[PATCH v2 2/5] rust: macros: support additional tokens in quote!

[Alice Ryhl <aliceryhl@google.com>]

This gives the quote! macro support for the following additional tokens:

* The = token.
* The _ token.
* Using #my_var with variables of type Ident.

Additionally, some type annotations are added to allow cases where
groups are empty. For example, quote! does support () in the input, but
only when it is *not* empty. When it is empty, the compiler cannot infer
the item type of `tokens`.

These additional quote! features are used by a new proc macro that
generates code looking like this:

	const _: () = {
	    if true {
	        ::kernel::bindings::#name
	    } else {
	        #name
	    };
	};

where #name has type Ident.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/macros/quote.rs | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/rust/macros/quote.rs b/rust/macros/quote.rs
index 33a199e4f176..c18960a91082 100644
--- a/rust/macros/quote.rs
+++ b/rust/macros/quote.rs
@@ -20,6 +20,12 @@ fn to_tokens(&self, tokens: &mut TokenStream) {
     }
 }
 
+impl ToTokens for proc_macro::Ident {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        tokens.extend([TokenTree::from(self.clone())]);
+    }
+}
+
 impl ToTokens for TokenTree {
     fn to_tokens(&self, tokens: &mut TokenStream) {
         tokens.extend([self.clone()]);
@@ -40,7 +46,7 @@ fn to_tokens(&self, tokens: &mut TokenStream) {
 /// `quote` crate but provides only just enough functionality needed by the current `macros` crate.
 macro_rules! quote_spanned {
     ($span:expr => $($tt:tt)*) => {{
-        let mut tokens;
+        let mut tokens: ::std::vec::Vec<::proc_macro::TokenTree>;
         #[allow(clippy::vec_init_then_push)]
         {
             tokens = ::std::vec::Vec::new();
@@ -65,7 +71,8 @@ macro_rules! quote_spanned {
         quote_spanned!(@proc $v $span $($tt)*);
     };
     (@proc $v:ident $span:ident ( $($inner:tt)* ) $($tt:tt)*) => {
-        let mut tokens = ::std::vec::Vec::new();
+        #[allow(unused_mut)]
+        let mut tokens = ::std::vec::Vec::<::proc_macro::TokenTree>::new();
         quote_spanned!(@proc tokens $span $($inner)*);
         $v.push(::proc_macro::TokenTree::Group(::proc_macro::Group::new(
             ::proc_macro::Delimiter::Parenthesis,
@@ -136,6 +143,16 @@ macro_rules! quote_spanned {
         ));
         quote_spanned!(@proc $v $span $($tt)*);
     };
+    (@proc $v:ident $span:ident = $($tt:tt)*) => {
+        $v.push(::proc_macro::TokenTree::Punct(
+                ::proc_macro::Punct::new('=', ::proc_macro::Spacing::Alone)
+        ));
+        quote_spanned!(@proc $v $span $($tt)*);
+    };
+    (@proc $v:ident $span:ident _ $($tt:tt)*) => {
+        $v.push(::proc_macro::TokenTree::Ident(::proc_macro::Ident::new("_", $span)));
+        quote_spanned!(@proc $v $span $($tt)*);
+    };
     (@proc $v:ident $span:ident $id:ident $($tt:tt)*) => {
         $v.push(::proc_macro::TokenTree::Ident(::proc_macro::Ident::new(stringify!($id), $span)));
         quote_spanned!(@proc $v $span $($tt)*);

-- 
2.48.1.711.g2feabab25a-goog