[Blocked on upstream] Reject leading grouped use-bounds in bare trait object types

src/lib/lib.rs

@@ -8,7 +8,6 @@
 #![feature(deref_patterns)]
 #![feature(derive_const)]
 #![feature(gen_blocks)]
-#![feature(generic_const_items)]
 #![feature(import_trait_associated_functions)]
 #![feature(macro_metavar_expr)]
 #![feature(mut_ref)]

src/lib/parser/test/ty.rs

@@ -231,14 +231,18 @@ fn bare_trait_object_tys() {
         Ok(ast::Ty::DynTrait(ast::DynKind::Bare, [ast::Bound::Use(_)]))
     );
 
-    // Indeed, even though you can't parenthesize precise-capturing lists
-    // in "normal" bounds, you can do so in bare trait object type bounds.
-    // If find it a bit janky. Might report upstream.
+    // Context: <https://github.com/rust-lang/rust/pull/162652>
     t!(
         parse_ty,
         Rust2015,
         "(use<>)+",
-        Ok(ast::Ty::DynTrait(ast::DynKind::Bare, [ast::Bound::Use(_)]))
+        Err([Error {
+            kind: ErrorKind::UnexpectedToken(
+                TokenKind::SinglePlus,
+                [Fragment::Token(TokenKind::EndOfInput)],
+            ),
+            ..
+        }])
     );
 
     // It's easy to accidentally accept the following code while trying to support the form above.

src/lib/parser/ty.rs

@@ -302,31 +302,22 @@ impl<'src> super::Parser<'_, '_, 'src> {
         inner_ty: &mut ast::Ty<'src>,
         p_policy: PlusPolicy,
     ) -> Result<Option<ast::Ty<'src>>> {
-        const EMPTY<'src>: ast::Path<'src, ast::UnambiguousGenericArgs> =
-            ast::Path { segs: Vec::new() };
-
-        let bound = match inner_ty {
+        let mut bounds = vec![match inner_ty {
             ast::Ty::Path(ast::ExtPath { ext: None, path }) => {
-                ast::Bound::from(mem::replace(path, EMPTY))
-            }
-            ast::Ty::DynTrait(ast::DynKind::Bare, [bound]) => {
-                match bound {
-                    ast::Bound::Outlives(_) => return Ok(None),
-                    // NOTE: I'm not happy about this since use-bounds can't be parenthesized "normally".
-                    ast::Bound::Use(captures) => ast::Bound::Use(mem::take(captures)),
-                    ast::Bound::Trait { bound_vars, modifiers, path } => ast::Bound::Trait {
-                        bound_vars: mem::take(bound_vars),
-                        modifiers: *modifiers,
-                        path: mem::replace(path, EMPTY),
-                    },
-                }
+                ast::Bound::from(mem::replace(path, ast::Path { segs: Vec::new() }))
             }
+            ast::Ty::DynTrait(
+                ast::DynKind::Bare,
+                [ast::Bound::Trait { bound_vars, modifiers, path }],
+            ) => ast::Bound::Trait {
+                bound_vars: mem::take(bound_vars),
+                modifiers: *modifiers,
+                path: mem::replace(path, ast::Path { segs: Vec::new() }),
+            },
             _ => return Ok(None),
-        };
+        }];
 
         self.parse_unchecked(TokenPrefix::Plus);
-
-        let mut bounds = vec![bound];
         self.parse_bounds_into(p_policy.maintain(), &mut bounds)?;
 
         Ok(Some(ast::Ty::DynTrait(ast::DynKind::Bare, bounds)))