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Date:   Wed, 13 Dec 2023 22:08:32 +0000
To:     Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        Benno Lossin <benno.lossin@proton.me>,
        Andreas Hindborg <a.hindborg@samsung.com>,
        Alice Ryhl <aliceryhl@google.com>,
        Sumera Priyadarsini <sylphrenadin@gmail.com>,
        Vincenzo Palazzo <vincenzopalazzodev@gmail.com>,
        Asahi Lina <lina@asahilina.net>,
        Martin Rodriguez Reboredo <yakoyoku@gmail.com>
From:   Benno Lossin <benno.lossin@proton.me>
Cc:     rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH v2 1/3] rust: macros: add `decl_generics` to `parse_generics()`
Message-ID: <20231213220447.3613500-1-benno.lossin@proton.me>
Feedback-ID: 71780778:user:proton
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Precedence: bulk

The generic parameters on a type definition can specify default values.
Currently `parse_generics()` cannot handle this though. For example when
parsing the following generics:

    <T: Clone, const N: usize =3D 0>

The `impl_generics` will be set to `T: Clone, const N: usize =3D 0` and
`ty_generics` will be set to `T, N`. Now using the `impl_generics` on an
impl block:

    impl<$($impl_generics)*> Foo {}

will result in invalid Rust code, because default values are only
available on type definitions.

Therefore add parsing support for generic parameter default values using
a new kind of generics called `decl_generics` and change the old
behavior of `impl_generics` to not contain the generic parameter default
values.

Now `Generics` has three fields:
- `impl_generics`: the generics with bounds
  (e.g. `T: Clone, const N: usize`)
- `decl_generics`: the generics with bounds and default values
  (e.g. `T: Clone, const N: usize =3D 0`)
- `ty_generics`:  contains the generics without bounds and without
  default values (e.g. `T, N`)

`impl_generics` is designed to be used on `impl<$impl_generics>`,
`decl_generics` for the type definition, so `struct Foo<$decl_generics>`
and `ty_generics` whenever you use the type, so `Foo<$ty_generics>`.

Here is an example that uses all three different types of generics:

    let (Generics { decl_generics, impl_generics, ty_generics }, rest) =3D =
parse_generics(input);
    quote! {
        struct Foo<$($decl_generics)*> {
            // ...
        }

        impl<$impl_generics> Foo<$ty_generics> {
            fn foo() {
                // ...
            }
        }
    }

The next commit contains a fix to the `#[pin_data]` macro making it
compatible with generic parameter default values by relying on this new
behavior.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>
---
v1 -> v2:
- improve documentation and commit message explanation
- add motivation to commit message

 rust/macros/helpers.rs  | 122 ++++++++++++++++++++++++++++++----------
 rust/macros/pin_data.rs |   1 +
 rust/macros/zeroable.rs |   1 +
 3 files changed, 94 insertions(+), 30 deletions(-)

diff --git a/rust/macros/helpers.rs b/rust/macros/helpers.rs
index afb0f2e3a36a..3f50a5c847c8 100644
--- a/rust/macros/helpers.rs
+++ b/rust/macros/helpers.rs
@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
=20
-use proc_macro::{token_stream, Group, Punct, Spacing, TokenStream, TokenTr=
ee};
+use proc_macro::{token_stream, Group, TokenStream, TokenTree};
=20
 pub(crate) fn try_ident(it: &mut token_stream::IntoIter) -> Option<String>=
 {
     if let Some(TokenTree::Ident(ident)) =3D it.next() {
@@ -70,8 +70,40 @@ pub(crate) fn expect_end(it: &mut token_stream::IntoIter=
) {
     }
 }
=20
+/// Parsed generics.
+///
+/// See the field documentation for an explanation what each of the fields=
 represents.
+///
+/// # Examples
+///
+/// ```rust,ignore
+/// # let input =3D todo!();
+/// let (Generics { decl_generics, impl_generics, ty_generics }, rest) =3D=
 parse_generics(input);
+/// quote! {
+///     struct Foo<$($decl_generics)*> {
+///         // ...
+///     }
+///
+///     impl<$impl_generics> Foo<$ty_generics> {
+///         fn foo() {
+///             // ...
+///         }
+///     }
+/// }
+/// ```
 pub(crate) struct Generics {
+    /// The generics with bounds and default values (e.g. `T: Clone, const=
 N: usize =3D 0`).
+    ///
+    /// Use this on type definitions e.g. `struct Foo<$decl_generics> ...`=
 (or `union`/`enum`).
+    pub(crate) decl_generics: Vec<TokenTree>,
+    /// The generics with bounds (e.g. `T: Clone, const N: usize`).
+    ///
+    /// Use this on `impl` blocks e.g. `impl<$impl_generics> Trait for ...=
`.
     pub(crate) impl_generics: Vec<TokenTree>,
+    /// The generics without bounds and without default values (e.g. `T, N=
`).
+    ///
+    /// Use this when you use the type that is declared with these generic=
s e.g.
+    /// `Foo<$ty_generics>`.
     pub(crate) ty_generics: Vec<TokenTree>,
 }
=20
@@ -81,6 +113,8 @@ pub(crate) struct Generics {
 pub(crate) fn parse_generics(input: TokenStream) -> (Generics, Vec<TokenTr=
ee>) {
     // `impl_generics`, the declared generics with their bounds.
     let mut impl_generics =3D vec![];
+    // The generics with bounds and default values.
+    let mut decl_generics =3D vec![];
     // Only the names of the generics, without any bounds.
     let mut ty_generics =3D vec![];
     // Tokens not related to the generics e.g. the `where` token and defin=
ition.
@@ -90,10 +124,17 @@ pub(crate) fn parse_generics(input: TokenStream) -> (G=
enerics, Vec<TokenTree>) {
     let mut toks =3D input.into_iter();
     // If we are at the beginning of a generic parameter.
     let mut at_start =3D true;
-    for tt in &mut toks {
+    let mut skip_until_comma =3D false;
+    while let Some(tt) =3D toks.next() {
+        if nesting =3D=3D 1 && matches!(&tt, TokenTree::Punct(p) if p.as_c=
har() =3D=3D '>') {
+            // Found the end of the generics.
+            break;
+        } else if nesting >=3D 1 {
+            decl_generics.push(tt.clone());
+        }
         match tt.clone() {
             TokenTree::Punct(p) if p.as_char() =3D=3D '<' =3D> {
-                if nesting >=3D 1 {
+                if nesting >=3D 1 && !skip_until_comma {
                     // This is inside of the generics and part of some bou=
nd.
                     impl_generics.push(tt);
                 }
@@ -105,49 +146,70 @@ pub(crate) fn parse_generics(input: TokenStream) -> (=
Generics, Vec<TokenTree>) {
                     break;
                 } else {
                     nesting -=3D 1;
-                    if nesting >=3D 1 {
+                    if nesting >=3D 1 && !skip_until_comma {
                         // We are still inside of the generics and part of=
 some bound.
                         impl_generics.push(tt);
                     }
-                    if nesting =3D=3D 0 {
-                        break;
-                    }
                 }
             }
-            tt =3D> {
+            TokenTree::Punct(p) if skip_until_comma && p.as_char() =3D=3D =
',' =3D> {
                 if nesting =3D=3D 1 {
-                    // Here depending on the token, it might be a generic =
variable name.
-                    match &tt {
-                        // Ignore const.
-                        TokenTree::Ident(i) if i.to_string() =3D=3D "const=
" =3D> {}
-                        TokenTree::Ident(_) if at_start =3D> {
-                            ty_generics.push(tt.clone());
-                            // We also already push the `,` token, this ma=
kes it easier to append
-                            // generics.
-                            ty_generics.push(TokenTree::Punct(Punct::new('=
,', Spacing::Alone)));
-                            at_start =3D false;
-                        }
-                        TokenTree::Punct(p) if p.as_char() =3D=3D ',' =3D>=
 at_start =3D true,
-                        // Lifetimes begin with `'`.
-                        TokenTree::Punct(p) if p.as_char() =3D=3D '\'' && =
at_start =3D> {
-                            ty_generics.push(tt.clone());
-                        }
-                        _ =3D> {}
-                    }
+                    impl_generics.push(TokenTree::Punct(p.clone()));
+                    ty_generics.push(TokenTree::Punct(p));
+                    skip_until_comma =3D false;
                 }
-                if nesting >=3D 1 {
-                    impl_generics.push(tt);
-                } else if nesting =3D=3D 0 {
+            }
+            tt if !skip_until_comma =3D> {
+                match nesting {
                     // If we haven't entered the generics yet, we still wa=
nt to keep these tokens.
-                    rest.push(tt);
+                    0 =3D> rest.push(tt),
+                    1 =3D> {
+                        // Here depending on the token, it might be a gene=
ric variable name.
+                        match tt {
+                            TokenTree::Ident(i) if at_start && i.to_string=
() =3D=3D "const" =3D> {
+                                let Some(name) =3D toks.next() else {
+                                    // Parsing error.
+                                    break;
+                                };
+                                impl_generics.push(TokenTree::Ident(i));
+                                impl_generics.push(name.clone());
+                                ty_generics.push(name.clone());
+                                decl_generics.push(name);
+                                at_start =3D false;
+                            }
+                            tt @ TokenTree::Ident(_) if at_start =3D> {
+                                impl_generics.push(tt.clone());
+                                ty_generics.push(tt);
+                                at_start =3D false;
+                            }
+                            TokenTree::Punct(p) if p.as_char() =3D=3D ',' =
=3D> {
+                                impl_generics.push(TokenTree::Punct(p.clon=
e()));
+                                ty_generics.push(TokenTree::Punct(p));
+                                at_start =3D true;
+                            }
+                            // Lifetimes begin with `'`.
+                            TokenTree::Punct(p) if p.as_char() =3D=3D '\''=
 && at_start =3D> {
+                                ty_generics.push(TokenTree::Punct(p.clone(=
)));
+                                impl_generics.push(TokenTree::Punct(p));
+                            }
+                            // Generics can have default values, we skip t=
hese.
+                            TokenTree::Punct(p) if p.as_char() =3D=3D '=3D=
' =3D> {
+                                skip_until_comma =3D true;
+                            }
+                            tt =3D> impl_generics.push(tt),
+                        }
+                    }
+                    _ =3D> impl_generics.push(tt),
                 }
             }
+            _ =3D> {}
         }
     }
     rest.extend(toks);
     (
         Generics {
             impl_generics,
+            decl_generics,
             ty_generics,
         },
         rest,
diff --git a/rust/macros/pin_data.rs b/rust/macros/pin_data.rs
index 6d58cfda9872..022e68e9720d 100644
--- a/rust/macros/pin_data.rs
+++ b/rust/macros/pin_data.rs
@@ -10,6 +10,7 @@ pub(crate) fn pin_data(args: TokenStream, input: TokenStr=
eam) -> TokenStream {
     let (
         Generics {
             impl_generics,
+            decl_generics: _,
             ty_generics,
         },
         rest,
diff --git a/rust/macros/zeroable.rs b/rust/macros/zeroable.rs
index 0d605c46ab3b..cfee2cec18d5 100644
--- a/rust/macros/zeroable.rs
+++ b/rust/macros/zeroable.rs
@@ -7,6 +7,7 @@ pub(crate) fn derive(input: TokenStream) -> TokenStream {
     let (
         Generics {
             impl_generics,
+            decl_generics: _,
             ty_generics,
         },
         mut rest,

base-commit: d9857c16cfc6bce7764e1b79956c6a028f97f4d0
--=20
2.42.0