[improper_ctypes] Point at definition of non-FFI-safe type if possible

Author: Robin Kruppe

src/librustc_lint/types.rs

@@ -666,6 +666,11 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                 if let Some(s) = help {
                     diag.help(s);
                 }
+                if let ty::TyAdt(def, _) = unsafe_ty.sty {
+                    if let Some(sp) = self.cx.tcx.hir.span_if_local(def.did) {
+                        diag.span_note(sp, "type defined here");
+                    }
+                }
                 diag.emit();
             }
         }

src/test/ui/lint-ctypes.stderr

@@ -10,6 +10,11 @@ note: lint level defined here
 11 | #![deny(improper_ctypes)]
    |         ^^^^^^^^^^^^^^^
    = help: consider adding a #[repr(C)] or #[repr(transparent)] attribute to this struct
+note: type defined here
+  --> $DIR/lint-ctypes.rs:32:1
+   |
+32 | pub struct Foo;
+   | ^^^^^^^^^^^^^^^
 
 error: `extern` block uses type `Foo` which is not FFI-safe: this struct has unspecified layout
   --> $DIR/lint-ctypes.rs:55:28
@@ -18,6 +23,11 @@ error: `extern` block uses type `Foo` which is not FFI-safe: this struct has uns
    |                            ^^^^^^^^^^
    |
    = help: consider adding a #[repr(C)] or #[repr(transparent)] attribute to this struct
+note: type defined here
+  --> $DIR/lint-ctypes.rs:32:1
+   |
+32 | pub struct Foo;
+   | ^^^^^^^^^^^^^^^
 
 error: `extern` block uses type `[u32]` which is not FFI-safe: slices have no C equivalent
   --> $DIR/lint-ctypes.rs:56:26
@@ -94,6 +104,11 @@ error: `extern` block uses type `ZeroSize` which is not FFI-safe: this struct ha
    |                         ^^^^^^^^
    |
    = help: consider adding a member to this struct
+note: type defined here
+  --> $DIR/lint-ctypes.rs:28:1
+   |
+28 | pub struct ZeroSize;
+   | ^^^^^^^^^^^^^^^^^^^^
 
 error: `extern` block uses type `ZeroSizeWithPhantomData` which is not FFI-safe: composed only of PhantomData
   --> $DIR/lint-ctypes.rs:66:33