error: `extern` block uses type `Foo`, which is not FFI-safe --> $DIR/issue-16250.rs:6:20 | LL | pub fn foo(x: (Foo)); | ^^^ not FFI-safe | = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct = note: this struct has unspecified layout note: the type is defined here --> $DIR/issue-16250.rs:3:1 | LL | pub struct Foo; | ^^^^^^^^^^^^^^ note: the lint level is defined here --> $DIR/issue-16250.rs:1:9 | LL | #![deny(warnings)] | ^^^^^^^^ = note: `#[deny(improper_ctypes)]` implied by `#[deny(warnings)]` error: aborting due to 1 previous error