[improper_ctypes] Suggest repr(transparent) for structs

The suggestion is unconditional, so following it could lead to further errors. This is already the case for the repr(C) suggestion, which makes this acceptable, though not *good*. Checking up-front whether the suggestion can help would be great but applies more broadly (and would require some refactoring to avoid duplicating the checks).

Author: Robin Kruppe

src/librustc_lint/types.rs

@@ -425,7 +425,8 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                             return FfiUnsafe {
                                 ty: ty,
                                 reason: "this struct has unspecified layout",
-                                help: Some("consider adding a #[repr(C)] attribute to this struct"),
+                                help: Some("consider adding a #[repr(C)] or #[repr(transparent)] \
+                                            attribute to this struct"),
                             };
                         }
 

src/test/ui/lint-ctypes.stderr

@@ -9,15 +9,15 @@ note: lint level defined here
    |
 11 | #![deny(improper_ctypes)]
    |         ^^^^^^^^^^^^^^^
-   = help: consider adding a #[repr(C)] attribute to this struct
+   = help: consider adding a #[repr(C)] or #[repr(transparent)] attribute to this struct
 
 error: `extern` block uses type `Foo` which is not FFI-safe: this struct has unspecified layout
   --> $DIR/lint-ctypes.rs:55:28
    |
 55 |     pub fn ptr_type2(size: *const Foo); //~ ERROR: uses type `Foo`
    |                            ^^^^^^^^^^
    |
-   = help: consider adding a #[repr(C)] attribute to this struct
+   = help: consider adding a #[repr(C)] or #[repr(transparent)] attribute to this struct
 
 error: `extern` block uses type `[u32]` which is not FFI-safe: slices have no C equivalent
   --> $DIR/lint-ctypes.rs:56:26
@@ -41,7 +41,7 @@ error: `extern` block uses type `std::boxed::Box<u32>` which is not FFI-safe: th
 58 |     pub fn box_type(p: Box<u32>); //~ ERROR uses type `std::boxed::Box<u32>`
    |                        ^^^^^^^^
    |
-   = help: consider adding a #[repr(C)] attribute to this struct
+   = help: consider adding a #[repr(C)] or #[repr(transparent)] attribute to this struct
 
 error: `extern` block uses type `char` which is not FFI-safe: the `char` type has no C equivalent
   --> $DIR/lint-ctypes.rs:59:25
@@ -129,7 +129,7 @@ error: `extern` block uses type `std::boxed::Box<u32>` which is not FFI-safe: th
 71 |     pub fn fn_contained(p: RustBadRet); //~ ERROR: uses type `std::boxed::Box<u32>`
    |                            ^^^^^^^^^^
    |
-   = help: consider adding a #[repr(C)] attribute to this struct
+   = help: consider adding a #[repr(C)] or #[repr(transparent)] attribute to this struct
 
 error: `extern` block uses type `i128` which is not FFI-safe: 128-bit integers don't currently have a known stable ABI
   --> $DIR/lint-ctypes.rs:72:32
@@ -151,7 +151,7 @@ error: `extern` block uses type `std::boxed::Box<u32>` which is not FFI-safe: th
 74 |     pub fn transparent_fn(p: TransparentBadFn); //~ ERROR: uses type `std::boxed::Box<u32>`
    |                              ^^^^^^^^^^^^^^^^
    |
-   = help: consider adding a #[repr(C)] attribute to this struct
+   = help: consider adding a #[repr(C)] or #[repr(transparent)] attribute to this struct
 
 error: aborting due to 20 previous errors