Allow trailing commas in type definitions

src/gen.rs

@@ -1,4 +1,4 @@
-use proc_macro2::{Span as Span2, TokenStream as TokenStream2};
+use proc_macro2::{Span as Span2, TokenStream as TokenStream2, TokenTree as TokenTree2};
 use quote::{ToTokens, TokenStreamExt};
 use syn::{
     punctuated::Punctuated, spanned::Spanned, Attribute, Error, FnArg, GenericParam, Ident,
@@ -246,6 +246,20 @@ fn gen_header(
             .skip(1) // the opening `<`
             .collect::<Vec<_>>();
         tts.pop(); // the closing `>`
+
+        // Pop a trailing comma if there is one
+        // We'll end up conditionally putting one back in shortly
+        if tts.last().and_then(|tt| {
+            if let TokenTree2::Punct(p) = tt {
+                Some(p.as_char())
+            } else {
+                None
+            }
+        }) == Some(',')
+        {
+            tts.pop();
+        }
+
         params.append_all(&tts);
 
         // Append proxy type parameter (if there aren't any parameters so far,

tests/compile-fail/super_trait_not_implemented.stderr

@@ -1,12 +1,11 @@
-error[E0277]: the trait bound `Box<Dog>: Supi` is not satisfied
+error[E0277]: the trait bound `Box<Dog>: Foo` is not satisfied
   --> tests/compile-fail/super_trait_not_implemented.rs:18:18
    |
 18 |     requires_foo(Box::new(Dog)); // shouldn't, because `Box<Dog>: Supi` is not satisfied
    |     ------------ ^^^^^^^^^^^^^ the trait `Supi` is not implemented for `Box<Dog>`
    |     |
    |     required by a bound introduced by this call
    |
-   = help: the trait `Supi` is implemented for `Dog`
 note: required for `Box<Dog>` to implement `Foo`
   --> tests/compile-fail/super_trait_not_implemented.rs:5:1
    |
@@ -20,3 +19,7 @@ note: required by a bound in `requires_foo`
 14 | fn requires_foo<T: Foo>(_: T) {}
    |                    ^^^ required by this bound in `requires_foo`
    = note: this error originates in the attribute macro `auto_impl` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider dereferencing here
+   |
+18 |     requires_foo(*Box::new(Dog)); // shouldn't, because `Box<Dog>: Supi` is not satisfied
+   |                  +

tests/compile-pass/trailing_comma.rs

@@ -0,0 +1,6 @@
+use auto_impl::auto_impl;
+
+#[auto_impl(&)]
+trait MyTrait<T,> {}
+
+fn main() { }