[v1,06/28] rust: macros: add `#[vtable]` proc macro

  From: Gary Guo <gary@garyguo.net>

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>
[Reworded, adapted for upstream and applied latest changes]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 rust/kernel/prelude.rs |  2 +-
 rust/macros/lib.rs     | 52 +++++++++++++++++++++++
 rust/macros/vtable.rs  | 95 ++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 148 insertions(+), 1 deletion(-)
 create mode 100644 rust/macros/vtable.rs
  

On Thu, 10 Nov 2022 at 11:43, Miguel Ojeda <ojeda@kernel.org> wrote:
>
> From: Gary Guo <gary@garyguo.net>
>
> 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>
> [Reworded, adapted for upstream and applied latest changes]
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
> ---
>  rust/kernel/prelude.rs |  2 +-
>  rust/macros/lib.rs     | 52 +++++++++++++++++++++++
>  rust/macros/vtable.rs  | 95 ++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 148 insertions(+), 1 deletion(-)
>  create mode 100644 rust/macros/vtable.rs
>
> diff --git a/rust/kernel/prelude.rs b/rust/kernel/prelude.rs
> index 6a1c6b38327f..7c4c35bf3c66 100644
> --- a/rust/kernel/prelude.rs
> +++ b/rust/kernel/prelude.rs
> @@ -15,7 +15,7 @@ pub use core::pin::Pin;
>
>  pub use alloc::{boxed::Box, vec::Vec};
>
> -pub use macros::module;
> +pub use macros::{module, vtable};
>
>  pub use super::{pr_alert, pr_crit, pr_debug, pr_emerg, pr_err, pr_info, pr_notice, pr_warn};
>
> diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs
> index 15555e7ff487..e40caaf0a656 100644
> --- a/rust/macros/lib.rs
> +++ b/rust/macros/lib.rs
> @@ -5,6 +5,7 @@
>  mod concat_idents;
>  mod helpers;
>  mod module;
> +mod vtable;
>
>  use proc_macro::TokenStream;
>
> @@ -72,6 +73,57 @@ pub fn module(ts: TokenStream) -> TokenStream {
>      module::module(ts)
>  }
>
> +/// Declares or implements a vtable trait.
> +///
> +/// Linux's use of pure vtables is very close to Rust traits, but they differ
> +/// in how unimplemented functions are represented. In Rust, traits can provide
> +/// default implementation for all non-required methods (and the default
> +/// implementation could just return `Error::EINVAL`); Linux typically use C
> +/// `NULL` pointers to represent these functions.
> +///
> +/// This attribute is intended to close the gap. Traits can be declared and
> +/// implemented with the `#[vtable]` attribute, and a `HAS_*` associated constant
> +/// will be generated for each method in the trait, indicating if the implementor
> +/// has overridden a method.
> +///
> +/// This attribute is not needed if all methods are required.
> +///
> +/// # Examples
> +///
> +/// ```ignore
> +/// use kernel::prelude::*;
> +///
> +/// // Declares a `#[vtable]` trait
> +/// #[vtable]
> +/// pub trait Operations: Send + Sync + Sized {
> +///     fn foo(&self) -> Result<()> {
> +///         Err(EINVAL)
> +///     }
> +///
> +///     fn bar(&self) -> Result<()> {
> +///         Err(EINVAL)
> +///     }
> +/// }
> +///
> +/// struct Foo;
> +///
> +/// // Implements the `#[vtable]` trait
> +/// #[vtable]
> +/// impl Operations for Foo {
> +///     fn foo(&self) -> Result<()> {
> +/// #        Err(EINVAL)
> +///         // ...
> +///     }
> +/// }
> +///
> +/// assert_eq!(<Foo as Operations>::HAS_FOO, true);
> +/// assert_eq!(<Foo as Operations>::HAS_BAR, false);
> +/// ```
> +#[proc_macro_attribute]
> +pub fn vtable(attr: TokenStream, ts: TokenStream) -> TokenStream {
> +    vtable::vtable(attr, ts)
> +}
> +
>  /// Concatenate two identifiers.
>  ///
>  /// This is useful in macros that need to declare or reference items with names
> 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()
> +}
> --
> 2.38.1
>
Reviewed-by: Sergio González Collado <sergio.collado@gmail.com>