rust: macros: add `#[vtable]` proc macro

This procedural macro attribute provides a simple way to declare
a trait with a set of operations that later users can partially
implement, providing compile-time `HAS_*` boolean associated
constants that indicate whether a particular operation was overridden.

This is useful as the Rust counterpart to structs like
`file_operations` where some pointers may be `NULL`, indicating
an operation is not provided.

For instance:

    #[vtable]
    trait Operations {
        fn read(...) -> Result<usize> {
            Err(EINVAL)
        }

        fn write(...) -> Result<usize> {
            Err(EINVAL)
        }
    }

    #[vtable]
    impl Operations for S {
        fn read(...) -> Result<usize> {
            ...
        }
    }

    assert_eq!(<S as Operations>::HAS_READ, true);
    assert_eq!(<S as Operations>::HAS_WRITE, false);

Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Sergio González Collado <sergio.collado@gmail.com>
[Reworded, adapted for upstream and applied latest changes]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 95 insertions, 0 deletions

diff --git a/rust/macros/vtable.rs b/rust/macros/vtable.rs
new file mode 100644
index 000000000000..34d5e7fb5768
--- /dev/null
+++ b/rust/macros/vtable.rs
@@ -0,0 +1,95 @@
+// SPDX-License-Identifier: GPL-2.0
+
+use proc_macro::{Delimiter, Group, TokenStream, TokenTree};
+use std::collections::HashSet;
+use std::fmt::Write;
+
+pub(crate) fn vtable(_attr: TokenStream, ts: TokenStream) -> TokenStream {
+    let mut tokens: Vec<_> = ts.into_iter().collect();
+
+    // Scan for the `trait` or `impl` keyword.
+    let is_trait = tokens
+        .iter()
+        .find_map(|token| match token {
+            TokenTree::Ident(ident) => match ident.to_string().as_str() {
+                "trait" => Some(true),
+                "impl" => Some(false),
+                _ => None,
+            },
+            _ => None,
+        })
+        .expect("#[vtable] attribute should only be applied to trait or impl block");
+
+    // Retrieve the main body. The main body should be the last token tree.
+    let body = match tokens.pop() {
+        Some(TokenTree::Group(group)) if group.delimiter() == Delimiter::Brace => group,
+        _ => panic!("cannot locate main body of trait or impl block"),
+    };
+
+    let mut body_it = body.stream().into_iter();
+    let mut functions = Vec::new();
+    let mut consts = HashSet::new();
+    while let Some(token) = body_it.next() {
+        match token {
+            TokenTree::Ident(ident) if ident.to_string() == "fn" => {
+                let fn_name = match body_it.next() {
+                    Some(TokenTree::Ident(ident)) => ident.to_string(),
+                    // Possibly we've encountered a fn pointer type instead.
+                    _ => continue,
+                };
+                functions.push(fn_name);
+            }
+            TokenTree::Ident(ident) if ident.to_string() == "const" => {
+                let const_name = match body_it.next() {
+                    Some(TokenTree::Ident(ident)) => ident.to_string(),
+                    // Possibly we've encountered an inline const block instead.
+                    _ => continue,
+                };
+                consts.insert(const_name);
+            }
+            _ => (),
+        }
+    }
+
+    let mut const_items;
+    if is_trait {
+        const_items = "
+                /// A marker to prevent implementors from forgetting to use [`#[vtable]`](vtable)
+                /// attribute when implementing this trait.
+                const USE_VTABLE_ATTR: ();
+        "
+        .to_owned();
+
+        for f in functions {
+            let gen_const_name = format!("HAS_{}", f.to_uppercase());
+            // Skip if it's declared already -- this allows user override.
+            if consts.contains(&gen_const_name) {
+                continue;
+            }
+            // We don't know on the implementation-site whether a method is required or provided
+            // so we have to generate a const for all methods.
+            write!(
+                const_items,
+                "/// Indicates if the `{f}` method is overridden by the implementor.
+                const {gen_const_name}: bool = false;",
+            )
+            .unwrap();
+        }
+    } else {
+        const_items = "const USE_VTABLE_ATTR: () = ();".to_owned();
+
+        for f in functions {
+            let gen_const_name = format!("HAS_{}", f.to_uppercase());
+            if consts.contains(&gen_const_name) {
+                continue;
+            }
+            write!(const_items, "const {gen_const_name}: bool = true;").unwrap();
+        }
+    }
+
+    let new_body = vec![const_items.parse().unwrap(), body.stream()]
+        .into_iter()
+        .collect();
+    tokens.push(TokenTree::Group(Group::new(Delimiter::Brace, new_body)));
+    tokens.into_iter().collect()
+}