Forbid opaque types in extern "C" blocks

src/librustc_lint/types.rs

@@ -624,7 +624,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                     AdtKind::Struct => {
                         if !def.repr.c() && !def.repr.transparent() {
                             return FfiUnsafe {
-                                ty: ty,
+                                ty,
                                 reason: "this struct has unspecified layout",
                                 help: Some("consider adding a `#[repr(C)]` or \
                                             `#[repr(transparent)]` attribute to this struct"),
@@ -633,7 +633,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
 
                         if def.non_enum_variant().fields.is_empty() {
                             return FfiUnsafe {
-                                ty: ty,
+                                ty,
                                 reason: "this struct has no fields",
                                 help: Some("consider adding a member to this struct"),
                             };
@@ -669,7 +669,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                     AdtKind::Union => {
                         if !def.repr.c() && !def.repr.transparent() {
                             return FfiUnsafe {
-                                ty: ty,
+                                ty,
                                 reason: "this union has unspecified layout",
                                 help: Some("consider adding a `#[repr(C)]` or \
                                             `#[repr(transparent)]` attribute to this union"),
@@ -678,7 +678,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
 
                         if def.non_enum_variant().fields.is_empty() {
                             return FfiUnsafe {
-                                ty: ty,
+                                ty,
                                 reason: "this union has no fields",
                                 help: Some("consider adding a field to this union"),
                             };
@@ -721,7 +721,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                             // Special-case types like `Option<extern fn()>`.
                             if !is_repr_nullable_ptr(cx, ty, def, substs) {
                                 return FfiUnsafe {
-                                    ty: ty,
+                                    ty,
                                     reason: "enum has no representation hint",
                                     help: Some("consider adding a `#[repr(C)]`, \
                                                 `#[repr(transparent)]`, or integer `#[repr(...)]` \
@@ -750,7 +750,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                                     }
                                     FfiPhantom(..) => {
                                         return FfiUnsafe {
-                                            ty: ty,
+                                            ty,
                                             reason: "this enum contains a PhantomData field",
                                             help: None,
                                         };
@@ -764,13 +764,13 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
             }
 
             ty::Char => FfiUnsafe {
-                ty: ty,
+                ty,
                 reason: "the `char` type has no C equivalent",
                 help: Some("consider using `u32` or `libc::wchar_t` instead"),
             },
 
             ty::Int(ast::IntTy::I128) | ty::Uint(ast::UintTy::U128) => FfiUnsafe {
-                ty: ty,
+                ty,
                 reason: "128-bit integers don't currently have a known stable ABI",
                 help: None,
             },
@@ -779,25 +779,25 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
             ty::Bool | ty::Int(..) | ty::Uint(..) | ty::Float(..) | ty::Never => FfiSafe,
 
             ty::Slice(_) => FfiUnsafe {
-                ty: ty,
+                ty,
                 reason: "slices have no C equivalent",
                 help: Some("consider using a raw pointer instead"),
             },
 
             ty::Dynamic(..) => FfiUnsafe {
-                ty: ty,
+                ty,
                 reason: "trait objects have no C equivalent",
                 help: None,
             },
 
             ty::Str => FfiUnsafe {
-                ty: ty,
+                ty,
                 reason: "string slices have no C equivalent",
                 help: Some("consider using `*const u8` and a length instead"),
             },
 
             ty::Tuple(..) => FfiUnsafe {
-                ty: ty,
+                ty,
                 reason: "tuples have unspecified layout",
                 help: Some("consider using a struct instead"),
             },
@@ -811,7 +811,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                 match sig.abi() {
                     Abi::Rust | Abi::RustIntrinsic | Abi::PlatformIntrinsic | Abi::RustCall => {
                         return FfiUnsafe {
-                            ty: ty,
+                            ty,
                             reason: "this function pointer has Rust-specific calling convention",
                             help: Some("consider using an `extern fn(...) -> ...` \
                                         function pointer instead"),
@@ -855,11 +855,44 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
             ty::UnnormalizedProjection(..) |
             ty::Projection(..) |
             ty::Opaque(..) |
-            ty::FnDef(..) => bug!("Unexpected type in foreign function"),
+            ty::FnDef(..) => bug!("unexpected type in foreign function: {:?}", ty),
         }
     }
 
+    fn check_for_opaque_ty(&mut self, sp: Span, ty: Ty<'tcx>) -> bool {
+        use crate::rustc::ty::TypeFoldable;
+
+        struct ProhibitOpaqueTypes<'a, 'tcx> {
+            cx: &'a LateContext<'a, 'tcx>,
+            sp: Span,
+        };
+
+        impl<'a, 'tcx> ty::fold::TypeVisitor<'tcx> for ProhibitOpaqueTypes<'a, 'tcx> {
+            fn visit_ty(&mut self, ty: Ty<'tcx>) -> bool {
+                if let ty::Opaque(..) = ty.sty {
+                    self.cx.span_lint(IMPROPER_CTYPES,
+                        self.sp,
+                        &format!("`extern` block uses type `{}` which is not FFI-safe: \
+                                  opaque types have no C equivalent", ty));
+                    true
+                } else {
+                    ty.super_visit_with(self)
+                }
+            }
+        }
+
+        let mut visitor = ProhibitOpaqueTypes { cx: self.cx, sp };
+        ty.visit_with(&mut visitor)
+    }
+
     fn check_type_for_ffi_and_report_errors(&mut self, sp: Span, ty: Ty<'tcx>) {
+        // We have to check for opaque types before `normalize_erasing_regions`,
+        // which will replace opaque types with their underlying concrete type.
+        if self.check_for_opaque_ty(sp, ty) {
+            // We've already emitted an error due to an opaque type.
+            return;
+        }
+
         // 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);
@@ -870,7 +903,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                 self.cx.span_lint(IMPROPER_CTYPES,
                                   sp,
                                   &format!("`extern` block uses type `{}` which is not FFI-safe: \
-                                            composed only of PhantomData", ty));
+                                            composed only of `PhantomData`", ty));
             }
             FfiResult::FfiUnsafe { ty: unsafe_ty, reason, help } => {
                 let msg = format!("`extern` block uses type `{}` which is not FFI-safe: {}",

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

@@ -1,7 +1,7 @@
-#![deny(improper_ctypes)]
 #![feature(rustc_private)]
 
 #![allow(private_in_public)]
+#![deny(improper_ctypes)]
 
 extern crate libc;
 
@@ -55,9 +55,9 @@ extern {
     pub fn tuple_type(p: (i32, i32)); //~ ERROR uses type `(i32, i32)`
     pub fn tuple_type2(p: I32Pair); //~ ERROR uses type `(i32, i32)`
     pub fn zero_size(p: ZeroSize); //~ ERROR struct has no fields
-    pub fn zero_size_phantom(p: ZeroSizeWithPhantomData); //~ ERROR composed only of PhantomData
+    pub fn zero_size_phantom(p: ZeroSizeWithPhantomData); //~ ERROR composed only of `PhantomData`
     pub fn zero_size_phantom_toplevel()
-        -> ::std::marker::PhantomData<bool>; //~ ERROR: composed only of PhantomData
+        -> ::std::marker::PhantomData<bool>; //~ ERROR: composed only of `PhantomData`
     pub fn fn_type(p: RustFn); //~ ERROR function pointer has Rust-specific
     pub fn fn_type2(p: fn()); //~ ERROR function pointer has Rust-specific
     pub fn fn_contained(p: RustBadRet); //~ ERROR: uses type `std::boxed::Box<u32>`

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

@@ -5,7 +5,7 @@ LL |     pub fn ptr_type1(size: *const Foo);
    |                            ^^^^^^^^^^
    |
 note: lint level defined here
-  --> $DIR/lint-ctypes.rs:1:9
+  --> $DIR/lint-ctypes.rs:4:9
    |
 LL | #![deny(improper_ctypes)]
    |         ^^^^^^^^^^^^^^^
@@ -108,13 +108,13 @@ note: type defined here
 LL | pub struct ZeroSize;
    | ^^^^^^^^^^^^^^^^^^^^
 
-error: `extern` block uses type `ZeroSizeWithPhantomData` which is not FFI-safe: composed only of PhantomData
+error: `extern` block uses type `ZeroSizeWithPhantomData` which is not FFI-safe: composed only of `PhantomData`
   --> $DIR/lint-ctypes.rs:58:33
    |
 LL |     pub fn zero_size_phantom(p: ZeroSizeWithPhantomData);
    |                                 ^^^^^^^^^^^^^^^^^^^^^^^
 
-error: `extern` block uses type `std::marker::PhantomData<bool>` which is not FFI-safe: composed only of PhantomData
+error: `extern` block uses type `std::marker::PhantomData<bool>` which is not FFI-safe: composed only of `PhantomData`
   --> $DIR/lint-ctypes.rs:60:12
    |
 LL |         -> ::std::marker::PhantomData<bool>;

src/test/ui/lint/opaque-ty-ffi-unsafe.rs

@@ -0,0 +1,16 @@
+#![feature(type_alias_impl_trait)]
+
+#![deny(improper_ctypes)]
+
+type A = impl Fn();
+
+pub fn ret_closure() -> A {
+    || {}
+}
+
+extern "C" {
+    pub fn a(_: A);
+    //~^ ERROR `extern` block uses type `A` which is not FFI-safe: opaque types have no C equivalent
+}
+
+fn main() {}

src/test/ui/lint/opaque-ty-ffi-unsafe.stderr

@@ -0,0 +1,14 @@
+error: `extern` block uses type `A` which is not FFI-safe: opaque types have no C equivalent
+  --> $DIR/opaque-ty-ffi-unsafe.rs:12:17
+   |
+LL |     pub fn a(_: A);
+   |                 ^
+   |
+note: lint level defined here
+  --> $DIR/opaque-ty-ffi-unsafe.rs:3:9
+   |
+LL | #![deny(improper_ctypes)]
+   |         ^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+