Rollup merge of #72700 - davidtwco:issue-66220-improper-ctypes-declarations, r=lcnr,varkor

`improper_ctypes_definitions` lint

Addresses #19834, #66220, and #66373.

This PR takes another attempt at #65134 (reverted in #66378). Instead of modifying the existing `improper_ctypes` lint to consider `extern "C" fn` definitions in addition to `extern "C" {}` declarations, this PR adds a new lint - `improper_ctypes_definitions` - which only applies to `extern "C" fn` definitions.

In addition, the `improper_ctype_definitions` lint differs from `improper_ctypes` by considering `*T` and `&T` (where `T: Sized`) FFI-safe (addressing #66220).

There wasn't a clear consensus in #66220 (where the issues with #65134 were primarily discussed) on the approach to take, but there has [been some discussion in Zulip](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/.2366220.20improper_ctypes.20definitions.20vs.20declarations/near/198903086). I fully expect that we'll want to iterate on this before landing.

cc @varkor + @shepmaster (from #19834) @hanna-kruppe (active in discussing #66220), @SimonSapin (#65134 caused problems for Servo, want to make sure that this PR doesn't)

src/liballoc/boxed.rs

@@ -92,11 +92,13 @@
 //! pub struct Foo;
 //!
 //! #[no_mangle]
+//! #[allow(improper_ctypes_definitions)]
 //! pub extern "C" fn foo_new() -> Box<Foo> {
 //!     Box::new(Foo)
 //! }
 //!
 //! #[no_mangle]
+//! #[allow(improper_ctypes_definitions)]
 //! pub extern "C" fn foo_delete(_: Option<Box<Foo>>) {}
 //! ```
 //!

src/libpanic_abort/lib.rs

@@ -21,6 +21,7 @@
 use core::any::Any;
 
 #[rustc_std_internal_symbol]
+#[cfg_attr(not(bootstrap), allow(improper_ctypes_definitions))]
 pub unsafe extern "C" fn __rust_panic_cleanup(_: *mut u8) -> *mut (dyn Any + Send + 'static) {
     unreachable!()
 }

src/libpanic_unwind/lib.rs

@@ -81,6 +81,7 @@ extern "C" {
 mod dwarf;
 
 #[rustc_std_internal_symbol]
+#[cfg_attr(not(bootstrap), allow(improper_ctypes_definitions))]
 pub unsafe extern "C" fn __rust_panic_cleanup(payload: *mut u8) -> *mut (dyn Any + Send + 'static) {
     Box::into_raw(imp::cleanup(payload))
 }

src/librustc_lint/lib.rs

@@ -167,7 +167,8 @@ macro_rules! late_lint_mod_passes {
             $args,
             [
                 HardwiredLints: HardwiredLints,
-                ImproperCTypes: ImproperCTypes,
+                ImproperCTypesDeclarations: ImproperCTypesDeclarations,
+                ImproperCTypesDefinitions: ImproperCTypesDefinitions,
                 VariantSizeDifferences: VariantSizeDifferences,
                 BoxPointers: BoxPointers,
                 PathStatements: PathStatements,

src/librustc_lint/types.rs

@@ -14,7 +14,7 @@ use rustc_middle::ty::subst::SubstsRef;
 use rustc_middle::ty::{self, AdtKind, ParamEnv, Ty, TyCtxt, TypeFoldable};
 use rustc_span::source_map;
 use rustc_span::symbol::sym;
-use rustc_span::Span;
+use rustc_span::{Span, DUMMY_SP};
 use rustc_target::abi::{Integer, LayoutOf, TagEncoding, VariantIdx, Variants};
 use rustc_target::spec::abi::Abi;
 
@@ -498,10 +498,24 @@ declare_lint! {
     "proper use of libc types in foreign modules"
 }
 
-declare_lint_pass!(ImproperCTypes => [IMPROPER_CTYPES]);
+declare_lint_pass!(ImproperCTypesDeclarations => [IMPROPER_CTYPES]);
+
+declare_lint! {
+    IMPROPER_CTYPES_DEFINITIONS,
+    Warn,
+    "proper use of libc types in foreign item definitions"
+}
+
+declare_lint_pass!(ImproperCTypesDefinitions => [IMPROPER_CTYPES_DEFINITIONS]);
+
+enum ImproperCTypesMode {
+    Declarations,
+    Definitions,
+}
 
 struct ImproperCTypesVisitor<'a, 'tcx> {
     cx: &'a LateContext<'a, 'tcx>,
+    mode: ImproperCTypesMode,
 }
 
 enum FfiResult<'tcx> {
@@ -804,6 +818,15 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                 help: Some("consider using a struct instead".into()),
             },
 
+            ty::RawPtr(ty::TypeAndMut { ty, .. }) | ty::Ref(_, ty, _)
+                if {
+                    matches!(self.mode, ImproperCTypesMode::Definitions)
+                        && ty.is_sized(self.cx.tcx.at(DUMMY_SP), self.cx.param_env)
+                } =>
+            {
+                FfiSafe
+            }
+
             ty::RawPtr(ty::TypeAndMut { ty, .. }) | ty::Ref(_, ty, _) => {
                 self.check_type_for_ffi(cache, ty)
             }
@@ -811,20 +834,16 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
             ty::Array(inner_ty, _) => self.check_type_for_ffi(cache, inner_ty),
 
             ty::FnPtr(sig) => {
-                match sig.abi() {
-                    Abi::Rust | Abi::RustIntrinsic | Abi::PlatformIntrinsic | Abi::RustCall => {
-                        return FfiUnsafe {
-                            ty,
-                            reason: "this function pointer has Rust-specific calling convention"
+                if self.is_internal_abi(sig.abi()) {
+                    return FfiUnsafe {
+                        ty,
+                        reason: "this function pointer has Rust-specific calling convention".into(),
+                        help: Some(
+                            "consider using an `extern fn(...) -> ...` \
+                                    function pointer instead"
                                 .into(),
-                            help: Some(
-                                "consider using an `extern fn(...) -> ...` \
-                                        function pointer instead"
-                                    .into(),
-                            ),
-                        };
-                    }
-                    _ => {}
+                        ),
+                    };
                 }
 
                 let sig = cx.erase_late_bound_regions(&sig);
@@ -857,7 +876,16 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                 FfiUnsafe { ty, reason: "opaque types have no C equivalent".into(), help: None }
             }
 
+            // `extern "C" fn` functions can have type parameters, which may or may not be FFI-safe,
+            //  so they are currently ignored for the purposes of this lint.
+            ty::Param(..) | ty::Projection(..)
+                if matches!(self.mode, ImproperCTypesMode::Definitions) =>
+            {
+                FfiSafe
+            }
+
             ty::Param(..)
+            | ty::Projection(..)
             | ty::Infer(..)
             | ty::Bound(..)
             | ty::Error(_)
@@ -865,7 +893,6 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
             | ty::Generator(..)
             | ty::GeneratorWitness(..)
             | ty::Placeholder(..)
-            | ty::Projection(..)
             | ty::FnDef(..) => bug!("unexpected type in foreign function: {:?}", ty),
         }
     }
@@ -877,9 +904,20 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
         note: &str,
         help: Option<&str>,
     ) {
-        self.cx.struct_span_lint(IMPROPER_CTYPES, sp, |lint| {
-            let mut diag =
-                lint.build(&format!("`extern` block uses type `{}`, which is not FFI-safe", ty));
+        let lint = match self.mode {
+            ImproperCTypesMode::Declarations => IMPROPER_CTYPES,
+            ImproperCTypesMode::Definitions => IMPROPER_CTYPES_DEFINITIONS,
+        };
+
+        self.cx.struct_span_lint(lint, sp, |lint| {
+            let item_description = match self.mode {
+                ImproperCTypesMode::Declarations => "block",
+                ImproperCTypesMode::Definitions => "fn",
+            };
+            let mut diag = lint.build(&format!(
+                "`extern` {} uses type `{}`, which is not FFI-safe",
+                item_description, ty
+            ));
             diag.span_label(sp, "not FFI-safe");
             if let Some(help) = help {
                 diag.help(help);
@@ -947,7 +985,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
 
         // it is only OK to use this function because extern fns cannot have
         // any generic types right now:
-        let ty = self.cx.tcx.normalize_erasing_regions(ParamEnv::reveal_all(), ty);
+        let ty = self.cx.tcx.normalize_erasing_regions(self.cx.param_env, ty);
 
         // C doesn't really support passing arrays by value - the only way to pass an array by value
         // is through a struct. So, first test that the top level isn't an array, and then
@@ -997,15 +1035,22 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
         let ty = self.cx.tcx.type_of(def_id);
         self.check_type_for_ffi_and_report_errors(span, ty, true, false);
     }
+
+    fn is_internal_abi(&self, abi: Abi) -> bool {
+        if let Abi::Rust | Abi::RustCall | Abi::RustIntrinsic | Abi::PlatformIntrinsic = abi {
+            true
+        } else {
+            false
+        }
+    }
 }
 
-impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ImproperCTypes {
+impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ImproperCTypesDeclarations {
     fn check_foreign_item(&mut self, cx: &LateContext<'_, '_>, it: &hir::ForeignItem<'_>) {
-        let mut vis = ImproperCTypesVisitor { cx };
+        let mut vis = ImproperCTypesVisitor { cx, mode: ImproperCTypesMode::Declarations };
         let abi = cx.tcx.hir().get_foreign_abi(it.hir_id);
-        if let Abi::Rust | Abi::RustCall | Abi::RustIntrinsic | Abi::PlatformIntrinsic = abi {
-            // Don't worry about types in internal ABIs.
-        } else {
+
+        if !vis.is_internal_abi(abi) {
             match it.kind {
                 hir::ForeignItemKind::Fn(ref decl, _, _) => {
                     vis.check_foreign_fn(it.hir_id, decl);
@@ -1019,6 +1064,31 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ImproperCTypes {
     }
 }
 
+impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ImproperCTypesDefinitions {
+    fn check_fn(
+        &mut self,
+        cx: &LateContext<'a, 'tcx>,
+        kind: hir::intravisit::FnKind<'tcx>,
+        decl: &'tcx hir::FnDecl<'_>,
+        _: &'tcx hir::Body<'_>,
+        _: Span,
+        hir_id: hir::HirId,
+    ) {
+        use hir::intravisit::FnKind;
+
+        let abi = match kind {
+            FnKind::ItemFn(_, _, header, ..) => header.abi,
+            FnKind::Method(_, sig, ..) => sig.header.abi,
+            _ => return,
+        };
+
+        let mut vis = ImproperCTypesVisitor { cx, mode: ImproperCTypesMode::Definitions };
+        if !vis.is_internal_abi(abi) {
+            vis.check_foreign_fn(hir_id, decl);
+        }
+    }
+}
+
 declare_lint_pass!(VariantSizeDifferences => [VARIANT_SIZE_DIFFERENCES]);
 
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for VariantSizeDifferences {

src/librustc_llvm/lib.rs

@@ -15,6 +15,7 @@ pub struct RustString {
 
 /// Appending to a Rust string -- used by RawRustStringOstream.
 #[no_mangle]
+#[cfg_attr(not(bootstrap), allow(improper_ctypes_definitions))]
 pub unsafe extern "C" fn LLVMRustStringWriteImpl(
     sr: &RustString,
     ptr: *const c_char,

src/libstd/sys/sgx/abi/mod.rs

@@ -56,6 +56,7 @@ unsafe extern "C" fn tcs_init(secondary: bool) {
 // able to specify this
 #[cfg(not(test))]
 #[no_mangle]
+#[allow(improper_ctypes_definitions)]
 extern "C" fn entry(p1: u64, p2: u64, p3: u64, secondary: bool, p4: u64, p5: u64) -> (u64, u64) {
     // FIXME: how to support TLS in library mode?
     let tls = Box::new(tls::Tls::new());

src/test/ui/abi/abi-sysv64-register-usage.rs

@@ -38,6 +38,7 @@ pub struct LargeStruct(i64, i64, i64, i64, i64, i64, i64, i64);
 
 #[cfg(target_arch = "x86_64")]
 #[inline(never)]
+#[allow(improper_ctypes_definitions)]
 pub extern "sysv64" fn large_struct_by_val(mut foo: LargeStruct) -> LargeStruct {
     foo.0 *= 1;
     foo.1 *= 2;

src/test/ui/align-with-extern-c-fn.rs

@@ -10,6 +10,7 @@
 #[repr(align(16))]
 pub struct A(i64);
 
+#[allow(improper_ctypes_definitions)]
 pub extern "C" fn foo(x: A) {}
 
 fn main() {

src/test/ui/issues/issue-16441.rs

@@ -5,6 +5,7 @@
 struct Empty;
 
 // This used to cause an ICE
+#[allow(improper_ctypes_definitions)]
 extern "C" fn ice(_a: Empty) {}
 
 fn main() {

src/test/ui/issues/issue-26997.rs

@@ -6,6 +6,7 @@ pub struct Foo {
 }
 
 impl Foo {
+    #[allow(improper_ctypes_definitions)]
     pub extern fn foo_new() -> Foo {
         Foo { x: 21, y: 33 }
     }

src/test/ui/issues/issue-28600.rs

@@ -6,6 +6,7 @@ struct Test;
 impl Test {
     #[allow(dead_code)]
     #[allow(unused_variables)]
+    #[allow(improper_ctypes_definitions)]
     pub extern fn test(val: &str) {
 
     }

src/test/ui/issues/issue-38763.rs

@@ -5,6 +5,7 @@
 pub struct Foo(i128);
 
 #[no_mangle]
+#[allow(improper_ctypes_definitions)]
 pub extern "C" fn foo(x: Foo) -> Foo { x }
 
 fn main() {

src/test/ui/issues/issue-51907.rs

@@ -6,7 +6,9 @@ trait Foo {
 
 struct Bar;
 impl Foo for Bar {
+    #[allow(improper_ctypes_definitions)]
     extern fn borrow(&self) {}
+    #[allow(improper_ctypes_definitions)]
     extern fn take(self: Box<Self>) {}
 }
 

src/test/ui/lint/lint-ctypes-fn.rs

@@ -0,0 +1,182 @@
+#![feature(rustc_private)]
+
+#![allow(private_in_public)]
+#![deny(improper_ctypes_definitions)]
+
+extern crate libc;
+
+use std::default::Default;
+use std::marker::PhantomData;
+
+trait Mirror { type It: ?Sized; }
+
+impl<T: ?Sized> Mirror for T { type It = Self; }
+
+#[repr(C)]
+pub struct StructWithProjection(*mut <StructWithProjection as Mirror>::It);
+
+#[repr(C)]
+pub struct StructWithProjectionAndLifetime<'a>(
+    &'a mut <StructWithProjectionAndLifetime<'a> as Mirror>::It
+);
+
+pub type I32Pair = (i32, i32);
+
+#[repr(C)]
+pub struct ZeroSize;
+
+pub type RustFn = fn();
+
+pub type RustBadRet = extern fn() -> Box<u32>;
+
+pub type CVoidRet = ();
+
+pub struct Foo;
+
+#[repr(transparent)]
+pub struct TransparentI128(i128);
+
+#[repr(transparent)]
+pub struct TransparentStr(&'static str);
+
+#[repr(transparent)]
+pub struct TransparentBadFn(RustBadRet);
+
+#[repr(transparent)]
+pub struct TransparentInt(u32);
+
+#[repr(transparent)]
+pub struct TransparentRef<'a>(&'a TransparentInt);
+
+#[repr(transparent)]
+pub struct TransparentLifetime<'a>(*const u8, PhantomData<&'a ()>);
+
+#[repr(transparent)]
+pub struct TransparentUnit<U>(f32, PhantomData<U>);
+
+#[repr(transparent)]
+pub struct TransparentCustomZst(i32, ZeroSize);
+
+#[repr(C)]
+pub struct ZeroSizeWithPhantomData(PhantomData<i32>);
+
+pub extern "C" fn ptr_type1(size: *const Foo) { }
+
+pub extern "C" fn ptr_type2(size: *const Foo) { }
+
+pub extern "C" fn slice_type(p: &[u32]) { }
+//~^ ERROR: uses type `[u32]`
+
+pub extern "C" fn str_type(p: &str) { }
+//~^ ERROR: uses type `str`
+
+pub extern "C" fn box_type(p: Box<u32>) { }
+//~^ ERROR uses type `std::boxed::Box<u32>`
+
+pub extern "C" fn char_type(p: char) { }
+//~^ ERROR uses type `char`
+
+pub extern "C" fn i128_type(p: i128) { }
+//~^ ERROR uses type `i128`
+
+pub extern "C" fn u128_type(p: u128) { }
+//~^ ERROR uses type `u128`
+
+pub extern "C" fn tuple_type(p: (i32, i32)) { }
+//~^ ERROR uses type `(i32, i32)`
+
+pub extern "C" fn tuple_type2(p: I32Pair) { }
+//~^ ERROR uses type `(i32, i32)`
+
+pub extern "C" fn zero_size(p: ZeroSize) { }
+//~^ ERROR uses type `ZeroSize`
+
+pub extern "C" fn zero_size_phantom(p: ZeroSizeWithPhantomData) { }
+//~^ ERROR uses type `ZeroSizeWithPhantomData`
+
+pub extern "C" fn zero_size_phantom_toplevel() -> PhantomData<bool> {
+//~^ ERROR uses type `std::marker::PhantomData<bool>`
+    Default::default()
+}
+
+pub extern "C" fn fn_type(p: RustFn) { }
+//~^ ERROR uses type `fn()`
+
+pub extern "C" fn fn_type2(p: fn()) { }
+//~^ ERROR uses type `fn()`
+
+pub extern "C" fn fn_contained(p: RustBadRet) { }
+//~^ ERROR: uses type `std::boxed::Box<u32>`
+
+pub extern "C" fn transparent_i128(p: TransparentI128) { }
+//~^ ERROR: uses type `i128`
+
+pub extern "C" fn transparent_str(p: TransparentStr) { }
+//~^ ERROR: uses type `str`
+
+pub extern "C" fn transparent_fn(p: TransparentBadFn) { }
+//~^ ERROR: uses type `std::boxed::Box<u32>`
+
+pub extern "C" fn good3(fptr: Option<extern fn()>) { }
+
+pub extern "C" fn good4(aptr: &[u8; 4 as usize]) { }
+
+pub extern "C" fn good5(s: StructWithProjection) { }
+
+pub extern "C" fn good6(s: StructWithProjectionAndLifetime) { }
+
+pub extern "C" fn good7(fptr: extern fn() -> ()) { }
+
+pub extern "C" fn good8(fptr: extern fn() -> !) { }
+
+pub extern "C" fn good9() -> () { }
+
+pub extern "C" fn good10() -> CVoidRet { }
+
+pub extern "C" fn good11(size: isize) { }
+
+pub extern "C" fn good12(size: usize) { }
+
+pub extern "C" fn good13(n: TransparentInt) { }
+
+pub extern "C" fn good14(p: TransparentRef) { }
+
+pub extern "C" fn good15(p: TransparentLifetime) { }
+
+pub extern "C" fn good16(p: TransparentUnit<ZeroSize>) { }
+
+pub extern "C" fn good17(p: TransparentCustomZst) { }
+
+#[allow(improper_ctypes_definitions)]
+pub extern "C" fn good18(_: &String) { }
+
+#[cfg(not(target_arch = "wasm32"))]
+pub extern "C" fn good1(size: *const libc::c_int) { }
+
+#[cfg(not(target_arch = "wasm32"))]
+pub extern "C" fn good2(size: *const libc::c_uint) { }
+
+pub extern "C" fn unused_generic1<T>(size: *const Foo) { }
+
+pub extern "C" fn unused_generic2<T>() -> PhantomData<bool> {
+//~^ ERROR uses type `std::marker::PhantomData<bool>`
+    Default::default()
+}
+
+pub extern "C" fn used_generic1<T>(x: T) { }
+
+pub extern "C" fn used_generic2<T>(x: T, size: *const Foo) { }
+
+pub extern "C" fn used_generic3<T: Default>() -> T {
+    Default::default()
+}
+
+pub extern "C" fn used_generic4<T>(x: Vec<T>) { }
+//~^ ERROR: uses type `std::vec::Vec<T>`
+
+pub extern "C" fn used_generic5<T>() -> Vec<T> {
+//~^ ERROR: uses type `std::vec::Vec<T>`
+    Default::default()
+}
+
+fn main() {}

src/test/ui/lint/lint-ctypes-fn.stderr

@@ -0,0 +1,191 @@
+error: `extern` fn uses type `[u32]`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:67:33
+   |
+LL | pub extern "C" fn slice_type(p: &[u32]) { }
+   |                                 ^^^^^^ not FFI-safe
+   |
+note: the lint level is defined here
+  --> $DIR/lint-ctypes-fn.rs:4:9
+   |
+LL | #![deny(improper_ctypes_definitions)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = help: consider using a raw pointer instead
+   = note: slices have no C equivalent
+
+error: `extern` fn uses type `str`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:70:31
+   |
+LL | pub extern "C" fn str_type(p: &str) { }
+   |                               ^^^^ not FFI-safe
+   |
+   = help: consider using `*const u8` and a length instead
+   = note: string slices have no C equivalent
+
+error: `extern` fn uses type `std::boxed::Box<u32>`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:73:31
+   |
+LL | pub extern "C" fn box_type(p: Box<u32>) { }
+   |                               ^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
+   = note: this struct has unspecified layout
+
+error: `extern` fn uses type `char`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:76:32
+   |
+LL | pub extern "C" fn char_type(p: char) { }
+   |                                ^^^^ not FFI-safe
+   |
+   = help: consider using `u32` or `libc::wchar_t` instead
+   = note: the `char` type has no C equivalent
+
+error: `extern` fn uses type `i128`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:79:32
+   |
+LL | pub extern "C" fn i128_type(p: i128) { }
+   |                                ^^^^ not FFI-safe
+   |
+   = note: 128-bit integers don't currently have a known stable ABI
+
+error: `extern` fn uses type `u128`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:82:32
+   |
+LL | pub extern "C" fn u128_type(p: u128) { }
+   |                                ^^^^ not FFI-safe
+   |
+   = note: 128-bit integers don't currently have a known stable ABI
+
+error: `extern` fn uses type `(i32, i32)`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:85:33
+   |
+LL | pub extern "C" fn tuple_type(p: (i32, i32)) { }
+   |                                 ^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider using a struct instead
+   = note: tuples have unspecified layout
+
+error: `extern` fn uses type `(i32, i32)`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:88:34
+   |
+LL | pub extern "C" fn tuple_type2(p: I32Pair) { }
+   |                                  ^^^^^^^ not FFI-safe
+   |
+   = help: consider using a struct instead
+   = note: tuples have unspecified layout
+
+error: `extern` fn uses type `ZeroSize`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:91:32
+   |
+LL | pub extern "C" fn zero_size(p: ZeroSize) { }
+   |                                ^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a member to this struct
+   = note: this struct has no fields
+note: the type is defined here
+  --> $DIR/lint-ctypes-fn.rs:26:1
+   |
+LL | pub struct ZeroSize;
+   | ^^^^^^^^^^^^^^^^^^^^
+
+error: `extern` fn uses type `ZeroSizeWithPhantomData`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:94:40
+   |
+LL | pub extern "C" fn zero_size_phantom(p: ZeroSizeWithPhantomData) { }
+   |                                        ^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = note: composed only of `PhantomData`
+note: the type is defined here
+  --> $DIR/lint-ctypes-fn.rs:61:1
+   |
+LL | pub struct ZeroSizeWithPhantomData(PhantomData<i32>);
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: `extern` fn uses type `std::marker::PhantomData<bool>`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:97:51
+   |
+LL | pub extern "C" fn zero_size_phantom_toplevel() -> PhantomData<bool> {
+   |                                                   ^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = note: composed only of `PhantomData`
+
+error: `extern` fn uses type `fn()`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:102:30
+   |
+LL | pub extern "C" fn fn_type(p: RustFn) { }
+   |                              ^^^^^^ not FFI-safe
+   |
+   = help: consider using an `extern fn(...) -> ...` function pointer instead
+   = note: this function pointer has Rust-specific calling convention
+
+error: `extern` fn uses type `fn()`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:105:31
+   |
+LL | pub extern "C" fn fn_type2(p: fn()) { }
+   |                               ^^^^ not FFI-safe
+   |
+   = help: consider using an `extern fn(...) -> ...` function pointer instead
+   = note: this function pointer has Rust-specific calling convention
+
+error: `extern` fn uses type `std::boxed::Box<u32>`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:108:35
+   |
+LL | pub extern "C" fn fn_contained(p: RustBadRet) { }
+   |                                   ^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
+   = note: this struct has unspecified layout
+
+error: `extern` fn uses type `i128`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:111:39
+   |
+LL | pub extern "C" fn transparent_i128(p: TransparentI128) { }
+   |                                       ^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = note: 128-bit integers don't currently have a known stable ABI
+
+error: `extern` fn uses type `str`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:114:38
+   |
+LL | pub extern "C" fn transparent_str(p: TransparentStr) { }
+   |                                      ^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider using `*const u8` and a length instead
+   = note: string slices have no C equivalent
+
+error: `extern` fn uses type `std::boxed::Box<u32>`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:117:37
+   |
+LL | pub extern "C" fn transparent_fn(p: TransparentBadFn) { }
+   |                                     ^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
+   = note: this struct has unspecified layout
+
+error: `extern` fn uses type `std::marker::PhantomData<bool>`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:161:43
+   |
+LL | pub extern "C" fn unused_generic2<T>() -> PhantomData<bool> {
+   |                                           ^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = note: composed only of `PhantomData`
+
+error: `extern` fn uses type `std::vec::Vec<T>`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:174:39
+   |
+LL | pub extern "C" fn used_generic4<T>(x: Vec<T>) { }
+   |                                       ^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
+   = note: this struct has unspecified layout
+
+error: `extern` fn uses type `std::vec::Vec<T>`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:177:41
+   |
+LL | pub extern "C" fn used_generic5<T>() -> Vec<T> {
+   |                                         ^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
+   = note: this struct has unspecified layout
+
+error: aborting due to 20 previous errors
+

src/test/ui/mir/mir_cast_fn_ret.rs

@@ -1,8 +1,10 @@
 // run-pass
+#[allow(improper_ctypes_definitions)]
 pub extern "C" fn tuple2() -> (u16, u8) {
     (1, 2)
 }
 
+#[allow(improper_ctypes_definitions)]
 pub extern "C" fn tuple3() -> (u8, u8, u8) {
     (1, 2, 3)
 }

src/test/ui/mir/mir_codegen_calls.rs

@@ -74,6 +74,7 @@ fn test8() -> isize {
     Two::two()
 }
 
+#[allow(improper_ctypes_definitions)]
 extern fn simple_extern(x: u32, y: (u32, u32)) -> u32 {
     x + y.0 * y.1
 }