address PR review feedback for unstable-book-gen

compiler/rustc_session/src/options.rs

@@ -604,6 +604,10 @@ pub fn is_target_modifier(flag_name: &str) -> bool {
 /// necessary code. The main gotcha of this macro is the `cgsetters` module which is a bunch of
 /// generated code to parse an option into its respective field in the struct. There are a few
 /// hand-written parsers for parsing specific types of values in this module.
+///
+/// Note: this macro's invocation is also parsed by a `syn`-based parser in
+/// `src/tools/unstable-book-gen/src/main.rs` to extract unstable option names and descriptions.
+/// If the format of this macro changes, that parser may need to be updated as well.
 macro_rules! options {
     ($struct_name:ident, $tmod_enum_name:ident, $stat:ident, $optmod:ident, $prefix:expr, $outputname:expr,
      $($( #[$attr:meta] )* $opt:ident : $t:ty = (

src/tools/unstable-book-gen/src/main.rs

@@ -158,7 +158,7 @@ fn parse(input: ParseStream<'_>) -> syn::Result<Self> {
             return Err(syn::Error::new(
                 name.span(),
                 format!(
-                    "unexpected field count for option `{name}`: expected 4 or 5, found {}",
+                    "unexpected field count for option `{name}`: expected {REQUIRED_FIELDS} or {OPTIONAL_FIELDS}, found {}",
                     tuple_fields.len()
                 ),
             ));