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

[Alice Ryhl]

On Fri, Feb 28, 2025 at 4:26 PM Tamir Duberstein <tamird at gmail.com> wrote:
>
> On Fri, Feb 28, 2025 at 7:42 AM Alice Ryhl <aliceryhl at google.com> wrote:
> >
> > 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 at 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)]
>
> It'd be nice to mention the need for this attribute in the commit
> message along with the added type annotations.

Adding a note to mention that when the () is empty, not only can't it
infer the item type, it's also never modified.

Alice