Hide niches in SIMD types, too

compiler/rustc_middle/src/ty/layout.rs

@@ -1092,12 +1092,7 @@ fn layout_of_uncached(&self, ty: Ty<'tcx>) -> Result<Layout<'tcx>, LayoutError<'
                                 hide_niches(a);
                                 hide_niches(b);
                             }
-                            Abi::Vector { element, count: _ } => {
-                                // Until we support types other than floats and integers in SIMD,
-                                // `element` must already be a full for its range, so there's nothing to
-                                // do here.
-                                assert!(element.is_always_valid(dl));
-                            }
+                            Abi::Vector { element, count: _ } => hide_niches(element),
                             Abi::Aggregate { sized: _ } => {}
                         }
                         st.largest_niche = None;

src/test/ui/layout/unsafe-cell-hides-niche.rs

@@ -5,6 +5,8 @@
 
 // run-pass
 
+#![feature(repr_simd)]
+
 use std::cell::{UnsafeCell, RefCell, Cell};
 use std::mem::size_of;
 use std::num::NonZeroU32 as N32;
@@ -47,4 +49,10 @@ fn main() {
     trait Trait {}
     assert_eq!(size_of::<       UnsafeCell<&dyn Trait> >(), 16);
     assert_eq!(size_of::<Option<UnsafeCell<&dyn Trait>>>(), 24); // (✗ niche opt)
+
+    #[repr(simd)]
+    pub struct Vec4<T>([T; 4]);
+
+    assert_eq!(size_of::<       UnsafeCell<Vec4<N32>> >(), 16);
+    assert_eq!(size_of::<Option<UnsafeCell<Vec4<N32>>>>(), 32); // (✗ niche opt)
 }