From d530864dcc6df62711ef4b74878d9f48d47b2374 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Wed, 6 Jan 2021 05:06:30 -0800
Subject: [PATCH 1/2] Parse empty supertrait list

---
 src/item.rs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/item.rs b/src/item.rs
index 883ecc799f..c5083d0501 100644
--- a/src/item.rs
+++ b/src/item.rs
@@ -2032,14 +2032,14 @@ pub mod parsing {
         let mut supertraits = Punctuated::new();
         if colon_token.is_some() {
             loop {
-                supertraits.push_value(input.parse()?);
                 if input.peek(Token![where]) || input.peek(token::Brace) {
                     break;
                 }
-                supertraits.push_punct(input.parse()?);
+                supertraits.push_value(input.parse()?);
                 if input.peek(Token![where]) || input.peek(token::Brace) {
                     break;
                 }
+                supertraits.push_punct(input.parse()?);
             }
         }
 

From d8c39a8247bc42be35bdc27f0c4fd1f6a0817fb7 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Wed, 6 Jan 2021 05:06:38 -0800
Subject: [PATCH 2/2] Add test of empty supertrait list

---
 tests/test_item.rs | 84 +++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 83 insertions(+), 1 deletion(-)

diff --git a/tests/test_item.rs b/tests/test_item.rs
index c28fc87fdc..7695f19906 100644
--- a/tests/test_item.rs
+++ b/tests/test_item.rs
@@ -4,7 +4,7 @@ mod macros;
 use proc_macro2::{Delimiter, Group, Ident, Span, TokenStream, TokenTree};
 use quote::quote;
 use std::iter::FromIterator;
-use syn::Item;
+use syn::{Item, ItemTrait};
 
 #[test]
 fn test_macro_variable_attr() {
@@ -159,3 +159,85 @@ fn test_macro_variable_impl() {
     }
     "###);
 }
+
+#[test]
+fn test_supertraits() {
+    // Rustc parses all of the following.
+
+    #[rustfmt::skip]
+    let tokens = quote!(trait Trait where {});
+    snapshot!(tokens as ItemTrait, @r###"
+    ItemTrait {
+        vis: Inherited,
+        ident: "Trait",
+        generics: Generics {
+            where_clause: Some(WhereClause),
+        },
+    }
+    "###);
+
+    #[rustfmt::skip]
+    let tokens = quote!(trait Trait: where {});
+    snapshot!(tokens as ItemTrait, @r###"
+    ItemTrait {
+        vis: Inherited,
+        ident: "Trait",
+        generics: Generics {
+            where_clause: Some(WhereClause),
+        },
+        colon_token: Some,
+    }
+    "###);
+
+    #[rustfmt::skip]
+    let tokens = quote!(trait Trait: Sized where {});
+    snapshot!(tokens as ItemTrait, @r###"
+    ItemTrait {
+        vis: Inherited,
+        ident: "Trait",
+        generics: Generics {
+            where_clause: Some(WhereClause),
+        },
+        colon_token: Some,
+        supertraits: [
+            Trait(TraitBound {
+                modifier: None,
+                path: Path {
+                    segments: [
+                        PathSegment {
+                            ident: "Sized",
+                            arguments: None,
+                        },
+                    ],
+                },
+            }),
+        ],
+    }
+    "###);
+
+    #[rustfmt::skip]
+    let tokens = quote!(trait Trait: Sized + where {});
+    snapshot!(tokens as ItemTrait, @r###"
+    ItemTrait {
+        vis: Inherited,
+        ident: "Trait",
+        generics: Generics {
+            where_clause: Some(WhereClause),
+        },
+        colon_token: Some,
+        supertraits: [
+            Trait(TraitBound {
+                modifier: None,
+                path: Path {
+                    segments: [
+                        PathSegment {
+                            ident: "Sized",
+                            arguments: None,
+                        },
+                    ],
+                },
+            }),
+        ],
+    }
+    "###);
+}