Pretty-print argument-position impl trait to name it.

compiler/rustc_ast_lowering/src/lib.rs

@@ -1421,12 +1421,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         ),
                     ImplTraitContext::Universal => {
                         let span = t.span;
-                        self.create_def(
-                            self.current_hir_id_owner.def_id,
-                            *def_node_id,
-                            DefPathData::ImplTrait,
-                            span,
-                        );
 
                         // HACK: pprust breaks strings with newlines when the type
                         // gets too long. We don't want these to show up in compiler
@@ -1437,6 +1431,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             span,
                         );
 
+                        self.create_def(
+                            self.current_hir_id_owner.def_id,
+                            *def_node_id,
+                            DefPathData::TypeNs(ident.name),
+                            span,
+                        );
                         let (param, bounds, path) = self.lower_universal_param_and_bounds(
                             *def_node_id,
                             span,

compiler/rustc_hir/src/definitions.rs

@@ -278,7 +278,8 @@ pub enum DefPathData {
     Ctor,
     /// A constant expression (see `{ast,hir}::AnonConst`).
     AnonConst,
-    /// An `impl Trait` type node.
+    /// An existential `impl Trait` type node.
+    /// Argument position `impl Trait` have a `TypeNs` with their pretty-printed name.
     ImplTrait,
     /// `impl Trait` generated associated type node.
     ImplTraitAssocTy,

compiler/rustc_middle/src/ty/mod.rs

@@ -2216,10 +2216,6 @@ impl<'tcx> TyCtxt<'tcx> {
                 // The name of a constructor is that of its parent.
                 rustc_hir::definitions::DefPathData::Ctor => self
                     .opt_item_name(DefId { krate: def_id.krate, index: def_key.parent.unwrap() }),
-                // The name of opaque types only exists in HIR.
-                rustc_hir::definitions::DefPathData::ImplTrait
-                    if let Some(def_id) = def_id.as_local() =>
-                    self.hir().opt_name(self.hir().local_def_id_to_hir_id(def_id)),
                 _ => def_key.get_opt_name(),
             }
         }

src/librustdoc/clean/utils.rs

@@ -151,7 +151,7 @@ pub(super) fn external_path<'tcx>(
     args: ty::Binder<'tcx, GenericArgsRef<'tcx>>,
 ) -> Path {
     let def_kind = cx.tcx.def_kind(did);
-    let name = cx.tcx.item_name(did);
+    let name = cx.tcx.opt_item_name(did).unwrap_or(kw::Empty);
     Path {
         res: Res::Def(def_kind, did),
         segments: thin_vec![PathSegment {