improper_ctypes_definitions: allow Box

This commit stops linting against `Box` in `extern "C" fn`s for the
`improper_ctypes_definitions` lint - boxes are documented to be
FFI-safe.

Signed-off-by: David Wood <david@davidtw.co>

src/librustc_lint/types.rs

@@ -531,6 +531,11 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
         match ty.kind {
             ty::FnPtr(_) => true,
             ty::Ref(..) => true,
+            ty::Adt(def, _)
+                if def.is_box() && matches!(self.mode, ImproperCTypesMode::Definitions) =>
+            {
+                true
+            }
             ty::Adt(def, substs) if def.repr.transparent() && !def.is_union() => {
                 let guaranteed_nonnull_optimization = self
                     .cx
@@ -558,7 +563,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
     }
 
     /// Check if this enum can be safely exported based on the "nullable pointer optimization".
-    /// Currently restricted to function pointers, references, `core::num::NonZero*`,
+    /// Currently restricted to function pointers, boxes, references, `core::num::NonZero*`,
     /// `core::ptr::NonNull`, and `#[repr(transparent)]` newtypes.
     fn is_repr_nullable_ptr(
         &self,
@@ -692,6 +697,12 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
         }
 
         match ty.kind {
+            ty::Adt(def, _)
+                if def.is_box() && matches!(self.mode, ImproperCTypesMode::Definitions) =>
+            {
+                FfiSafe
+            }
+
             ty::Adt(def, substs) => {
                 if def.is_phantom_data() {
                     return FfiPhantom(ty);

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

@@ -71,7 +71,8 @@ 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 opt_box_type(p: Option<Box<u32>>) { }
 
 pub extern "C" fn char_type(p: char) { }
 //~^ ERROR uses type `char`
@@ -106,7 +107,6 @@ 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`
@@ -115,7 +115,6 @@ 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()>) { }
 

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

@@ -21,17 +21,8 @@ LL | pub extern "C" fn str_type(p: &str) { }
    = 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
+  --> $DIR/lint-ctypes-fn.rs:77:32
    |
 LL | pub extern "C" fn char_type(p: char) { }
    |                                ^^^^ not FFI-safe
@@ -40,23 +31,23 @@ LL | pub extern "C" fn char_type(p: char) { }
    = 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
+  --> $DIR/lint-ctypes-fn.rs:80:32
    |
 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
+  --> $DIR/lint-ctypes-fn.rs:83:32
    |
 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
+  --> $DIR/lint-ctypes-fn.rs:86:33
    |
 LL | pub extern "C" fn tuple_type(p: (i32, i32)) { }
    |                                 ^^^^^^^^^^ not FFI-safe
@@ -65,7 +56,7 @@ LL | pub extern "C" fn tuple_type(p: (i32, i32)) { }
    = note: tuples have unspecified layout
 
 error: `extern` fn uses type `(i32, i32)`, which is not FFI-safe
-  --> $DIR/lint-ctypes-fn.rs:88:34
+  --> $DIR/lint-ctypes-fn.rs:89:34
    |
 LL | pub extern "C" fn tuple_type2(p: I32Pair) { }
    |                                  ^^^^^^^ not FFI-safe
@@ -74,7 +65,7 @@ LL | pub extern "C" fn tuple_type2(p: I32Pair) { }
    = note: tuples have unspecified layout
 
 error: `extern` fn uses type `ZeroSize`, which is not FFI-safe
-  --> $DIR/lint-ctypes-fn.rs:91:32
+  --> $DIR/lint-ctypes-fn.rs:92:32
    |
 LL | pub extern "C" fn zero_size(p: ZeroSize) { }
    |                                ^^^^^^^^ not FFI-safe
@@ -88,7 +79,7 @@ LL | pub struct ZeroSize;
    | ^^^^^^^^^^^^^^^^^^^^
 
 error: `extern` fn uses type `ZeroSizeWithPhantomData`, which is not FFI-safe
-  --> $DIR/lint-ctypes-fn.rs:94:40
+  --> $DIR/lint-ctypes-fn.rs:95:40
    |
 LL | pub extern "C" fn zero_size_phantom(p: ZeroSizeWithPhantomData) { }
    |                                        ^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -101,15 +92,15 @@ 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
+  --> $DIR/lint-ctypes-fn.rs:98:51
    |
 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
+  --> $DIR/lint-ctypes-fn.rs:103:30
    |
 LL | pub extern "C" fn fn_type(p: RustFn) { }
    |                              ^^^^^^ not FFI-safe
@@ -118,23 +109,14 @@ LL | pub extern "C" fn fn_type(p: RustFn) { }
    = 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
+  --> $DIR/lint-ctypes-fn.rs:106:31
    |
 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
    |
@@ -152,25 +134,16 @@ LL | pub extern "C" fn transparent_str(p: TransparentStr) { }
    = 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
+  --> $DIR/lint-ctypes-fn.rs:160:43
    |
 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
+  --> $DIR/lint-ctypes-fn.rs:173:39
    |
 LL | pub extern "C" fn used_generic4<T>(x: Vec<T>) { }
    |                                       ^^^^^^ not FFI-safe
@@ -179,13 +152,13 @@ LL | pub extern "C" fn used_generic4<T>(x: Vec<T>) { }
    = 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
+  --> $DIR/lint-ctypes-fn.rs:176:41
    |
 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
+error: aborting due to 17 previous errors
 

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

@@ -48,6 +48,8 @@ extern {
     pub fn slice_type(p: &[u32]); //~ ERROR: uses type `[u32]`
     pub fn str_type(p: &str); //~ ERROR: uses type `str`
     pub fn box_type(p: Box<u32>); //~ ERROR uses type `std::boxed::Box<u32>`
+    pub fn opt_box_type(p: Option<Box<u32>>);
+    //~^ ERROR uses type `std::option::Option<std::boxed::Box<u32>>`
     pub fn char_type(p: char); //~ ERROR uses type `char`
     pub fn i128_type(p: i128); //~ ERROR uses type `i128`
     pub fn u128_type(p: u128); //~ ERROR uses type `u128`