implement nits

compiler/rustc_ast_lowering/src/lib.rs

@@ -2210,7 +2210,7 @@ fn lower_generic_param(
                         .attrs
                         .iter()
                         .filter(|attr| self.sess.check_name(attr, sym::rustc_synthetic))
-                        .map(|_| hir::SyntheticTyParamKind::Rustc)
+                        .map(|_| hir::SyntheticTyParamKind::FromAttr)
                         .next(),
                 };
 

compiler/rustc_hir/src/hir.rs

@@ -509,7 +509,7 @@ pub fn spans(&self) -> MultiSpan {
 pub enum SyntheticTyParamKind {
     ImplTrait,
     // Created by the `#[rustc_synthetic]` attribute.
-    Rustc,
+    FromAttr,
 }
 
 /// A where-clause in a definition.

compiler/rustc_passes/src/hir_id_validator.rs

@@ -170,7 +170,8 @@ fn visit_generic_param(&mut self, param: &'hir hir::GenericParam<'hir>) {
             ..
         } = param.kind
         {
-            // Do nothing because bodging is fun.
+            // Synthetic impl trait parameters are owned by the node of the desugared type.
+            // This means it is correct for them to have a different owner.
         } else {
             intravisit::walk_generic_param(self, param);
         }

compiler/rustc_typeck/src/astconv/generics.rs

@@ -548,14 +548,18 @@ pub(crate) fn check_impl_trait(
         generics: &ty::Generics,
     ) -> bool {
         let explicit = !seg.infer_args;
-        let impl_trait = generics.params.iter().any(|param| match param.kind {
-            ty::GenericParamDefKind::Type {
-                synthetic:
-                    Some(hir::SyntheticTyParamKind::ImplTrait | hir::SyntheticTyParamKind::Rustc),
-                ..
-            } => true,
-            _ => false,
-        });
+        let impl_trait =
+            generics.params.iter().any(|param| match param.kind {
+                ty::GenericParamDefKind::Type {
+                    synthetic:
+                        Some(
+                            hir::SyntheticTyParamKind::ImplTrait
+                            | hir::SyntheticTyParamKind::FromAttr,
+                        ),
+                    ..
+                } => true,
+                _ => false,
+            });
 
         if explicit && impl_trait {
             let spans = seg