From 0b11ad436630fa8700845337f4d2197853ed0b21 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Sun, 28 Mar 2021 17:07:15 -0400
Subject: [PATCH] Limit const generic default value expr to lit or braced expr

---
 src/generics.rs   |  2 +-
 src/path.rs       | 48 ++++++++++++++++++++++++++++-------------------
 tests/repo/mod.rs |  9 ---------
 3 files changed, 30 insertions(+), 29 deletions(-)

diff --git a/src/generics.rs b/src/generics.rs
index 87f7fdd865..de0772a704 100644
--- a/src/generics.rs
+++ b/src/generics.rs
@@ -880,7 +880,7 @@ pub mod parsing {
                 eq_token: {
                     if input.peek(Token![=]) {
                         let eq_token = input.parse()?;
-                        default = Some(input.parse::<Expr>()?);
+                        default = Some(path::parsing::const_argument(input)?);
                         Some(eq_token)
                     } else {
                         None
diff --git a/src/path.rs b/src/path.rs
index 6c72f9cffa..90e2970d30 100644
--- a/src/path.rs
+++ b/src/path.rs
@@ -241,31 +241,41 @@ pub mod parsing {
                 }
             }
 
-            if input.peek(Lit) {
-                let lit = input.parse()?;
-                return Ok(GenericArgument::Const(Expr::Lit(lit)));
+            if input.peek(Lit) || input.peek(token::Brace) {
+                return const_argument(input).map(GenericArgument::Const);
             }
 
-            if input.peek(token::Brace) {
-                #[cfg(feature = "full")]
-                {
-                    let block: ExprBlock = input.parse()?;
-                    return Ok(GenericArgument::Const(Expr::Block(block)));
-                }
+            input.parse().map(GenericArgument::Type)
+        }
+    }
 
-                #[cfg(not(feature = "full"))]
-                {
-                    let begin = input.fork();
-                    let content;
-                    braced!(content in input);
-                    content.parse::<Expr>()?;
-                    let verbatim = verbatim::between(begin, input);
-                    return Ok(GenericArgument::Const(Expr::Verbatim(verbatim)));
-                }
+    pub fn const_argument(input: ParseStream) -> Result<Expr> {
+        let lookahead = input.lookahead1();
+
+        if input.peek(Lit) {
+            let lit = input.parse()?;
+            return Ok(Expr::Lit(lit));
+        }
+
+        if input.peek(token::Brace) {
+            #[cfg(feature = "full")]
+            {
+                let block: ExprBlock = input.parse()?;
+                return Ok(Expr::Block(block));
             }
 
-            input.parse().map(GenericArgument::Type)
+            #[cfg(not(feature = "full"))]
+            {
+                let begin = input.fork();
+                let content;
+                braced!(content in input);
+                content.parse::<Expr>()?;
+                let verbatim = verbatim::between(begin, input);
+                return Ok(Expr::Verbatim(verbatim));
+            }
         }
+
+        Err(lookahead.error())
     }
 
     #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
diff --git a/tests/repo/mod.rs b/tests/repo/mod.rs
index d27893b6cc..eeebbb8279 100644
--- a/tests/repo/mod.rs
+++ b/tests/repo/mod.rs
@@ -18,15 +18,6 @@ static EXCLUDE: &[&str] = &[
     "src/test/ui/generic-associated-types/generic-associated-type-bounds.rs",
     "src/test/ui/generic-associated-types/issue-80433-reduced.rs",
 
-    // TODO: const generics parameter default value
-    // https://github.com/dtolnay/syn/issues/980
-    "src/test/ui/const-generics/defaults/auxiliary/const_defaulty.rs",
-    "src/test/ui/const-generics/defaults/const-default.rs",
-    "src/test/ui/const-generics/defaults/default-annotation.rs",
-    "src/test/ui/const-generics/defaults/external.rs",
-    "src/test/ui/const-generics/defaults/pretty-printing-ast.rs",
-    "src/test/ui/const-generics/min_const_generics/type_and_const_defaults.rs",
-
     // Compile-fail expr parameter in const generic position: f::<1 + 2>()
     "src/test/ui/const-generics/closing-args-token.rs",
     "src/test/ui/const-generics/const-expression-parameter.rs",