rust: syn: import crate

This is a subset of the Rust `syn` crate, version 2.0.106 (released
2025-08-16), licensed under "Apache-2.0 OR MIT", from:

    https://github.com/dtolnay/syn/raw/2.0.106/src

The files are copied as-is, with no modifications whatsoever (not even
adding the SPDX identifiers).

For copyright details, please see:

    https://github.com/dtolnay/syn/blob/2.0.106/README.md#license
    https://github.com/dtolnay/syn/blob/2.0.106/LICENSE-APACHE
    https://github.com/dtolnay/syn/blob/2.0.106/LICENSE-MIT

The next two patches modify these files as needed for use within the
kernel. This patch split allows reviewers to double-check the import
and to clearly see the differences introduced.

The following script may be used to verify the contents:

    for path in $(cd rust/syn/ && find . -type f -name '*.rs'); do
        curl --silent --show-error --location \
            https://github.com/dtolnay/syn/raw/2.0.106/src/$path \
            | diff --unified rust/syn/$path - && echo $path: OK
    done

Reviewed-by: Gary Guo <gary@garyguo.net>
Tested-by: Gary Guo <gary@garyguo.net>
Tested-by: Jesung Yang <y.j3ms.n@gmail.com>
Link: https://patch.msgid.link/20251124151837.2184382-16-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 58 insertions, 0 deletions

diff --git a/rust/syn/drops.rs b/rust/syn/drops.rs
new file mode 100644
index 000000000000..c54308f02c13
--- /dev/null
+++ b/rust/syn/drops.rs
@@ -0,0 +1,58 @@
+use std::iter;
+use std::mem::ManuallyDrop;
+use std::ops::{Deref, DerefMut};
+use std::option;
+use std::slice;
+
+#[repr(transparent)]
+pub(crate) struct NoDrop<T: ?Sized>(ManuallyDrop<T>);
+
+impl<T> NoDrop<T> {
+    pub(crate) fn new(value: T) -> Self
+    where
+        T: TrivialDrop,
+    {
+        NoDrop(ManuallyDrop::new(value))
+    }
+}
+
+impl<T: ?Sized> Deref for NoDrop<T> {
+    type Target = T;
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<T: ?Sized> DerefMut for NoDrop<T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
+pub(crate) trait TrivialDrop {}
+
+impl<T> TrivialDrop for iter::Empty<T> {}
+impl<T> TrivialDrop for slice::Iter<'_, T> {}
+impl<T> TrivialDrop for slice::IterMut<'_, T> {}
+impl<T> TrivialDrop for option::IntoIter<&T> {}
+impl<T> TrivialDrop for option::IntoIter<&mut T> {}
+
+#[test]
+fn test_needs_drop() {
+    use std::mem::needs_drop;
+
+    struct NeedsDrop;
+
+    impl Drop for NeedsDrop {
+        fn drop(&mut self) {}
+    }
+
+    assert!(needs_drop::<NeedsDrop>());
+
+    // Test each of the types with a handwritten TrivialDrop impl above.
+    assert!(!needs_drop::<iter::Empty<NeedsDrop>>());
+    assert!(!needs_drop::<slice::Iter<NeedsDrop>>());
+    assert!(!needs_drop::<slice::IterMut<NeedsDrop>>());
+    assert!(!needs_drop::<option::IntoIter<&NeedsDrop>>());
+    assert!(!needs_drop::<option::IntoIter<&mut NeedsDrop>>());
+}