rust: init: add `PinnedDrop` trait and macros

The `PinnedDrop` trait that facilitates destruction of pinned types.
It has to be implemented via the `#[pinned_drop]` macro, since the
`drop` function should not be called by normal code, only by other
destructors. It also only works on structs that are annotated with
`#[pin_data(PinnedDrop)]`.

Co-developed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Link: https://lore.kernel.org/r/20230408122429.1103522-10-y86-dev@protonmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 49 insertions, 0 deletions

diff --git a/rust/macros/pinned_drop.rs b/rust/macros/pinned_drop.rs
new file mode 100644
index 000000000000..88fb72b20660
--- /dev/null
+++ b/rust/macros/pinned_drop.rs
@@ -0,0 +1,49 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+use proc_macro::{TokenStream, TokenTree};
+
+pub(crate) fn pinned_drop(_args: TokenStream, input: TokenStream) -> TokenStream {
+    let mut toks = input.into_iter().collect::<Vec<_>>();
+    assert!(!toks.is_empty());
+    // Ensure that we have an `impl` item.
+    assert!(matches!(&toks[0], TokenTree::Ident(i) if i.to_string() == "impl"));
+    // Ensure that we are implementing `PinnedDrop`.
+    let mut nesting: usize = 0;
+    let mut pinned_drop_idx = None;
+    for (i, tt) in toks.iter().enumerate() {
+        match tt {
+            TokenTree::Punct(p) if p.as_char() == '<' => {
+                nesting += 1;
+            }
+            TokenTree::Punct(p) if p.as_char() == '>' => {
+                nesting = nesting.checked_sub(1).unwrap();
+                continue;
+            }
+            _ => {}
+        }
+        if i >= 1 && nesting == 0 {
+            // Found the end of the generics, this should be `PinnedDrop`.
+            assert!(
+                matches!(tt, TokenTree::Ident(i) if i.to_string() == "PinnedDrop"),
+                "expected 'PinnedDrop', found: '{:?}'",
+                tt
+            );
+            pinned_drop_idx = Some(i);
+            break;
+        }
+    }
+    let idx = pinned_drop_idx
+        .unwrap_or_else(|| panic!("Expected an `impl` block implementing `PinnedDrop`."));
+    // Fully qualify the `PinnedDrop`, as to avoid any tampering.
+    toks.splice(idx..idx, quote!(::kernel::init::));
+    // Take the `{}` body and call the declarative macro.
+    if let Some(TokenTree::Group(last)) = toks.pop() {
+        let last = last.stream();
+        quote!(::kernel::__pinned_drop! {
+            @impl_sig(#(#toks)*),
+            @impl_body(#last),
+        })
+    } else {
+        TokenStream::from_iter(toks)
+    }
+}