rust: add derive macro for `Zeroable`

Add a derive proc-macro for the `Zeroable` trait. The macro supports
structs where every field implements the `Zeroable` trait. This way
`unsafe` implementations can be avoided.

The macro is split into two parts:
- a proc-macro to parse generics into impl and ty generics,
- a declarative macro that expands to the impl block.

Suggested-by: Asahi Lina <lina@asahilina.net>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Link: https://lore.kernel.org/r/20230814084602.25699-4-benno.lossin@proton.me
[ Added `ignore` to the `lib.rs` example and cleaned trivial nit. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 72 insertions, 0 deletions

diff --git a/rust/macros/zeroable.rs b/rust/macros/zeroable.rs
new file mode 100644
index 000000000000..0d605c46ab3b
--- /dev/null
+++ b/rust/macros/zeroable.rs
@@ -0,0 +1,72 @@
+// SPDX-License-Identifier: GPL-2.0
+
+use crate::helpers::{parse_generics, Generics};
+use proc_macro::{TokenStream, TokenTree};
+
+pub(crate) fn derive(input: TokenStream) -> TokenStream {
+    let (
+        Generics {
+            impl_generics,
+            ty_generics,
+        },
+        mut rest,
+    ) = parse_generics(input);
+    // This should be the body of the struct `{...}`.
+    let last = rest.pop();
+    // Now we insert `Zeroable` as a bound for every generic parameter in `impl_generics`.
+    let mut new_impl_generics = Vec::with_capacity(impl_generics.len());
+    // Are we inside of a generic where we want to add `Zeroable`?
+    let mut in_generic = !impl_generics.is_empty();
+    // Have we already inserted `Zeroable`?
+    let mut inserted = false;
+    // Level of `<>` nestings.
+    let mut nested = 0;
+    for tt in impl_generics {
+        match &tt {
+            // If we find a `,`, then we have finished a generic/constant/lifetime parameter.
+            TokenTree::Punct(p) if nested == 0 && p.as_char() == ',' => {
+                if in_generic && !inserted {
+                    new_impl_generics.extend(quote! { : ::kernel::init::Zeroable });
+                }
+                in_generic = true;
+                inserted = false;
+                new_impl_generics.push(tt);
+            }
+            // If we find `'`, then we are entering a lifetime.
+            TokenTree::Punct(p) if nested == 0 && p.as_char() == '\'' => {
+                in_generic = false;
+                new_impl_generics.push(tt);
+            }
+            TokenTree::Punct(p) if nested == 0 && p.as_char() == ':' => {
+                new_impl_generics.push(tt);
+                if in_generic {
+                    new_impl_generics.extend(quote! { ::kernel::init::Zeroable + });
+                    inserted = true;
+                }
+            }
+            TokenTree::Punct(p) if p.as_char() == '<' => {
+                nested += 1;
+                new_impl_generics.push(tt);
+            }
+            TokenTree::Punct(p) if p.as_char() == '>' => {
+                assert!(nested > 0);
+                nested -= 1;
+                new_impl_generics.push(tt);
+            }
+            _ => new_impl_generics.push(tt),
+        }
+    }
+    assert_eq!(nested, 0);
+    if in_generic && !inserted {
+        new_impl_generics.extend(quote! { : ::kernel::init::Zeroable });
+    }
+    quote! {
+        ::kernel::__derive_zeroable!(
+            parse_input:
+                @sig(#(#rest)*),
+                @impl_generics(#(#new_impl_generics)*),
+                @ty_generics(#(#ty_generics)*),
+                @body(#last),
+        );
+    }
+}