1 files changed, 417 insertions, 0 deletions
diff --git a/tests/ui/lint/clashing-extern-fn.rs b/tests/ui/lint/clashing-extern-fn.rs
new file mode 100644
index 00000000000..809e0602671
--- /dev/null
+++ b/tests/ui/lint/clashing-extern-fn.rs
@@ -0,0 +1,417 @@
+// check-pass
+// aux-build:external_extern_fn.rs
+#![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;
+mod extern_no_clash {
+    // Should not clash with external_extern_fn::extern_fn.
+    extern "C" {
+        fn extern_fn(x: u8);
+    }
+}
+
+extern "C" {
+    fn some_other_new_name(x: i16);
+
+    #[link_name = "extern_link_name"]
+    fn some_new_name(x: i16);
+
+    #[link_name = "link_name_same"]
+    fn both_names_different(x: i16);
+}
+
+fn link_name_clash() {
+    extern "C" {
+        fn extern_link_name(x: u32);
+        //~^ WARN `extern_link_name` redeclared with a different signature
+
+        #[link_name = "some_other_new_name"]
+        //~^ WARN `some_other_extern_link_name` redeclares `some_other_new_name` with a different
+        fn some_other_extern_link_name(x: u32);
+
+        #[link_name = "link_name_same"]
+        //~^ WARN `other_both_names_different` redeclares `link_name_same` with a different
+        fn other_both_names_different(x: u32);
+    }
+}
+
+mod a {
+    extern "C" {
+        fn different_mod(x: u8);
+    }
+}
+mod b {
+    extern "C" {
+        fn different_mod(x: u64); //~ WARN `different_mod` redeclared with a different signature
+    }
+}
+
+extern "C" {
+    fn variadic_decl(x: u8, ...);
+}
+
+fn variadic_clash() {
+    extern "C" {
+        fn variadic_decl(x: u8); //~ WARN `variadic_decl` redeclared with a different signature
+    }
+}
+
+#[no_mangle]
+fn no_mangle_name(x: u8) {}
+
+extern "C" {
+    #[link_name = "unique_link_name"]
+    fn link_name_specified(x: u8);
+}
+
+fn tricky_no_clash() {
+    extern "C" {
+        // Shouldn't warn, because the declaration above actually declares a different symbol (and
+        // Rust's name resolution rules around shadowing will handle this gracefully).
+        fn link_name_specified() -> u32;
+
+        // The case of a no_mangle name colliding with an extern decl (see #28179) is related but
+        // shouldn't be reported by ClashingExternDeclarations, because this is an example of
+        // unmangled name clash causing bad behaviour in functions with a defined body.
+        fn no_mangle_name() -> u32;
+    }
+}
+
+mod banana {
+    mod one {
+        #[repr(C)]
+        struct Banana {
+            weight: u32,
+            length: u16,
+        }
+        extern "C" {
+            fn weigh_banana(count: *const Banana) -> u64;
+        }
+    }
+
+    mod two {
+        #[repr(C)]
+        struct Banana {
+            weight: u32,
+            length: u16,
+        } // note: distinct type
+          // This should not trigger the lint because two::Banana is structurally equivalent to
+          // one::Banana.
+        extern "C" {
+            fn weigh_banana(count: *const Banana) -> u64;
+        }
+    }
+
+    mod three {
+        // This _should_ trigger the lint, because repr(packed) should generate a struct that has a
+        // different layout.
+        #[repr(packed)]
+        struct Banana {
+            weight: u32,
+            length: u16,
+        }
+        #[allow(improper_ctypes)]
+        extern "C" {
+            fn weigh_banana(count: *const Banana) -> u64;
+            //~^ WARN `weigh_banana` redeclared with a different signature
+        }
+    }
+}
+
+mod sameish_members {
+    mod a {
+        #[repr(C)]
+        struct Point {
+            x: i16,
+            y: i16,
+        }
+
+        extern "C" {
+            fn draw_point(p: Point);
+        }
+    }
+    mod b {
+        #[repr(C)]
+        struct Point {
+            coordinates: [i16; 2],
+        }
+
+        // It's possible we are overconservative for this case, as accessing the elements of the
+        // coordinates array might end up correctly accessing `.x` and `.y`. However, this may not
+        // 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 signature
+        }
+    }
+}
+
+mod same_sized_members_clash {
+    mod a {
+        #[repr(C)]
+        struct Point3 {
+            x: f32,
+            y: f32,
+            z: f32,
+        }
+        extern "C" {
+            fn origin() -> Point3;
+        }
+    }
+    mod b {
+        #[repr(C)]
+        struct Point3 {
+            x: i32,
+            y: i32,
+            z: i32, // NOTE: Incorrectly redeclared as i32
+        }
+        extern "C" {
+            fn origin() -> Point3; //~ WARN `origin` redeclared with a different signature
+        }
+    }
+}
+
+mod transparent {
+    #[repr(transparent)]
+    struct T(usize);
+    mod a {
+        use super::T;
+        extern "C" {
+            fn transparent() -> T;
+            fn transparent_incorrect() -> 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;
+
+            // Should warn, because there's a signedness conversion here:
+            fn transparent_incorrect() -> isize;
+            //~^ WARN `transparent_incorrect` redeclared with a different signature
+        }
+    }
+}
+
+mod missing_return_type {
+    mod a {
+        extern "C" {
+            fn missing_return_type() -> usize;
+        }
+    }
+
+    mod b {
+        extern "C" {
+            // This should output a warning 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" {
+            // If there's a clash in either of these cases 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
+        }
+    }
+}
+
+// See #75739
+mod non_zero_transparent {
+    mod a1 {
+        use std::num::NonZeroUsize;
+        extern "C" {
+            fn f1() -> NonZeroUsize;
+        }
+    }
+
+    mod b1 {
+        #[repr(transparent)]
+        struct X(NonZeroUsize);
+        use std::num::NonZeroUsize;
+        extern "C" {
+            fn f1() -> X;
+        }
+    }
+
+    mod a2 {
+        use std::num::NonZeroUsize;
+        extern "C" {
+            fn f2() -> NonZeroUsize;
+        }
+    }
+
+    mod b2 {
+        #[repr(transparent)]
+        struct X1(NonZeroUsize);
+
+        #[repr(transparent)]
+        struct X(X1);
+
+        use std::num::NonZeroUsize;
+        extern "C" {
+            // Same case as above, but with two layers of newtyping.
+            fn f2() -> X;
+        }
+    }
+
+    mod a3 {
+        #[repr(transparent)]
+        struct X(core::ptr::NonNull<i32>);
+
+        use std::num::NonZeroUsize;
+        extern "C" {
+            fn f3() -> X;
+        }
+    }
+
+    mod b3 {
+        extern "C" {
+            fn f3() -> core::ptr::NonNull<i32>;
+        }
+    }
+
+    mod a4 {
+        #[repr(transparent)]
+        enum E {
+            X(std::num::NonZeroUsize),
+        }
+        extern "C" {
+            fn f4() -> E;
+        }
+    }
+
+    mod b4 {
+        extern "C" {
+            fn f4() -> std::num::NonZeroUsize;
+        }
+    }
+}
+
+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
+        }
+    }
+}
+
+#[allow(improper_ctypes)]
+mod unknown_layout {
+    mod a {
+        extern "C" {
+            pub fn generic(l: Link<u32>);
+        }
+        pub struct Link<T> {
+            pub item: T,
+            pub next: *const Link<T>,
+        }
+    }
+
+    mod b {
+        extern "C" {
+            pub fn generic(l: Link<u32>);
+        }
+        pub struct Link<T> {
+            pub item: T,
+            pub next: *const Link<T>,
+        }
+    }
+}
+
+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(transparent)]
+        struct TransparentNoNiche { y: UnsafeCell<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
+        }
+    }
+}