diff --git a/src/test/ui/lint/auxiliary/external_extern_fn.rs b/src/test/ui/lint/auxiliary/external_extern_fn.rs
index b2caebc6fee..c2a8cadc6b5 100644
--- a/src/test/ui/lint/auxiliary/external_extern_fn.rs
+++ b/src/test/ui/lint/auxiliary/external_extern_fn.rs
@@ -1,3 +1,3 @@
-extern {
+extern "C" {
     pub fn extern_fn(x: u8);
 }
diff --git a/src/test/ui/lint/clashing-extern-fn.rs b/src/test/ui/lint/clashing-extern-fn.rs
index 544614100ba..72c5d3da4b0 100644
--- a/src/test/ui/lint/clashing-extern-fn.rs
+++ b/src/test/ui/lint/clashing-extern-fn.rs
@@ -3,52 +3,40 @@
 #![crate_type = "lib"]
 #![warn(clashing_extern_declarations)]
 
+mod redeclared_different_signature {
+    mod a {
+        extern "C" {
+            fn clash(x: u8);
+        }
+    }
+    mod b {
+        extern "C" {
+            fn clash(x: u64); //~ WARN `clash` redeclared with a different signature
+        }
+    }
+}
+
+mod redeclared_same_signature {
+    mod a {
+        extern "C" {
+            fn no_clash(x: u8);
+        }
+    }
+    mod b {
+        extern "C" {
+            fn no_clash(x: u8);
+        }
+    }
+}
+
 extern crate external_extern_fn;
-
-extern "C" {
-    fn clash(x: u8);
-    fn no_clash(x: u8);
-}
-
-fn redeclared_different_signature() {
+mod extern_no_clash {
+    // Should not clash with external_extern_fn::extern_fn.
     extern "C" {
-        fn clash(x: u64); //~ WARN `clash` redeclared with a different signature
-    }
-
-    unsafe {
-        clash(123);
-        no_clash(123);
+        fn extern_fn(x: u8);
     }
 }
 
-fn redeclared_same_signature() {
-    extern "C" {
-        fn no_clash(x: u8);
-    }
-    unsafe {
-        no_clash(123);
-    }
-}
-
-extern "C" {
-    fn extern_fn(x: u64);
-}
-
-fn extern_clash() {
-    extern "C" {
-        fn extern_fn(x: u32); //~ WARN `extern_fn` redeclared with a different signature
-    }
-    unsafe {
-        extern_fn(123);
-    }
-}
-
-fn extern_no_clash() {
-    unsafe {
-        external_extern_fn::extern_fn(123);
-        crate::extern_fn(123);
-    }
-}
 extern "C" {
     fn some_other_new_name(x: i16);
 
@@ -134,9 +122,9 @@ mod banana {
             weight: u32,
             length: u16,
         } // note: distinct type
-        extern "C" {
           // This should not trigger the lint because two::Banana is structurally equivalent to
           // one::Banana.
+        extern "C" {
             fn weigh_banana(count: *const Banana) -> u64;
         }
     }
@@ -180,7 +168,93 @@ mod sameish_members {
         // always be the case, for every architecture and situation. This is also a really odd
         // thing to do anyway.
         extern "C" {
-            fn draw_point(p: Point); //~ WARN `draw_point` redeclared with a different
+            fn draw_point(p: Point);
+            //~^ WARN `draw_point` redeclared with a different signature
+        }
+    }
+}
+
+mod transparent {
+    #[repr(transparent)]
+    struct T(usize);
+    mod a {
+        use super::T;
+        extern "C" {
+            fn transparent() -> T;
+        }
+    }
+
+    mod b {
+        extern "C" {
+            // Shouldn't warn here, because repr(transparent) guarantees that T's layout is the
+            // same as just the usize.
+            fn transparent() -> usize;
+        }
+    }
+}
+
+mod missing_return_type {
+    mod a {
+        extern "C" {
+            fn missing_return_type() -> usize;
+        }
+    }
+
+    mod b {
+        extern "C" {
+            // This should warn, because we can't assume that the first declaration is the correct
+            // one -- if this one is the correct one, then calling the usize-returning version
+            // would allow reads into uninitialised memory.
+            fn missing_return_type();
+            //~^ WARN `missing_return_type` redeclared with a different signature
+        }
+    }
+}
+
+mod non_zero_and_non_null {
+    mod a {
+        extern "C" {
+            fn non_zero_usize() -> core::num::NonZeroUsize;
+            fn non_null_ptr() -> core::ptr::NonNull<usize>;
+        }
+    }
+    mod b {
+        extern "C" {
+            // For both of these cases, if there's a clash, you're either gaining an incorrect
+            // invariant that the value is non-zero, or you're missing out on that invariant. Both
+            // cases are warning for, from both a caller-convenience and optimisation perspective.
+            fn non_zero_usize() -> usize;
+            //~^ WARN `non_zero_usize` redeclared with a different signature
+            fn non_null_ptr() -> *const usize;
+            //~^ WARN `non_null_ptr` redeclared with a different signature
+        }
+    }
+}
+
+mod null_optimised_enums {
+    mod a {
+        extern "C" {
+            fn option_non_zero_usize() -> usize;
+            fn option_non_zero_isize() -> isize;
+            fn option_non_null_ptr() -> *const usize;
+
+            fn option_non_zero_usize_incorrect() -> usize;
+            fn option_non_null_ptr_incorrect() -> *const usize;
+        }
+    }
+    mod b {
+        extern "C" {
+            // This should be allowed, because these conversions are guaranteed to be FFI-safe (see
+            // #60300)
+            fn option_non_zero_usize() -> Option<core::num::NonZeroUsize>;
+            fn option_non_zero_isize() -> Option<core::num::NonZeroIsize>;
+            fn option_non_null_ptr() -> Option<core::ptr::NonNull<usize>>;
+
+            // However, these should be incorrect (note isize instead of usize)
+            fn option_non_zero_usize_incorrect() -> isize;
+            //~^ WARN `option_non_zero_usize_incorrect` redeclared with a different signature
+            fn option_non_null_ptr_incorrect() -> *const isize;
+            //~^ WARN `option_non_null_ptr_incorrect` redeclared with a different signature
         }
     }
 }
diff --git a/src/test/ui/lint/clashing-extern-fn.stderr b/src/test/ui/lint/clashing-extern-fn.stderr
index 96e51ab5a3e..a2aaaba7173 100644
--- a/src/test/ui/lint/clashing-extern-fn.stderr
+++ b/src/test/ui/lint/clashing-extern-fn.stderr
@@ -1,11 +1,11 @@
 warning: `clash` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:15:9
+  --> $DIR/clashing-extern-fn.rs:14:13
    |
-LL |     fn clash(x: u8);
-   |     ---------------- `clash` previously declared here
+LL |             fn clash(x: u8);
+   |             ---------------- `clash` previously declared here
 ...
-LL |         fn clash(x: u64);
-   |         ^^^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
+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
@@ -15,20 +15,8 @@ LL | #![warn(clashing_extern_declarations)]
    = note: expected `unsafe extern "C" fn(u8)`
               found `unsafe extern "C" fn(u64)`
 
-warning: `extern_fn` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:39:9
-   |
-LL |     fn extern_fn(x: u64);
-   |     --------------------- `extern_fn` previously declared here
-...
-LL |         fn extern_fn(x: u32);
-   |         ^^^^^^^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
-   |
-   = note: expected `unsafe extern "C" fn(u64)`
-              found `unsafe extern "C" fn(u32)`
-
 warning: `extern_link_name` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:64:9
+  --> $DIR/clashing-extern-fn.rs:52:9
    |
 LL | /     #[link_name = "extern_link_name"]
 LL | |     fn some_new_name(x: i16);
@@ -41,7 +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:67:9
+  --> $DIR/clashing-extern-fn.rs:55:9
    |
 LL |       fn some_other_new_name(x: i16);
    |       ------------------------------- `some_other_new_name` previously declared here
@@ -55,7 +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:71:9
+  --> $DIR/clashing-extern-fn.rs:59:9
    |
 LL | /     #[link_name = "link_name_same"]
 LL | |     fn both_names_different(x: i16);
@@ -70,7 +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:84:9
+  --> $DIR/clashing-extern-fn.rs:72:9
    |
 LL |         fn different_mod(x: u8);
    |         ------------------------ `different_mod` previously declared here
@@ -82,7 +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:94:9
+  --> $DIR/clashing-extern-fn.rs:82:9
    |
 LL |     fn variadic_decl(x: u8, ...);
    |     ----------------------------- `variadic_decl` previously declared here
@@ -94,7 +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:154:13
+  --> $DIR/clashing-extern-fn.rs:142:13
    |
 LL |             fn weigh_banana(count: *const Banana) -> u64;
    |             --------------------------------------------- `weigh_banana` previously declared here
@@ -106,7 +94,7 @@ LL |             fn weigh_banana(count: *const Banana) -> u64;
               found `unsafe extern "C" fn(*const banana::three::Banana) -> u64`
 
 warning: `draw_point` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:183:13
+  --> $DIR/clashing-extern-fn.rs:171:13
    |
 LL |             fn draw_point(p: Point);
    |             ------------------------ `draw_point` previously declared here
@@ -117,5 +105,5 @@ LL |             fn draw_point(p: Point);
    = note: expected `unsafe extern "C" fn(sameish_members::a::Point)`
               found `unsafe extern "C" fn(sameish_members::b::Point)`
 
-warning: 9 warnings emitted
+warning: 8 warnings emitted