Replace custom const keyword parsing with new darling builtin key word parsing

bon-macros/src/builder/builder_gen/input_fn/validation.rs

@@ -30,14 +30,12 @@ impl super::FnInputCtx<'_> {
             }
         }
 
-        if let Some(const_) = &self.config.const_ {
-            if self.fn_item.orig.sig.constness.is_none() {
-                bail!(
-                    &const_,
-                    "#[builder(const)] requires the underlying function to be \
-                    marked as `const fn`"
-                );
-            }
+        if self.config.const_.is_present() && self.fn_item.orig.sig.constness.is_none() {
+            bail!(
+                &self.config.const_.span(),
+                "#[builder(const)] requires the underlying function to be \
+                marked as `const fn`"
+            );
         }
 
         Ok(())

bon-macros/src/builder/builder_gen/member/config/mod.rs

@@ -209,7 +209,7 @@ impl MemberConfig {
     }
 
     pub(crate) fn validate(&self, top_config: &TopLevelConfig, origin: MemberOrigin) -> Result {
-        if top_config.const_.is_some() {
+        if top_config.const_.is_present() {
             self.require_const_compat()?;
         }
 

bon-macros/src/builder/builder_gen/models.rs

@@ -181,7 +181,7 @@ pub(super) struct BuilderGenCtxParams<'a> {
     pub(super) members: Vec<Member>,
 
     pub(super) allow_attrs: Vec<syn::Attribute>,
-    pub(super) const_: Option<syn::Token![const]>,
+    pub(super) const_: darling::util::Flag,
     pub(super) on: Vec<OnConfig>,
 
     /// This is the visibility of the original item that the builder is generated for.
@@ -357,6 +357,10 @@ impl BuilderGenCtx {
             }
         });
 
+        let const_ = const_
+            .is_present()
+            .then(|| syn::Token![const](const_.span()));
+
         Ok(Self {
             bon,
             state_var,

bon-macros/src/builder/builder_gen/top_level_config/mod.rs

@@ -6,7 +6,6 @@ use crate::parsing::{BonCratePath, ItemSigConfig, ItemSigConfigParsing, SpannedK
 use crate::util::prelude::*;
 use darling::ast::NestedMeta;
 use darling::FromMeta;
-use syn::parse::Parser;
 use syn::punctuated::Punctuated;
 use syn::ItemFn;
 
@@ -44,13 +43,9 @@ fn parse_start_fn(meta: &syn::Meta) -> Result<ItemSigConfig> {
 
 #[derive(Debug, FromMeta)]
 pub(crate) struct TopLevelConfig {
-    /// Specifies whether the generated functions should be `const`.
-    ///
-    /// It is marked as `#[darling(skip)]` because `const` is a keyword, that
-    /// can't be parsed as a `syn::Ident` and therefore as a `syn::Meta` item.
-    /// We manually parse it from the beginning `builder(...)`.
-    #[darling(skip)]
-    pub(crate) const_: Option<syn::Token![const]>,
+    /// Specifies whether the generated functions should be `const`
+    #[darling(rename = "const")]
+    pub(crate) const_: darling::util::Flag,
 
     /// Overrides the path to the `bon` crate. This is useful when the macro is
     /// wrapped in another macro that also reexports `bon`.
@@ -129,30 +124,7 @@ impl TopLevelConfig {
         Self::parse_for_any(configs)
     }
 
-    fn parse_for_any(mut configs: Vec<TokenStream>) -> Result<Self> {
-        fn parse_const_prefix(
-            parse: syn::parse::ParseStream<'_>,
-        ) -> syn::Result<(Option<syn::Token![const]>, TokenStream)> {
-            let const_ = parse.parse().ok();
-            if const_.is_some() && !parse.is_empty() {
-                parse.parse::<syn::Token![,]>()?;
-            }
-
-            let rest = parse.parse()?;
-            Ok((const_, rest))
-        }
-
-        // Try to parse the first token of the first config as `const` token.
-        // We have to do this manually because `syn` doesn't support parsing
-        // keywords in the `syn::Meta` keys. Yeah, unfortunately it means that
-        // the users must ensure they place `const` right at the beginning of
-        // their `#[builder(...)]` attributes.
-        let mut const_ = None;
-
-        if let Some(config) = configs.first_mut() {
-            (const_, *config) = parse_const_prefix.parse2(std::mem::take(config))?;
-        }
-
+    fn parse_for_any(configs: Vec<TokenStream>) -> Result<Self> {
         let configs = configs
             .into_iter()
             .map(NestedMeta::parse_meta_list)
@@ -188,10 +160,7 @@ impl TopLevelConfig {
             }
         }
 
-        let me = Self {
-            const_,
-            ..Self::from_list(&configs)?
-        };
+        let me = Self::from_list(&configs)?;
 
         if let Some(on) = me.on.iter().skip(1).find(|on| on.required.is_present()) {
             bail!(

bon/tests/integration/builder/attr_const.rs

@@ -9,7 +9,8 @@ mod msrv_1_61 {
         #[test]
         const fn test_struct() {
             #[derive(Builder)]
-            #[builder(const)]
+            // Make sure `const` is parsed if it's not the first attribute
+            #[builder(builder_type(vis = ""), const)]
             struct Sut {
                 #[builder(start_fn)]
                 x1: u32,