From 0a218a84a813375cfb38e0ee5c5f8445afb2765c Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Fri, 18 Dec 2020 12:16:28 -0800
Subject: [PATCH 1/2] Add test of anyhow::Error in ffi signature

Currently:

    warning: `extern` fn uses type `anyhow::Error`, which is not FFI-safe
     --> tests/test_ffi.rs:6:32
      |
    6 | pub extern "C" fn anyhow1(err: anyhow::Error) {
      |                                ^^^^^^^^^^^^^ not FFI-safe
      |
    note: the lint level is defined here
     --> tests/test_ffi.rs:1:26
      |
    1 | #![warn(improper_ctypes, improper_ctypes_definitions)]
      |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^
      = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
      = note: this struct has unspecified layout

    warning: `extern` fn uses type `Option<anyhow::Error>`, which is not FFI-safe
      --> tests/test_ffi.rs:16:32
       |
    16 | pub extern "C" fn anyhow3() -> Option<anyhow::Error> {
       |                                ^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
       |
       = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
       = note: enum has no representation hint
---
 tests/test_ffi.rs | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)
 create mode 100644 tests/test_ffi.rs

diff --git a/tests/test_ffi.rs b/tests/test_ffi.rs
new file mode 100644
index 0000000..88bfc97
--- /dev/null
+++ b/tests/test_ffi.rs
@@ -0,0 +1,18 @@
+#![warn(improper_ctypes, improper_ctypes_definitions)]
+
+use anyhow::anyhow;
+
+#[no_mangle]
+pub extern "C" fn anyhow1(err: anyhow::Error) {
+    println!("{:?}", err);
+}
+
+#[no_mangle]
+pub extern "C" fn anyhow2(err: &mut Option<anyhow::Error>) {
+    *err = Some(anyhow!("ffi error"));
+}
+
+#[no_mangle]
+pub extern "C" fn anyhow3() -> Option<anyhow::Error> {
+    Some(anyhow!("ffi error"))
+}

From 3105a739d97042f3f831c24b10e4baab16f819b4 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Fri, 18 Dec 2020 12:16:54 -0800
Subject: [PATCH 2/2] Mark Error repr transparent

---
 src/lib.rs        | 1 +
 tests/test_ffi.rs | 2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/lib.rs b/src/lib.rs
index a211b22..d124b1e 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -361,6 +361,7 @@ pub use anyhow as format_err;
 ///     # Ok(())
 /// }
 /// ```
+#[repr(transparent)]
 pub struct Error {
     inner: ManuallyDrop<Box<ErrorImpl<()>>>,
 }
diff --git a/tests/test_ffi.rs b/tests/test_ffi.rs
index 88bfc97..0321fc1 100644
--- a/tests/test_ffi.rs
+++ b/tests/test_ffi.rs
@@ -1,4 +1,4 @@
-#![warn(improper_ctypes, improper_ctypes_definitions)]
+#![deny(improper_ctypes, improper_ctypes_definitions)]
 
 use anyhow::anyhow;