linux/rust/macros/vtable.rs
Gary Guo b44becc5ee 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>
2022-12-04 01:59:15 +01:00

96 lines
3.5 KiB
Rust

// 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()
}