[derive] Test with trivial_bounds feature

The nightly `trivial_bounds` causes some derive-emitted code to compile
which is designed to fail compilation. This commit adds a test that
ensures that the emitted code is still sound even with `trivial_bounds`
enabled.

Affects #61

Closes #500

Author: joshlf

zerocopy-derive/tests/ui-nightly/trivial_bounds.rs

@@ -0,0 +1,84 @@
+// Copyright 2023 The Fuchsia Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#![feature(trivial_bounds)]
+
+extern crate zerocopy;
+
+#[path = "../util.rs"]
+mod util;
+
+use static_assertions::assert_impl_all;
+use zerocopy::{AsBytes, FromBytes, FromZeroes, Unaligned};
+
+use self::util::{NotZerocopy, AU16};
+
+fn main() {}
+
+// These tests are compiled with the `trivial_bounds` feature enabled. We emit
+// code which is designed to fail compilation for types we wish to reject. E.g.:
+//
+//   #[derive(FromZeroes)]
+//   struct Foo(NotZerocopy);
+//
+// It would be unsound to emit an impl of `FromZeroes for Foo` in this case. Our
+// custom derives are designed to cause compilation to fail for this type.
+// `trivial_bounds` instead allows this code to succeed by simply not emitting
+// `impl FromZeroes for Foo`. This isn't a soundness issue, but is likely
+// confusing to users.
+//
+// As of this writing, the code in this file only fails to compile thanks to the
+// use of `assert_impl_all!`, which depends upon the incorrect trait bounds.
+//
+// TODO(https://github.com/rust-lang/rust/issues/48214): If support is ever
+// added for `#[deny(trivially_false_bounds)]` as discussed in the
+// `trivial_bounds` tracking issue, we should emit that annotation in our
+// derives. That will result in the `#[derive(...)]` annotations in this file
+// failing to compile, not just the `assert_impl_all!` calls.
+
+//
+// FromZeroes errors
+//
+
+#[derive(FromZeroes)]
+struct FromZeroes1 {
+    value: NotZerocopy,
+}
+
+assert_impl_all!(FromZeroes1: FromZeroes);
+
+//
+// FromBytes errors
+//
+
+#[derive(FromBytes)]
+struct FromBytes1 {
+    value: NotZerocopy,
+}
+
+assert_impl_all!(FromBytes1: FromBytes);
+
+//
+// AsBytes errors
+//
+
+#[derive(AsBytes)]
+#[repr(C)]
+struct AsBytes1 {
+    value: NotZerocopy,
+}
+
+assert_impl_all!(AsBytes1: AsBytes);
+
+//
+// Unaligned errors
+//
+
+#[derive(Unaligned)]
+#[repr(C)]
+struct Unaligned1 {
+    aligned: AU16,
+}
+
+assert_impl_all!(Unaligned1: Unaligned);

zerocopy-derive/tests/ui-nightly/trivial_bounds.stderr

@@ -0,0 +1,22 @@
+error[E0277]: the trait bound `FromBytes1: FromZeroes` is not satisfied
+  --> tests/ui-nightly/trivial_bounds.rs:55:10
+   |
+55 | #[derive(FromBytes)]
+   |          ^^^^^^^^^ the trait `FromZeroes` is not implemented for `FromBytes1`
+   |
+   = help: the following other types implement trait `FromZeroes`:
+             bool
+             char
+             isize
+             i8
+             i16
+             i32
+             i64
+             i128
+           and $N others
+note: required by a bound in `FromBytes`
+  --> $WORKSPACE/src/lib.rs
+   |
+   | pub unsafe trait FromBytes: FromZeroes {
+   |                             ^^^^^^^^^^ required by this bound in `FromBytes`
+   = note: this error originates in the derive macro `FromBytes` (in Nightly builds, run with -Z macro-backtrace for more info)