Adding diagnostics for use on non-nightly.

engine/Cargo.toml

@@ -51,6 +51,9 @@ regex = "1.5"
 indexmap = "1.8"
 prettyplease = { version = "0.1.15", features = ["verbatim"] }
 
+[build-dependencies]
+rustc_version = "0.4"
+
 [dependencies.syn]
 version = "1.0.39"
 features = [ "full", "printing" ]

engine/build.rs

@@ -0,0 +1,15 @@
+// Copyright 2022 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use rustc_version::{version_meta, Channel};
+
+fn main() {
+    if version_meta().unwrap().channel == Channel::Nightly {
+        println!("cargo:rustc-cfg=nightly");
+    }
+}

engine/src/lib.rs

@@ -125,6 +125,8 @@ pub enum Error {
     NoAutoCxxInc,
     #[error(transparent)]
     Conversion(conversion::ConvertError),
+    #[error("Using `unsafe_references_wrapped` requires the Rust nightly `arbitrary_self_types` feature")]
+    WrappedReferencesButNoArbitrarySelfTypes,
 }
 
 /// Result type.
@@ -398,6 +400,14 @@ impl IncludeCppEngine {
             State::Generated(_) => panic!("Only call generate once"),
         }
 
+        if matches!(
+            self.config.unsafe_policy,
+            UnsafePolicy::ReferencesWrappedAllFunctionsSafe
+        ) && cfg!(not(nightly))
+        {
+            return Err(Error::WrappedReferencesButNoArbitrarySelfTypes);
+        }
+
         let mod_name = self.config.get_mod_name();
         let mut builder = self.make_bindgen_builder(&inc_dirs, extra_clang_args);
         if let Some(dep_recorder) = dep_recorder {

src/lib.rs

@@ -267,8 +267,9 @@ macro_rules! concrete {
 /// `safety!(unsafe_references_wrapped)`
 /// This policy treats C++ references as scary and requires
 /// them to be wrapped in a `CppRef` type: see [`CppRef`].
-/// This only works practically on nightly Rust because it
-/// depends upon an unstable feature. However, it should
+/// This only works on nightly Rust because it
+/// depends upon an unstable feature
+/// (`arbitrary_self_types`). However, it should
 /// eliminate all undefined behavior related to Rust's
 /// stricter aliasing rules than C++.
 #[macro_export]