rust: macros: refactor generics parsing of `#[pin_data]` into its own function

Other macros might also want to parse generics. Additionally this makes
the code easier to read, as the next commit will introduce more code in
`#[pin_data]`. Also add more comments to explain how parsing generics
work.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Link: https://lore.kernel.org/r/20230424081112.99890-2-benno.lossin@proton.me


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/macros/helpers.rs

 // SPDX-License-Identifier: GPL-2.0
 
-use proc_macro::{token_stream, Group, TokenTree};
+use proc_macro::{token_stream, Group, Punct, Spacing, TokenStream, TokenTree};
 
 pub(crate) fn try_ident(it: &mut token_stream::IntoIter) -> Option<String> {
     if let Some(TokenTree::Ident(ident)) = it.next() {
@@ -69,3 +69,87 @@ pub(crate) fn expect_end(it: &mut token_stream::IntoIter) {
         panic!("Expected end");
     }
 }
+
+pub(crate) struct Generics {
+    pub(crate) impl_generics: Vec<TokenTree>,
+    pub(crate) ty_generics: Vec<TokenTree>,
+}
+
+/// Parses the given `TokenStream` into `Generics` and the rest.
+///
+/// The generics are not present in the rest, but a where clause might remain.
+pub(crate) fn parse_generics(input: TokenStream) -> (Generics, Vec<TokenTree>) {
+    // `impl_generics`, the declared generics with their bounds.
+    let mut impl_generics = vec![];
+    // Only the names of the generics, without any bounds.
+    let mut ty_generics = vec![];
+    // Tokens not related to the generics e.g. the `where` token and definition.
+    let mut rest = vec![];
+    // The current level of `<`.
+    let mut nesting = 0;
+    let mut toks = input.into_iter();
+    // If we are at the beginning of a generic parameter.
+    let mut at_start = true;
+    for tt in &mut toks {
+        match tt.clone() {
+            TokenTree::Punct(p) if p.as_char() == '<' => {
+                if nesting >= 1 {
+                    // This is inside of the generics and part of some bound.
+                    impl_generics.push(tt);
+                }
+                nesting += 1;
+            }
+            TokenTree::Punct(p) if p.as_char() == '>' => {
+                // This is a parsing error, so we just end it here.
+                if nesting == 0 {
+                    break;
+                } else {
+                    nesting -= 1;
+                    if nesting >= 1 {
+                        // We are still inside of the generics and part of some bound.
+                        impl_generics.push(tt);
+                    }
+                    if nesting == 0 {
+                        break;
+                    }
+                }
+            }
+            tt => {
+                if nesting == 1 {
+                    // Here depending on the token, it might be a generic variable name.
+                    match &tt {
+                        // Ignore const.
+                        TokenTree::Ident(i) if i.to_string() == "const" => {}
+                        TokenTree::Ident(_) if at_start => {
+                            ty_generics.push(tt.clone());
+                            // We also already push the `,` token, this makes it easier to append
+                            // generics.
+                            ty_generics.push(TokenTree::Punct(Punct::new(',', Spacing::Alone)));
+                            at_start = false;
+                        }
+                        TokenTree::Punct(p) if p.as_char() == ',' => at_start = true,
+                        // Lifetimes begin with `'`.
+                        TokenTree::Punct(p) if p.as_char() == '\'' && at_start => {
+                            ty_generics.push(tt.clone());
+                        }
+                        _ => {}
+                    }
+                }
+                if nesting >= 1 {
+                    impl_generics.push(tt);
+                } else if nesting == 0 {
+                    // If we haven't entered the generics yet, we still want to keep these tokens.
+                    rest.push(tt);
+                }
+            }
+        }
+    }
+    rest.extend(toks);
+    (
+        Generics {
+            impl_generics,
+            ty_generics,
+        },
+        rest,
+    )
+}

rust/macros/pin_data.rs

 // SPDX-License-Identifier: Apache-2.0 OR MIT
 
-use proc_macro::{Punct, Spacing, TokenStream, TokenTree};
+use crate::helpers::{parse_generics, Generics};
+use proc_macro::TokenStream;
 
 pub(crate) fn pin_data(args: TokenStream, input: TokenStream) -> TokenStream {
     // This proc-macro only does some pre-parsing and then delegates the actual parsing to
     // `kernel::__pin_data!`.
-    //
-    // In here we only collect the generics, since parsing them in declarative macros is very
-    // elaborate. We also do not need to analyse their structure, we only need to collect them.
 
-    // `impl_generics`, the declared generics with their bounds.
-    let mut impl_generics = vec![];
-    // Only the names of the generics, without any bounds.
-    let mut ty_generics = vec![];
-    // Tokens not related to the generics e.g. the `impl` token.
-    let mut rest = vec![];
-    // The current level of `<`.
-    let mut nesting = 0;
-    let mut toks = input.into_iter();
-    // If we are at the beginning of a generic parameter.
-    let mut at_start = true;
-    for tt in &mut toks {
-        match tt.clone() {
-            TokenTree::Punct(p) if p.as_char() == '<' => {
-                if nesting >= 1 {
-                    impl_generics.push(tt);
-                }
-                nesting += 1;
-            }
-            TokenTree::Punct(p) if p.as_char() == '>' => {
-                if nesting == 0 {
-                    break;
-                } else {
-                    nesting -= 1;
-                    if nesting >= 1 {
-                        impl_generics.push(tt);
-                    }
-                    if nesting == 0 {
-                        break;
-                    }
-                }
-            }
-            tt => {
-                if nesting == 1 {
-                    match &tt {
-                        TokenTree::Ident(i) if i.to_string() == "const" => {}
-                        TokenTree::Ident(_) if at_start => {
-                            ty_generics.push(tt.clone());
-                            ty_generics.push(TokenTree::Punct(Punct::new(',', Spacing::Alone)));
-                            at_start = false;
-                        }
-                        TokenTree::Punct(p) if p.as_char() == ',' => at_start = true,
-                        TokenTree::Punct(p) if p.as_char() == '\'' && at_start => {
-                            ty_generics.push(tt.clone());
-                        }
-                        _ => {}
-                    }
-                }
-                if nesting >= 1 {
-                    impl_generics.push(tt);
-                } else if nesting == 0 {
-                    rest.push(tt);
-                }
-            }
-        }
-    }
-    rest.extend(toks);
+    let (
+        Generics {
+            impl_generics,
+            ty_generics,
+        },
+        mut rest,
+    ) = parse_generics(input);
     // This should be the body of the struct `{...}`.
     let last = rest.pop();
     quote!(::kernel::__pin_data! {