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b44becc5ee
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>
95 lines
3.5 KiB
Rust
95 lines
3.5 KiB
Rust
// SPDX-License-Identifier: GPL-2.0
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use proc_macro::{Delimiter, Group, TokenStream, TokenTree};
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use std::collections::HashSet;
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use std::fmt::Write;
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pub(crate) fn vtable(_attr: TokenStream, ts: TokenStream) -> TokenStream {
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let mut tokens: Vec<_> = ts.into_iter().collect();
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// Scan for the `trait` or `impl` keyword.
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let is_trait = tokens
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.iter()
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.find_map(|token| match token {
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TokenTree::Ident(ident) => match ident.to_string().as_str() {
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"trait" => Some(true),
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"impl" => Some(false),
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_ => None,
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},
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_ => None,
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})
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.expect("#[vtable] attribute should only be applied to trait or impl block");
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// Retrieve the main body. The main body should be the last token tree.
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let body = match tokens.pop() {
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Some(TokenTree::Group(group)) if group.delimiter() == Delimiter::Brace => group,
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_ => panic!("cannot locate main body of trait or impl block"),
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};
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let mut body_it = body.stream().into_iter();
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let mut functions = Vec::new();
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let mut consts = HashSet::new();
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while let Some(token) = body_it.next() {
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match token {
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TokenTree::Ident(ident) if ident.to_string() == "fn" => {
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let fn_name = match body_it.next() {
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Some(TokenTree::Ident(ident)) => ident.to_string(),
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// Possibly we've encountered a fn pointer type instead.
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_ => continue,
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};
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functions.push(fn_name);
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}
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TokenTree::Ident(ident) if ident.to_string() == "const" => {
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let const_name = match body_it.next() {
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Some(TokenTree::Ident(ident)) => ident.to_string(),
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// Possibly we've encountered an inline const block instead.
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_ => continue,
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};
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consts.insert(const_name);
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}
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_ => (),
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}
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}
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let mut const_items;
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if is_trait {
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const_items = "
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/// A marker to prevent implementors from forgetting to use [`#[vtable]`](vtable)
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/// attribute when implementing this trait.
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const USE_VTABLE_ATTR: ();
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"
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.to_owned();
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for f in functions {
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let gen_const_name = format!("HAS_{}", f.to_uppercase());
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// Skip if it's declared already -- this allows user override.
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if consts.contains(&gen_const_name) {
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continue;
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}
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// We don't know on the implementation-site whether a method is required or provided
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// so we have to generate a const for all methods.
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write!(
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const_items,
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"/// Indicates if the `{f}` method is overridden by the implementor.
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const {gen_const_name}: bool = false;",
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)
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.unwrap();
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}
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} else {
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const_items = "const USE_VTABLE_ATTR: () = ();".to_owned();
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for f in functions {
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let gen_const_name = format!("HAS_{}", f.to_uppercase());
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if consts.contains(&gen_const_name) {
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continue;
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}
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write!(const_items, "const {gen_const_name}: bool = true;").unwrap();
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}
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}
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let new_body = vec![const_items.parse().unwrap(), body.stream()]
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.into_iter()
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.collect();
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tokens.push(TokenTree::Group(Group::new(Delimiter::Brace, new_body)));
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tokens.into_iter().collect()
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}
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