[PATCH v3 2/5] rust: macros: support additional tokens in quote!

[Alice Ryhl]

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

* The = token.
* The _ token.
* The # token. (when not followed by an identifier)
* 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, there are zero `.push`
calls, so the compiler can't infer the item type and also emits a
warning about it not needing to be mutable.

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.

Reviewed-by: Andreas Hindborg <a.hindborg at kernel.org>
Reviewed-by: Tamir Duberstein <tamird at gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Signed-off-by: Alice Ryhl <aliceryhl at google.com>
---
 rust/macros/quote.rs | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/rust/macros/quote.rs b/rust/macros/quote.rs
index 33a199e4f176..31b7ebe504f4 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 +143,22 @@ 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::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