diff --git a/src/test/ui/layout/unsafe-cell-hides-niche.rs b/src/test/ui/layout/unsafe-cell-hides-niche.rs
index 1d0e1fce178..865fc2ae297 100644
--- a/src/test/ui/layout/unsafe-cell-hides-niche.rs
+++ b/src/test/ui/layout/unsafe-cell-hides-niche.rs
@@ -4,6 +4,7 @@
 // size in memory as an `Option<UnsafeCell<u32>>` (namely, 8 bytes).
 
 // check-pass
+// compile-flags: --crate-type=lib
 
 #![feature(repr_simd)]
 
@@ -23,59 +24,57 @@
 
 // Overwriting the runtime assertion and making it a compile-time assertion
 macro_rules! assert_size {
-    ($a:ty, $b:expr) => {{
+    ($a:ty, $b:expr) => {
         const _: Size::<{$b}> = Size::<{size_of::<$a>()}>;
-    }};
+    };
 }
 
 const PTR_SIZE: usize = std::mem::size_of::<*const ()>();
 
-fn main() {
-    assert_size!(Option<Wrapper<u32>>,     8);
-    assert_size!(Option<Wrapper<N32>>,     4); // (✓ niche opt)
-    assert_size!(Option<Transparent<u32>>, 8);
-    assert_size!(Option<Transparent<N32>>, 4); // (✓ niche opt)
-    assert_size!(Option<NoNiche<u32>>,     8);
-    assert_size!(Option<NoNiche<N32>>,     8); // (✗ niche opt)
+assert_size!(Option<Wrapper<u32>>,     8);
+assert_size!(Option<Wrapper<N32>>,     4); // (✓ niche opt)
+assert_size!(Option<Transparent<u32>>, 8);
+assert_size!(Option<Transparent<N32>>, 4); // (✓ niche opt)
+assert_size!(Option<NoNiche<u32>>,     8);
+assert_size!(Option<NoNiche<N32>>,     8); // (✗ niche opt)
 
-    assert_size!(Option<UnsafeCell<u32>>,  8);
-    assert_size!(Option<UnsafeCell<N32>>,  8); // (✗ niche opt)
+assert_size!(Option<UnsafeCell<u32>>,  8);
+assert_size!(Option<UnsafeCell<N32>>,  8); // (✗ niche opt)
 
-    assert_size!(       UnsafeCell<&()> , PTR_SIZE);
-    assert_size!(Option<UnsafeCell<&()>>, PTR_SIZE * 2); // (✗ niche opt)
-    assert_size!(             Cell<&()> , PTR_SIZE);
-    assert_size!(Option<      Cell<&()>>, PTR_SIZE * 2); // (✗ niche opt)
-    assert_size!(          RefCell<&()> , PTR_SIZE * 2);
-    assert_size!(Option<   RefCell<&()>>, PTR_SIZE * 3); // (✗ niche opt)
-    assert_size!(
-        RwLock<&()>,
-        if cfg!(target_pointer_width = "32") { 16 } else { 24 }
-    );
-    assert_size!(
-        Option<RwLock<&()>>,
-        if cfg!(target_pointer_width = "32") { 20 } else { 32 }
-    ); // (✗ niche opt)
-    assert_size!(
-        Mutex<&()> ,
-        if cfg!(target_pointer_width = "32") { 12 } else { 16 }
-    );
-    assert_size!(
-        Option<Mutex<&()>>,
-        if cfg!(target_pointer_width = "32") { 16 } else { 24 }
-    ); // (✗ niche opt)
+assert_size!(       UnsafeCell<&()> , PTR_SIZE);
+assert_size!(Option<UnsafeCell<&()>>, PTR_SIZE * 2); // (✗ niche opt)
+assert_size!(             Cell<&()> , PTR_SIZE);
+assert_size!(Option<      Cell<&()>>, PTR_SIZE * 2); // (✗ niche opt)
+assert_size!(          RefCell<&()> , PTR_SIZE * 2);
+assert_size!(Option<   RefCell<&()>>, PTR_SIZE * 3); // (✗ niche opt)
+assert_size!(
+    RwLock<&()>,
+    if cfg!(target_pointer_width = "32") { 16 } else { 24 }
+);
+assert_size!(
+    Option<RwLock<&()>>,
+    if cfg!(target_pointer_width = "32") { 20 } else { 32 }
+); // (✗ niche opt)
+assert_size!(
+    Mutex<&()> ,
+    if cfg!(target_pointer_width = "32") { 12 } else { 16 }
+);
+assert_size!(
+    Option<Mutex<&()>>,
+    if cfg!(target_pointer_width = "32") { 16 } else { 24 }
+); // (✗ niche opt)
 
-    assert_size!(       UnsafeCell<&[i32]> , PTR_SIZE * 2);
-    assert_size!(Option<UnsafeCell<&[i32]>>, PTR_SIZE * 3); // (✗ niche opt)
-    assert_size!(       UnsafeCell<(&(), &())> , PTR_SIZE * 2);
-    assert_size!(Option<UnsafeCell<(&(), &())>>, PTR_SIZE * 3); // (✗ niche opt)
+assert_size!(       UnsafeCell<&[i32]> , PTR_SIZE * 2);
+assert_size!(Option<UnsafeCell<&[i32]>>, PTR_SIZE * 3); // (✗ niche opt)
+assert_size!(       UnsafeCell<(&(), &())> , PTR_SIZE * 2);
+assert_size!(Option<UnsafeCell<(&(), &())>>, PTR_SIZE * 3); // (✗ niche opt)
 
-    trait Trait {}
-    assert_size!(       UnsafeCell<&dyn Trait> , PTR_SIZE * 2);
-    assert_size!(Option<UnsafeCell<&dyn Trait>>, PTR_SIZE * 3); // (✗ niche opt)
+trait Trait {}
+assert_size!(       UnsafeCell<&dyn Trait> , PTR_SIZE * 2);
+assert_size!(Option<UnsafeCell<&dyn Trait>>, PTR_SIZE * 3); // (✗ niche opt)
 
-    #[repr(simd)]
-    pub struct Vec4<T>([T; 4]);
+#[repr(simd)]
+pub struct Vec4<T>([T; 4]);
 
-    assert_size!(       UnsafeCell<Vec4<N32>> , 16);
-    assert_size!(Option<UnsafeCell<Vec4<N32>>>, 32); // (✗ niche opt)
-}
+assert_size!(       UnsafeCell<Vec4<N32>> , 16);
+assert_size!(Option<UnsafeCell<Vec4<N32>>>, 32); // (✗ niche opt)