Update improper_ctypes warning to add info about extern "Rust"

src/librustc_lint/types.rs

@@ -989,6 +989,9 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                 diag.help(help);
             }
             diag.note(note);
+            diag.note(
+                r#"if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`"#,
+            );
             if let ty::Adt(def, _) = ty.kind {
                 if let Some(sp) = self.cx.tcx.hir().span_if_local(def.did) {
                     diag.span_note(sp, "the type is defined here");

src/test/ui/issues/issue-14309.stderr

@@ -11,6 +11,7 @@ LL | #![deny(improper_ctypes)]
    |         ^^^^^^^^^^^^^^^
    = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
    = note: this struct has unspecified layout
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 note: the type is defined here
   --> $DIR/issue-14309.rs:4:1
    |
@@ -27,6 +28,7 @@ LL |     fn bar(x: B);
    |
    = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
    = note: this struct has unspecified layout
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 note: the type is defined here
   --> $DIR/issue-14309.rs:4:1
    |
@@ -43,6 +45,7 @@ LL |     fn qux(x: A2);
    |
    = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
    = note: this struct has unspecified layout
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 note: the type is defined here
   --> $DIR/issue-14309.rs:4:1
    |
@@ -59,6 +62,7 @@ LL |     fn quux(x: B2);
    |
    = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
    = note: this struct has unspecified layout
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 note: the type is defined here
   --> $DIR/issue-14309.rs:4:1
    |
@@ -75,6 +79,7 @@ LL |     fn fred(x: D);
    |
    = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
    = note: this struct has unspecified layout
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 note: the type is defined here
   --> $DIR/issue-14309.rs:4:1
    |

src/test/ui/issues/issue-16250.stderr

@@ -12,6 +12,7 @@ LL | #![deny(warnings)]
    = note: `#[deny(improper_ctypes)]` implied by `#[deny(warnings)]`
    = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
    = note: this struct has unspecified layout
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 note: the type is defined here
   --> $DIR/issue-16250.rs:3:1
    |

src/test/ui/lint/lint-ctypes-73249-2.stderr

@@ -10,6 +10,7 @@ note: the lint level is defined here
 LL | #![deny(improper_ctypes)]
    |         ^^^^^^^^^^^^^^^
    = note: opaque types have no C equivalent
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: aborting due to previous error
 

src/test/ui/lint/lint-ctypes-73249-3.stderr

@@ -10,6 +10,7 @@ note: the lint level is defined here
 LL | #![deny(improper_ctypes)]
    |         ^^^^^^^^^^^^^^^
    = note: opaque types have no C equivalent
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: aborting due to previous error
 

src/test/ui/lint/lint-ctypes-73249-5.stderr

@@ -10,6 +10,7 @@ note: the lint level is defined here
 LL | #![deny(improper_ctypes)]
    |         ^^^^^^^^^^^^^^^
    = note: opaque types have no C equivalent
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: aborting due to previous error
 

src/test/ui/lint/lint-ctypes-73251-1.stderr

@@ -10,6 +10,7 @@ note: the lint level is defined here
 LL | #![deny(improper_ctypes)]
    |         ^^^^^^^^^^^^^^^
    = note: opaque types have no C equivalent
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: aborting due to previous error
 

src/test/ui/lint/lint-ctypes-73251-2.stderr

@@ -10,6 +10,7 @@ note: the lint level is defined here
 LL | #![deny(improper_ctypes)]
    |         ^^^^^^^^^^^^^^^
    = note: opaque types have no C equivalent
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: aborting due to previous error
 

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

@@ -11,6 +11,7 @@ LL | #![deny(improper_ctypes)]
    |         ^^^^^^^^^^^^^^^
    = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
    = note: enum has no representation hint
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 note: the type is defined here
   --> $DIR/lint-ctypes-enum.rs:9:1
    |
@@ -25,6 +26,7 @@ LL |    fn bf(x: B);
    |
    = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
    = note: enum has no representation hint
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 note: the type is defined here
   --> $DIR/lint-ctypes-enum.rs:10:1
    |
@@ -39,6 +41,7 @@ LL |    fn tf(x: T);
    |
    = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
    = note: enum has no representation hint
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 note: the type is defined here
   --> $DIR/lint-ctypes-enum.rs:11:1
    |
@@ -53,6 +56,7 @@ LL |    fn unique(x: Option<std::ptr::Unique<u8>>);
    |
    = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
    = note: enum has no representation hint
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: `extern` block uses type `u128`, which is not FFI-safe
   --> $DIR/lint-ctypes-enum.rs:54:23
@@ -61,6 +65,7 @@ LL |    fn nonzero_u128(x: Option<num::NonZeroU128>);
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
    |
    = note: 128-bit integers don't currently have a known stable ABI
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: `extern` block uses type `i128`, which is not FFI-safe
   --> $DIR/lint-ctypes-enum.rs:61:23
@@ -69,6 +74,7 @@ LL |    fn nonzero_i128(x: Option<num::NonZeroI128>);
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
    |
    = note: 128-bit integers don't currently have a known stable ABI
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: `extern` block uses type `std::option::Option<TransparentUnion<std::num::NonZeroU8>>`, which is not FFI-safe
   --> $DIR/lint-ctypes-enum.rs:66:28
@@ -78,6 +84,7 @@ LL |    fn transparent_union(x: Option<TransparentUnion<num::NonZeroU8>>);
    |
    = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
    = note: enum has no representation hint
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: `extern` block uses type `std::option::Option<Rust<std::num::NonZeroU8>>`, which is not FFI-safe
   --> $DIR/lint-ctypes-enum.rs:68:20
@@ -87,6 +94,7 @@ LL |    fn repr_rust(x: Option<Rust<num::NonZeroU8>>);
    |
    = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
    = note: enum has no representation hint
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: `extern` block uses type `std::result::Result<(), std::num::NonZeroI32>`, which is not FFI-safe
   --> $DIR/lint-ctypes-enum.rs:69:20
@@ -96,6 +104,7 @@ LL |    fn no_result(x: Result<(), num::NonZeroI32>);
    |
    = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
    = note: enum has no representation hint
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: aborting due to 9 previous errors
 

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

@@ -11,6 +11,7 @@ LL | #![deny(improper_ctypes_definitions)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = help: consider using a raw pointer instead
    = note: slices have no C equivalent
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: `extern` fn uses type `str`, which is not FFI-safe
   --> $DIR/lint-ctypes-fn.rs:70:31
@@ -20,6 +21,7 @@ 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
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: `extern` fn uses type `char`, which is not FFI-safe
   --> $DIR/lint-ctypes-fn.rs:77:32
@@ -29,6 +31,7 @@ LL | pub extern "C" fn char_type(p: char) { }
    |
    = help: consider using `u32` or `libc::wchar_t` instead
    = note: the `char` type has no C equivalent
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: `extern` fn uses type `i128`, which is not FFI-safe
   --> $DIR/lint-ctypes-fn.rs:80:32
@@ -37,6 +40,7 @@ LL | pub extern "C" fn i128_type(p: i128) { }
    |                                ^^^^ not FFI-safe
    |
    = note: 128-bit integers don't currently have a known stable ABI
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: `extern` fn uses type `u128`, which is not FFI-safe
   --> $DIR/lint-ctypes-fn.rs:83:32
@@ -45,6 +49,7 @@ LL | pub extern "C" fn u128_type(p: u128) { }
    |                                ^^^^ not FFI-safe
    |
    = note: 128-bit integers don't currently have a known stable ABI
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: `extern` fn uses type `(i32, i32)`, which is not FFI-safe
   --> $DIR/lint-ctypes-fn.rs:86:33
@@ -54,6 +59,7 @@ LL | pub extern "C" fn tuple_type(p: (i32, i32)) { }
    |
    = help: consider using a struct instead
    = note: tuples have unspecified layout
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: `extern` fn uses type `(i32, i32)`, which is not FFI-safe
   --> $DIR/lint-ctypes-fn.rs:89:34
@@ -63,6 +69,7 @@ LL | pub extern "C" fn tuple_type2(p: I32Pair) { }
    |
    = help: consider using a struct instead
    = note: tuples have unspecified layout
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: `extern` fn uses type `ZeroSize`, which is not FFI-safe
   --> $DIR/lint-ctypes-fn.rs:92:32
@@ -72,6 +79,7 @@ LL | pub extern "C" fn zero_size(p: ZeroSize) { }
    |
    = help: consider adding a member to this struct
    = note: this struct has no fields
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 note: the type is defined here
   --> $DIR/lint-ctypes-fn.rs:26:1
    |
@@ -85,6 +93,7 @@ LL | pub extern "C" fn zero_size_phantom(p: ZeroSizeWithPhantomData) { }
    |                                        ^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
    |
    = note: composed only of `PhantomData`
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 note: the type is defined here
   --> $DIR/lint-ctypes-fn.rs:61:1
    |
@@ -98,6 +107,7 @@ LL | pub extern "C" fn zero_size_phantom_toplevel() -> PhantomData<bool> {
    |                                                   ^^^^^^^^^^^^^^^^^ not FFI-safe
    |
    = note: composed only of `PhantomData`
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: `extern` fn uses type `fn()`, which is not FFI-safe
   --> $DIR/lint-ctypes-fn.rs:103:30
@@ -107,6 +117,7 @@ LL | pub extern "C" fn fn_type(p: RustFn) { }
    |
    = help: consider using an `extern fn(...) -> ...` function pointer instead
    = note: this function pointer has Rust-specific calling convention
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: `extern` fn uses type `fn()`, which is not FFI-safe
   --> $DIR/lint-ctypes-fn.rs:106:31
@@ -116,6 +127,7 @@ LL | pub extern "C" fn fn_type2(p: fn()) { }
    |
    = help: consider using an `extern fn(...) -> ...` function pointer instead
    = note: this function pointer has Rust-specific calling convention
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: `extern` fn uses type `i128`, which is not FFI-safe
   --> $DIR/lint-ctypes-fn.rs:111:39
@@ -124,6 +136,7 @@ LL | pub extern "C" fn transparent_i128(p: TransparentI128) { }
    |                                       ^^^^^^^^^^^^^^^ not FFI-safe
    |
    = note: 128-bit integers don't currently have a known stable ABI
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: `extern` fn uses type `str`, which is not FFI-safe
   --> $DIR/lint-ctypes-fn.rs:114:38
@@ -133,6 +146,7 @@ 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
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: `extern` fn uses type `std::marker::PhantomData<bool>`, which is not FFI-safe
   --> $DIR/lint-ctypes-fn.rs:160:43
@@ -141,6 +155,7 @@ LL | pub extern "C" fn unused_generic2<T>() -> PhantomData<bool> {
    |                                           ^^^^^^^^^^^^^^^^^ not FFI-safe
    |
    = note: composed only of `PhantomData`
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: `extern` fn uses type `std::vec::Vec<T>`, which is not FFI-safe
   --> $DIR/lint-ctypes-fn.rs:173:39
@@ -150,6 +165,7 @@ LL | pub extern "C" fn used_generic4<T>(x: Vec<T>) { }
    |
    = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
    = note: this struct has unspecified layout
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: `extern` fn uses type `std::vec::Vec<T>`, which is not FFI-safe
   --> $DIR/lint-ctypes-fn.rs:176:41
@@ -159,6 +175,7 @@ LL | pub extern "C" fn used_generic5<T>() -> Vec<T> {
    |
    = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
    = note: this struct has unspecified layout
+   = note: if intentionally using the unstable Rust ABI, specify it with `extern "Rust"`
 
 error: aborting due to 17 previous errors