    // FIXME: only clone the parts that interest us (starting at offset, going to offset + size)
     let mut alloc = alloc.clone();
-    alloc.align = align;
+    // we take `mplace.layout.align` instead of `mplace.align`
+    // as this function is essentially copying the value
+    // out of the larger allocation, so we lose all information about
+    // potential surrounding types with different alignment.
+    alloc.align = mplace.layout.align;
     // FIXME shouldnt it be the case that `intern_static` has already
     // interned this?  I thought that is the entire point of that `FinishStatic` stuff?
     let alloc = ecx.tcx.intern_const_alloc(alloc);