[PATCH v3 3/4] rust: macros: Enable use from macro_rules!

["Ethan D. Twardy" <ethan.twardy@gmail.com>]

According to the rustdoc for the proc_macro crate[1], tokens captured
from a "macro variable" (e.g. from within macro_rules!) may be delimited
by invisible tokens and be contained within a proc_macro::Group.
Previously, this scenario was not handled by macros::paste, which caused
a proc-macro panic when the corresponding tests are enabled. Enable the
tests, and handle this case by making macros::paste::concat recursive.

Link: https://doc.rust-lang.org/stable/proc_macro/enum.Delimiter.html [1]
Signed-off-by: Ethan D. Twardy <ethan.twardy@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>

diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs
index 1c4ae5789cfa..3278f7c8aa5e 100644
--- a/rust/macros/lib.rs
+++ b/rust/macros/lib.rs
@@ -268,12 +268,25 @@ pub fn pinned_drop(args: TokenStream, input: TokenStream) -> TokenStream {
 ///
 /// # Example
 ///
-/// ```ignore
-/// use kernel::macro::paste;
-///
+/// ```
+/// # const binder_driver_return_protocol_BR_OK: u32 = 0;
+/// # const binder_driver_return_protocol_BR_ERROR: u32 = 1;
+/// # const binder_driver_return_protocol_BR_TRANSACTION: u32 = 2;
+/// # const binder_driver_return_protocol_BR_REPLY: u32 = 3;
+/// # const binder_driver_return_protocol_BR_DEAD_REPLY: u32 = 4;
+/// # const binder_driver_return_protocol_BR_TRANSACTION_COMPLETE: u32 = 5;
+/// # const binder_driver_return_protocol_BR_INCREFS: u32 = 6;
+/// # const binder_driver_return_protocol_BR_ACQUIRE: u32 = 7;
+/// # const binder_driver_return_protocol_BR_RELEASE: u32 = 8;
+/// # const binder_driver_return_protocol_BR_DECREFS: u32 = 9;
+/// # const binder_driver_return_protocol_BR_NOOP: u32 = 10;
+/// # const binder_driver_return_protocol_BR_SPAWN_LOOPER: u32 = 11;
+/// # const binder_driver_return_protocol_BR_DEAD_BINDER: u32 = 12;
+/// # const binder_driver_return_protocol_BR_CLEAR_DEATH_NOTIFICATION_DONE: u32 = 13;
+/// # const binder_driver_return_protocol_BR_FAILED_REPLY: u32 = 14;
 /// macro_rules! pub_no_prefix {
 ///     ($prefix:ident, $($newname:ident),+) => {
-///         paste! {
+///         kernel::macros::paste! {
 ///             $(pub(crate) const $newname: u32 = [<$prefix $newname>];)+
 ///         }
 ///     };
@@ -312,13 +325,28 @@ pub fn pinned_drop(args: TokenStream, input: TokenStream) -> TokenStream {
 /// * `lower`: change the identifier to lower case.
 /// * `upper`: change the identifier to upper case.
 ///
-/// ```ignore
-/// use kernel::macro::paste;
-///
+/// ```rust
+/// # const binder_driver_return_protocol_BR_OK: u32 = 0;
+/// # const binder_driver_return_protocol_BR_ERROR: u32 = 1;
+/// # const binder_driver_return_protocol_BR_TRANSACTION: u32 = 2;
+/// # const binder_driver_return_protocol_BR_REPLY: u32 = 3;
+/// # const binder_driver_return_protocol_BR_DEAD_REPLY: u32 = 4;
+/// # const binder_driver_return_protocol_BR_TRANSACTION_COMPLETE: u32 = 5;
+/// # const binder_driver_return_protocol_BR_INCREFS: u32 = 6;
+/// # const binder_driver_return_protocol_BR_ACQUIRE: u32 = 7;
+/// # const binder_driver_return_protocol_BR_RELEASE: u32 = 8;
+/// # const binder_driver_return_protocol_BR_DECREFS: u32 = 9;
+/// # const binder_driver_return_protocol_BR_NOOP: u32 = 10;
+/// # const binder_driver_return_protocol_BR_SPAWN_LOOPER: u32 = 11;
+/// # const binder_driver_return_protocol_BR_DEAD_BINDER: u32 = 12;
+/// # const binder_driver_return_protocol_BR_CLEAR_DEATH_NOTIFICATION_DONE: u32 = 13;
+/// # const binder_driver_return_protocol_BR_FAILED_REPLY: u32 = 14;
 /// macro_rules! pub_no_prefix {
 ///     ($prefix:ident, $($newname:ident),+) => {
 ///         kernel::macros::paste! {
-///             $(pub(crate) const fn [<$newname:lower:span>]: u32 = [<$prefix $newname:span>];)+
+///             $(pub(crate) const fn [<$newname:lower:span>]() -> u32 {
+///                 [<$prefix $newname:span>]
+///             })+
 ///         }
 ///     };
 /// }
@@ -349,7 +377,7 @@ pub fn pinned_drop(args: TokenStream, input: TokenStream) -> TokenStream {
 ///
 /// Literals can also be concatenated with other identifiers:
 ///
-/// ```ignore
+/// ```rust
 /// macro_rules! create_numbered_fn {
 ///     ($name:literal, $val:literal) => {
 ///         kernel::macros::paste! {
diff --git a/rust/macros/paste.rs b/rust/macros/paste.rs
index f40d42b35b58..6529a387673f 100644
--- a/rust/macros/paste.rs
+++ b/rust/macros/paste.rs
@@ -2,7 +2,7 @@
 
 use proc_macro::{Delimiter, Group, Ident, Spacing, Span, TokenTree};
 
-fn concat(tokens: &[TokenTree], group_span: Span) -> TokenTree {
+fn concat_helper(tokens: &[TokenTree]) -> Vec<(String, Span)> {
     let mut tokens = tokens.iter();
     let mut segments = Vec::new();
     let mut span = None;
@@ -46,12 +46,21 @@ fn concat(tokens: &[TokenTree], group_span: Span) -> TokenTree {
                 };
                 segments.push((value, sp));
             }
-            _ => panic!("unexpected token in paste segments"),
+            Some(TokenTree::Group(group)) if group.delimiter() == Delimiter::None => {
+                let tokens = group.stream().into_iter().collect::<Vec<TokenTree>>();
+                segments.append(&mut concat_helper(tokens.as_slice()));
+            }
+            token => panic!("unexpected token in paste segments: {:?}", token),
         };
     }
 
+    segments
+}
+
+fn concat(tokens: &[TokenTree], group_span: Span) -> TokenTree {
+    let segments = concat_helper(tokens);
     let pasted: String = segments.into_iter().map(|x| x.0).collect();
-    TokenTree::Ident(Ident::new(&pasted, span.unwrap_or(group_span)))
+    TokenTree::Ident(Ident::new(&pasted, group_span))
 }
 
 pub(crate) fn expand(tokens: &mut Vec<TokenTree>) {
-- 
2.44.2