Fix bug when using a delagateable trait with const generics

ambassador/src/register.rs

@@ -137,7 +137,7 @@ fn param_to_matcher(param: &GenericParam) -> TokenStream {
             let ident = &lifetime.ident;
             quote!($ #ident : lifetime,)
         }
-        GenericParam::Const(ConstParam { ident, .. }) => quote!($ #ident : ident,),
+        GenericParam::Const(ConstParam { ident, .. }) => quote!($ #ident : expr,),
     }
 }
 
@@ -181,7 +181,7 @@ fn make_assoc_ty_bound(
     }
 }
 
-// Replaces the identifiers in gen_idents with there macro args (X => $X) ('a => $a)
+// Replaces the identifiers in gen_idents with their macro args (X => $X) ('a => $a)
 fn replace_gen_idents(tokens: TokenStream, gen_idents: &[&Ident]) -> TokenStream {
     let mut res = TokenStream::new();
     let mut apostrophe: Option<proc_macro2::Punct> = None;
@@ -217,6 +217,7 @@ fn build_method(
 ) -> TokenStream {
     quote! {
         #[inline]
+        #[allow(unused_braces)]
         #extr_attrs
         #method_sig {
             #method_invocation

ambassador/tests/run-pass/const_generic.rs

@@ -0,0 +1,31 @@
+extern crate ambassador;
+
+use ambassador::{delegatable_trait, Delegate};
+
+#[delegatable_trait]
+trait S<const N: usize> {
+    fn handle(&self, x: [u8; N]);
+}
+
+struct Base;
+
+impl<const N: usize> S<N> for Base {
+    fn handle(&self, x: [u8; N]) {
+        println!("{x:?}");
+    }
+}
+
+const C: usize = 2;
+
+#[derive(Delegate)]
+#[delegate(S<1>)]
+#[delegate(S<C>)]
+#[delegate(S<{C+1}>)]
+struct Wrap(Base);
+
+fn main() {
+    let w = Wrap(Base);
+    w.handle([1]);
+    w.handle([1, 2]);
+    w.handle([1, 2, 3]);
+}