split SyntheticTyParamKind

compiler/rustc_ast_lowering/src/lib.rs

@@ -2210,7 +2210,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         .attrs
                         .iter()
                         .filter(|attr| self.sess.check_name(attr, sym::rustc_synthetic))
-                        .map(|_| hir::SyntheticTyParamKind::ImplTrait)
+                        .map(|_| hir::SyntheticTyParamKind::Rustc)
                         .next(),
                 };
 

compiler/rustc_hir/src/hir.rs

@@ -508,6 +508,8 @@ impl Generics<'hir> {
 #[derive(HashStable_Generic)]
 pub enum SyntheticTyParamKind {
     ImplTrait,
+    // Created by the `#[rustc_synthetic]` attribute.
+    Rustc,
 }
 
 /// A where-clause in a definition.

compiler/rustc_typeck/src/astconv/generics.rs

@@ -550,7 +550,8 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         let explicit = !seg.infer_args;
         let impl_trait = generics.params.iter().any(|param| match param.kind {
             ty::GenericParamDefKind::Type {
-                synthetic: Some(hir::SyntheticTyParamKind::ImplTrait),
+                synthetic:
+                    Some(hir::SyntheticTyParamKind::ImplTrait | hir::SyntheticTyParamKind::Rustc),
                 ..
             } => true,
             _ => false,