diff --git a/compiler/rustc_lint/src/types.rs b/compiler/rustc_lint/src/types.rs
index 784b36c2837b7..af0f556557209 100644
--- a/compiler/rustc_lint/src/types.rs
+++ b/compiler/rustc_lint/src/types.rs
@@ -672,7 +672,7 @@ pub fn transparent_newtype_field<'a, 'tcx>(
 }
 
 /// Is type known to be non-null?
-crate fn ty_is_known_nonnull<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>, mode: CItemKind) -> bool {
+fn ty_is_known_nonnull<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>, mode: CItemKind) -> bool {
     let tcx = cx.tcx;
     match ty.kind() {
         ty::FnPtr(_) => true,
@@ -685,6 +685,12 @@ crate fn ty_is_known_nonnull<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>, mode: C
                 return true;
             }
 
+            // Types with a `#[repr(no_niche)]` attribute have their niche hidden.
+            // The attribute is used by the UnsafeCell for example (the only use so far).
+            if def.repr.hide_niche() {
+                return false;
+            }
+
             for variant in &def.variants {
                 if let Some(field) = transparent_newtype_field(cx.tcx, variant) {
                     if ty_is_known_nonnull(cx, field.ty(tcx, substs), mode) {
diff --git a/src/test/ui/lint/clashing-extern-fn.rs b/src/test/ui/lint/clashing-extern-fn.rs
index 41f0baecf24a8..2ce4dd56eab0d 100644
--- a/src/test/ui/lint/clashing-extern-fn.rs
+++ b/src/test/ui/lint/clashing-extern-fn.rs
@@ -1,6 +1,7 @@
 // check-pass
 // aux-build:external_extern_fn.rs
 #![crate_type = "lib"]
+#![feature(no_niche)]
 #![warn(clashing_extern_declarations)]
 
 mod redeclared_different_signature {
@@ -383,3 +384,36 @@ mod unknown_layout {
         }
     }
 }
+
+mod hidden_niche {
+    mod a {
+        extern "C" {
+            fn hidden_niche_transparent() -> usize;
+            fn hidden_niche_transparent_no_niche() -> usize;
+            fn hidden_niche_unsafe_cell() -> usize;
+        }
+    }
+    mod b {
+        use std::cell::UnsafeCell;
+        use std::num::NonZeroUsize;
+
+        #[repr(transparent)]
+        struct Transparent { x: NonZeroUsize }
+
+        #[repr(no_niche)]
+        #[repr(transparent)]
+        struct TransparentNoNiche { y: NonZeroUsize }
+
+        extern "C" {
+            fn hidden_niche_transparent() -> Option<Transparent>;
+
+            fn hidden_niche_transparent_no_niche() -> Option<TransparentNoNiche>;
+            //~^ WARN redeclared with a different signature
+            //~| WARN block uses type `Option<TransparentNoNiche>`, which is not FFI-safe
+
+            fn hidden_niche_unsafe_cell() -> Option<UnsafeCell<NonZeroUsize>>;
+            //~^ WARN redeclared with a different signature
+            //~| WARN block uses type `Option<UnsafeCell<NonZeroUsize>>`, which is not FFI-safe
+        }
+    }
+}
diff --git a/src/test/ui/lint/clashing-extern-fn.stderr b/src/test/ui/lint/clashing-extern-fn.stderr
index a48b0d008f913..a856de322c8ca 100644
--- a/src/test/ui/lint/clashing-extern-fn.stderr
+++ b/src/test/ui/lint/clashing-extern-fn.stderr
@@ -1,5 +1,5 @@
 warning: `clash` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:14:13
+  --> $DIR/clashing-extern-fn.rs:15:13
    |
 LL |             fn clash(x: u8);
    |             ---------------- `clash` previously declared here
@@ -8,7 +8,7 @@ LL |             fn clash(x: u64);
    |             ^^^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
    |
 note: the lint level is defined here
-  --> $DIR/clashing-extern-fn.rs:4:9
+  --> $DIR/clashing-extern-fn.rs:5:9
    |
 LL | #![warn(clashing_extern_declarations)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -16,7 +16,7 @@ LL | #![warn(clashing_extern_declarations)]
               found `unsafe extern "C" fn(u64)`
 
 warning: `extern_link_name` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:52:9
+  --> $DIR/clashing-extern-fn.rs:53:9
    |
 LL | /     #[link_name = "extern_link_name"]
 LL | |     fn some_new_name(x: i16);
@@ -29,7 +29,7 @@ LL |           fn extern_link_name(x: u32);
               found `unsafe extern "C" fn(u32)`
 
 warning: `some_other_extern_link_name` redeclares `some_other_new_name` with a different signature
-  --> $DIR/clashing-extern-fn.rs:55:9
+  --> $DIR/clashing-extern-fn.rs:56:9
    |
 LL |       fn some_other_new_name(x: i16);
    |       ------------------------------- `some_other_new_name` previously declared here
@@ -43,7 +43,7 @@ LL | |         fn some_other_extern_link_name(x: u32);
               found `unsafe extern "C" fn(u32)`
 
 warning: `other_both_names_different` redeclares `link_name_same` with a different signature
-  --> $DIR/clashing-extern-fn.rs:59:9
+  --> $DIR/clashing-extern-fn.rs:60:9
    |
 LL | /     #[link_name = "link_name_same"]
 LL | |     fn both_names_different(x: i16);
@@ -58,7 +58,7 @@ LL | |         fn other_both_names_different(x: u32);
               found `unsafe extern "C" fn(u32)`
 
 warning: `different_mod` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:72:9
+  --> $DIR/clashing-extern-fn.rs:73:9
    |
 LL |         fn different_mod(x: u8);
    |         ------------------------ `different_mod` previously declared here
@@ -70,7 +70,7 @@ LL |         fn different_mod(x: u64);
               found `unsafe extern "C" fn(u64)`
 
 warning: `variadic_decl` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:82:9
+  --> $DIR/clashing-extern-fn.rs:83:9
    |
 LL |     fn variadic_decl(x: u8, ...);
    |     ----------------------------- `variadic_decl` previously declared here
@@ -82,7 +82,7 @@ LL |         fn variadic_decl(x: u8);
               found `unsafe extern "C" fn(u8)`
 
 warning: `weigh_banana` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:142:13
+  --> $DIR/clashing-extern-fn.rs:143:13
    |
 LL |             fn weigh_banana(count: *const Banana) -> u64;
    |             --------------------------------------------- `weigh_banana` previously declared here
@@ -94,7 +94,7 @@ LL |             fn weigh_banana(count: *const Banana) -> u64;
               found `unsafe extern "C" fn(*const three::Banana) -> u64`
 
 warning: `draw_point` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:171:13
+  --> $DIR/clashing-extern-fn.rs:172:13
    |
 LL |             fn draw_point(p: Point);
    |             ------------------------ `draw_point` previously declared here
@@ -106,7 +106,7 @@ LL |             fn draw_point(p: Point);
               found `unsafe extern "C" fn(sameish_members::b::Point)`
 
 warning: `origin` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:197:13
+  --> $DIR/clashing-extern-fn.rs:198:13
    |
 LL |             fn origin() -> Point3;
    |             ---------------------- `origin` previously declared here
@@ -118,7 +118,7 @@ LL |             fn origin() -> Point3;
               found `unsafe extern "C" fn() -> same_sized_members_clash::b::Point3`
 
 warning: `transparent_incorrect` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:220:13
+  --> $DIR/clashing-extern-fn.rs:221:13
    |
 LL |             fn transparent_incorrect() -> T;
    |             -------------------------------- `transparent_incorrect` previously declared here
@@ -130,7 +130,7 @@ LL |             fn transparent_incorrect() -> isize;
               found `unsafe extern "C" fn() -> isize`
 
 warning: `missing_return_type` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:238:13
+  --> $DIR/clashing-extern-fn.rs:239:13
    |
 LL |             fn missing_return_type() -> usize;
    |             ---------------------------------- `missing_return_type` previously declared here
@@ -142,7 +142,7 @@ LL |             fn missing_return_type();
               found `unsafe extern "C" fn()`
 
 warning: `non_zero_usize` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:256:13
+  --> $DIR/clashing-extern-fn.rs:257:13
    |
 LL |             fn non_zero_usize() -> core::num::NonZeroUsize;
    |             ----------------------------------------------- `non_zero_usize` previously declared here
@@ -154,7 +154,7 @@ LL |             fn non_zero_usize() -> usize;
               found `unsafe extern "C" fn() -> usize`
 
 warning: `non_null_ptr` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:258:13
+  --> $DIR/clashing-extern-fn.rs:259:13
    |
 LL |             fn non_null_ptr() -> core::ptr::NonNull<usize>;
    |             ----------------------------------------------- `non_null_ptr` previously declared here
@@ -166,7 +166,7 @@ LL |             fn non_null_ptr() -> *const usize;
               found `unsafe extern "C" fn() -> *const usize`
 
 warning: `option_non_zero_usize_incorrect` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:356:13
+  --> $DIR/clashing-extern-fn.rs:357:13
    |
 LL |             fn option_non_zero_usize_incorrect() -> usize;
    |             ---------------------------------------------- `option_non_zero_usize_incorrect` previously declared here
@@ -178,7 +178,7 @@ LL |             fn option_non_zero_usize_incorrect() -> isize;
               found `unsafe extern "C" fn() -> isize`
 
 warning: `option_non_null_ptr_incorrect` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:358:13
+  --> $DIR/clashing-extern-fn.rs:359:13
    |
 LL |             fn option_non_null_ptr_incorrect() -> *const usize;
    |             --------------------------------------------------- `option_non_null_ptr_incorrect` previously declared here
@@ -189,5 +189,48 @@ LL |             fn option_non_null_ptr_incorrect() -> *const isize;
    = note: expected `unsafe extern "C" fn() -> *const usize`
               found `unsafe extern "C" fn() -> *const isize`
 
-warning: 15 warnings emitted
+warning: `hidden_niche_transparent_no_niche` redeclared with a different signature
+  --> $DIR/clashing-extern-fn.rs:410:13
+   |
+LL |             fn hidden_niche_transparent_no_niche() -> usize;
+   |             ------------------------------------------------ `hidden_niche_transparent_no_niche` previously declared here
+...
+LL |             fn hidden_niche_transparent_no_niche() -> Option<TransparentNoNiche>;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
+   |
+   = note: expected `unsafe extern "C" fn() -> usize`
+              found `unsafe extern "C" fn() -> Option<TransparentNoNiche>`
+
+warning: `hidden_niche_unsafe_cell` redeclared with a different signature
+  --> $DIR/clashing-extern-fn.rs:414:13
+   |
+LL |             fn hidden_niche_unsafe_cell() -> usize;
+   |             --------------------------------------- `hidden_niche_unsafe_cell` previously declared here
+...
+LL |             fn hidden_niche_unsafe_cell() -> Option<UnsafeCell<NonZeroUsize>>;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
+   |
+   = note: expected `unsafe extern "C" fn() -> usize`
+              found `unsafe extern "C" fn() -> Option<UnsafeCell<NonZeroUsize>>`
+
+warning: `extern` block uses type `Option<TransparentNoNiche>`, which is not FFI-safe
+  --> $DIR/clashing-extern-fn.rs:410:55
+   |
+LL |             fn hidden_niche_transparent_no_niche() -> Option<TransparentNoNiche>;
+   |                                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = note: `#[warn(improper_ctypes)]` on by default
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+warning: `extern` block uses type `Option<UnsafeCell<NonZeroUsize>>`, which is not FFI-safe
+  --> $DIR/clashing-extern-fn.rs:414:46
+   |
+LL |             fn hidden_niche_unsafe_cell() -> Option<UnsafeCell<NonZeroUsize>>;
+   |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+warning: 19 warnings emitted
 
diff --git a/src/test/ui/lint/lint-ctypes.rs b/src/test/ui/lint/lint-ctypes.rs
index fabbdcf689182..a291471f56670 100644
--- a/src/test/ui/lint/lint-ctypes.rs
+++ b/src/test/ui/lint/lint-ctypes.rs
@@ -5,6 +5,7 @@
 
 extern crate libc;
 
+use std::cell::UnsafeCell;
 use std::marker::PhantomData;
 
 trait Bar { }
@@ -70,6 +71,11 @@ extern "C" {
     pub fn transparent_fn(p: TransparentBadFn); //~ ERROR: uses type `Box<u32>`
     pub fn raw_array(arr: [u8; 8]); //~ ERROR: uses type `[u8; 8]`
 
+    pub fn no_niche_a(a: Option<UnsafeCell<extern fn()>>);
+    //~^ ERROR: uses type `Option<UnsafeCell<extern "C" fn()>>`
+    pub fn no_niche_b(b: Option<UnsafeCell<&i32>>);
+    //~^ ERROR: uses type `Option<UnsafeCell<&i32>>`
+
     pub static static_u128_type: u128; //~ ERROR: uses type `u128`
     pub static static_u128_array_type: [u128; 16]; //~ ERROR: uses type `u128`
 
diff --git a/src/test/ui/lint/lint-ctypes.stderr b/src/test/ui/lint/lint-ctypes.stderr
index 6a968fca92280..7f21e412c385c 100644
--- a/src/test/ui/lint/lint-ctypes.stderr
+++ b/src/test/ui/lint/lint-ctypes.stderr
@@ -1,5 +1,5 @@
 error: `extern` block uses type `Foo`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:47:28
+  --> $DIR/lint-ctypes.rs:48:28
    |
 LL |     pub fn ptr_type1(size: *const Foo);
    |                            ^^^^^^^^^^ not FFI-safe
@@ -12,13 +12,13 @@ LL | #![deny(improper_ctypes)]
    = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
    = note: this struct has unspecified layout
 note: the type is defined here
-  --> $DIR/lint-ctypes.rs:25:1
+  --> $DIR/lint-ctypes.rs:26:1
    |
 LL | pub struct Foo;
    | ^^^^^^^^^^^^^^^
 
 error: `extern` block uses type `Foo`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:48:28
+  --> $DIR/lint-ctypes.rs:49:28
    |
 LL |     pub fn ptr_type2(size: *const Foo);
    |                            ^^^^^^^^^^ not FFI-safe
@@ -26,13 +26,13 @@ LL |     pub fn ptr_type2(size: *const Foo);
    = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
    = note: this struct has unspecified layout
 note: the type is defined here
-  --> $DIR/lint-ctypes.rs:25:1
+  --> $DIR/lint-ctypes.rs:26:1
    |
 LL | pub struct Foo;
    | ^^^^^^^^^^^^^^^
 
 error: `extern` block uses type `[u32]`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:49:26
+  --> $DIR/lint-ctypes.rs:50:26
    |
 LL |     pub fn slice_type(p: &[u32]);
    |                          ^^^^^^ not FFI-safe
@@ -41,7 +41,7 @@ LL |     pub fn slice_type(p: &[u32]);
    = note: slices have no C equivalent
 
 error: `extern` block uses type `str`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:50:24
+  --> $DIR/lint-ctypes.rs:51:24
    |
 LL |     pub fn str_type(p: &str);
    |                        ^^^^ not FFI-safe
@@ -50,7 +50,7 @@ LL |     pub fn str_type(p: &str);
    = note: string slices have no C equivalent
 
 error: `extern` block uses type `Box<u32>`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:51:24
+  --> $DIR/lint-ctypes.rs:52:24
    |
 LL |     pub fn box_type(p: Box<u32>);
    |                        ^^^^^^^^ not FFI-safe
@@ -59,7 +59,7 @@ LL |     pub fn box_type(p: Box<u32>);
    = note: this struct has unspecified layout
 
 error: `extern` block uses type `Option<Box<u32>>`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:52:28
+  --> $DIR/lint-ctypes.rs:53:28
    |
 LL |     pub fn opt_box_type(p: Option<Box<u32>>);
    |                            ^^^^^^^^^^^^^^^^ not FFI-safe
@@ -68,7 +68,7 @@ LL |     pub fn opt_box_type(p: Option<Box<u32>>);
    = note: enum has no representation hint
 
 error: `extern` block uses type `char`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:54:25
+  --> $DIR/lint-ctypes.rs:55:25
    |
 LL |     pub fn char_type(p: char);
    |                         ^^^^ not FFI-safe
@@ -77,7 +77,7 @@ LL |     pub fn char_type(p: char);
    = note: the `char` type has no C equivalent
 
 error: `extern` block uses type `i128`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:55:25
+  --> $DIR/lint-ctypes.rs:56:25
    |
 LL |     pub fn i128_type(p: i128);
    |                         ^^^^ not FFI-safe
@@ -85,7 +85,7 @@ LL |     pub fn i128_type(p: i128);
    = note: 128-bit integers don't currently have a known stable ABI
 
 error: `extern` block uses type `u128`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:56:25
+  --> $DIR/lint-ctypes.rs:57:25
    |
 LL |     pub fn u128_type(p: u128);
    |                         ^^^^ not FFI-safe
@@ -93,7 +93,7 @@ LL |     pub fn u128_type(p: u128);
    = note: 128-bit integers don't currently have a known stable ABI
 
 error: `extern` block uses type `dyn Bar`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:57:26
+  --> $DIR/lint-ctypes.rs:58:26
    |
 LL |     pub fn trait_type(p: &dyn Bar);
    |                          ^^^^^^^^ not FFI-safe
@@ -101,7 +101,7 @@ LL |     pub fn trait_type(p: &dyn Bar);
    = note: trait objects have no C equivalent
 
 error: `extern` block uses type `(i32, i32)`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:58:26
+  --> $DIR/lint-ctypes.rs:59:26
    |
 LL |     pub fn tuple_type(p: (i32, i32));
    |                          ^^^^^^^^^^ not FFI-safe
@@ -110,7 +110,7 @@ LL |     pub fn tuple_type(p: (i32, i32));
    = note: tuples have unspecified layout
 
 error: `extern` block uses type `(i32, i32)`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:59:27
+  --> $DIR/lint-ctypes.rs:60:27
    |
 LL |     pub fn tuple_type2(p: I32Pair);
    |                           ^^^^^^^ not FFI-safe
@@ -119,7 +119,7 @@ LL |     pub fn tuple_type2(p: I32Pair);
    = note: tuples have unspecified layout
 
 error: `extern` block uses type `ZeroSize`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:60:25
+  --> $DIR/lint-ctypes.rs:61:25
    |
 LL |     pub fn zero_size(p: ZeroSize);
    |                         ^^^^^^^^ not FFI-safe
@@ -127,26 +127,26 @@ LL |     pub fn zero_size(p: ZeroSize);
    = help: consider adding a member to this struct
    = note: this struct has no fields
 note: the type is defined here
-  --> $DIR/lint-ctypes.rs:21:1
+  --> $DIR/lint-ctypes.rs:22:1
    |
 LL | pub struct ZeroSize;
    | ^^^^^^^^^^^^^^^^^^^^
 
 error: `extern` block uses type `ZeroSizeWithPhantomData`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:61:33
+  --> $DIR/lint-ctypes.rs:62:33
    |
 LL |     pub fn zero_size_phantom(p: ZeroSizeWithPhantomData);
    |                                 ^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
    |
    = note: composed only of `PhantomData`
 note: the type is defined here
-  --> $DIR/lint-ctypes.rs:44:1
+  --> $DIR/lint-ctypes.rs:45:1
    |
 LL | pub struct ZeroSizeWithPhantomData(::std::marker::PhantomData<i32>);
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: `extern` block uses type `PhantomData<bool>`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:64:12
+  --> $DIR/lint-ctypes.rs:65:12
    |
 LL |         -> ::std::marker::PhantomData<bool>;
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -154,7 +154,7 @@ LL |         -> ::std::marker::PhantomData<bool>;
    = note: composed only of `PhantomData`
 
 error: `extern` block uses type `fn()`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:65:23
+  --> $DIR/lint-ctypes.rs:66:23
    |
 LL |     pub fn fn_type(p: RustFn);
    |                       ^^^^^^ not FFI-safe
@@ -163,7 +163,7 @@ LL |     pub fn fn_type(p: RustFn);
    = note: this function pointer has Rust-specific calling convention
 
 error: `extern` block uses type `fn()`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:66:24
+  --> $DIR/lint-ctypes.rs:67:24
    |
 LL |     pub fn fn_type2(p: fn());
    |                        ^^^^ not FFI-safe
@@ -172,7 +172,7 @@ LL |     pub fn fn_type2(p: fn());
    = note: this function pointer has Rust-specific calling convention
 
 error: `extern` block uses type `Box<u32>`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:67:28
+  --> $DIR/lint-ctypes.rs:68:28
    |
 LL |     pub fn fn_contained(p: RustBadRet);
    |                            ^^^^^^^^^^ not FFI-safe
@@ -181,7 +181,7 @@ LL |     pub fn fn_contained(p: RustBadRet);
    = note: this struct has unspecified layout
 
 error: `extern` block uses type `i128`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:68:32
+  --> $DIR/lint-ctypes.rs:69:32
    |
 LL |     pub fn transparent_i128(p: TransparentI128);
    |                                ^^^^^^^^^^^^^^^ not FFI-safe
@@ -189,7 +189,7 @@ LL |     pub fn transparent_i128(p: TransparentI128);
    = note: 128-bit integers don't currently have a known stable ABI
 
 error: `extern` block uses type `str`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:69:31
+  --> $DIR/lint-ctypes.rs:70:31
    |
 LL |     pub fn transparent_str(p: TransparentStr);
    |                               ^^^^^^^^^^^^^^ not FFI-safe
@@ -198,7 +198,7 @@ LL |     pub fn transparent_str(p: TransparentStr);
    = note: string slices have no C equivalent
 
 error: `extern` block uses type `Box<u32>`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:70:30
+  --> $DIR/lint-ctypes.rs:71:30
    |
 LL |     pub fn transparent_fn(p: TransparentBadFn);
    |                              ^^^^^^^^^^^^^^^^ not FFI-safe
@@ -207,7 +207,7 @@ LL |     pub fn transparent_fn(p: TransparentBadFn);
    = note: this struct has unspecified layout
 
 error: `extern` block uses type `[u8; 8]`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:71:27
+  --> $DIR/lint-ctypes.rs:72:27
    |
 LL |     pub fn raw_array(arr: [u8; 8]);
    |                           ^^^^^^^ not FFI-safe
@@ -215,8 +215,26 @@ LL |     pub fn raw_array(arr: [u8; 8]);
    = help: consider passing a pointer to the array
    = note: passing raw arrays by value is not FFI-safe
 
+error: `extern` block uses type `Option<UnsafeCell<extern "C" fn()>>`, which is not FFI-safe
+  --> $DIR/lint-ctypes.rs:74:26
+   |
+LL |     pub fn no_niche_a(a: Option<UnsafeCell<extern fn()>>);
+   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: `extern` block uses type `Option<UnsafeCell<&i32>>`, which is not FFI-safe
+  --> $DIR/lint-ctypes.rs:76:26
+   |
+LL |     pub fn no_niche_b(b: Option<UnsafeCell<&i32>>);
+   |                          ^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
 error: `extern` block uses type `u128`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:73:34
+  --> $DIR/lint-ctypes.rs:79:34
    |
 LL |     pub static static_u128_type: u128;
    |                                  ^^^^ not FFI-safe
@@ -224,12 +242,12 @@ LL |     pub static static_u128_type: u128;
    = note: 128-bit integers don't currently have a known stable ABI
 
 error: `extern` block uses type `u128`, which is not FFI-safe
-  --> $DIR/lint-ctypes.rs:74:40
+  --> $DIR/lint-ctypes.rs:80:40
    |
 LL |     pub static static_u128_array_type: [u128; 16];
    |                                        ^^^^^^^^^^ not FFI-safe
    |
    = note: 128-bit integers don't currently have a known stable ABI
 
-error: aborting due to 24 previous errors
+error: aborting due to 26 previous errors