From 1383a6ae8d34a53e2b1b5d22a130def17807618b Mon Sep 17 00:00:00 2001
From: Andy Yang <71013200+Y-jiji@users.noreply.github.com>
Date: Mon, 11 Dec 2023 01:22:18 +0800
Subject: [PATCH] Feature: naive support for const param generics.  (#106)

* Feature: const generic parameter.

* Feature: support for const generic parameters.

* Fix: removing dummy expand.rs generated in testing.

* Use an imported type in the example.

---------

Co-authored-by: y-jiji <y.jijiji.data.science@gmail.com>
Co-authored-by: joshua-maros <60271685+joshua-maros@users.noreply.github.com>
---
 examples/src/lib.rs          | 10 ++++++++++
 ouroboros_macro/src/utils.rs | 17 +++++++++++++++--
 2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/examples/src/lib.rs b/examples/src/lib.rs
index 3198651..df5475d 100644
--- a/examples/src/lib.rs
+++ b/examples/src/lib.rs
@@ -8,6 +8,16 @@ use ouroboros::self_referencing;
 #[cfg(test)]
 mod ok_tests;
 
+pub struct Ext<'this, T, const REV: bool>(&'this Box<T>);
+
+#[self_referencing(pub_extras)]
+pub struct WithConstParam<T: 'static, const REV: bool> {
+    data: core::cell::RefCell<T>,
+    #[borrows(data)]
+    #[not_covariant]
+    dref: Option<Box<Ext<'this, T, REV>>>,
+}
+
 #[self_referencing]
 /// A simple struct which contains an `i32` and a `&'this i32`.
 pub struct DataAndRef {
diff --git a/ouroboros_macro/src/utils.rs b/ouroboros_macro/src/utils.rs
index ec527df..c62ad29 100644
--- a/ouroboros_macro/src/utils.rs
+++ b/ouroboros_macro/src/utils.rs
@@ -28,7 +28,17 @@ pub fn make_generic_consumers(generics: &Generics) -> impl Iterator<Item = (Toke
                     format_ident!("_consume_template_lifetime_{}", ident),
                 )
             }
-            GenericParam::Const(..) => unimplemented!(),
+            // rustc don't require constants to consume, so we just skip it. 
+            GenericParam::Const(ct) => {
+                let ident = ct.ident;
+                (
+                    quote! { () },
+                    format_ident!(
+                        "_comsume_template_const_parameter_{}",
+                        ident.to_string().to_snake_case()
+                    ),
+                )
+            },
         })
 }
 
@@ -45,7 +55,10 @@ pub fn make_generic_arguments(generics: Vec<&GenericParam>) -> Vec<TokenStream>
                 let lifetime = &lt.lifetime;
                 arguments.push(quote! { #lifetime });
             }
-            GenericParam::Const(_) => unimplemented!("Const generics are not supported yet."),
+            GenericParam::Const(ct) => {
+                let ident = &ct.ident;
+                arguments.push(quote! { #ident });
+            },
         }
     }
     arguments