Display only crate name for external trait impl restrictions

Author: CoCo-Japan-pan

compiler/rustc_middle/src/ty/trait_def.rs

@@ -140,7 +140,7 @@ impl ImplRestrictionKind {
                 if restricted_to.krate == rustc_hir::def_id::LOCAL_CRATE {
                     with_crate_prefix!(with_no_trimmed_paths!(tcx.def_path_str(restricted_to)))
                 } else {
-                    with_no_trimmed_paths!(tcx.def_path_str(restricted_to))
+                    tcx.def_path_str(restricted_to.krate.as_mod_def_id())
                 }
             }
         }

tests/ui/impl-restriction/impl-restriction-check.e2015.stderr

@@ -10,7 +10,7 @@ note: trait restricted here
 LL | pub impl(crate) trait TopLevel {}
    |     ^^^^^^^^^^^
 
-error: trait cannot be implemented outside `external::inner`
+error: trait cannot be implemented outside `external`
   --> $DIR/impl-restriction-check.rs:13:1
    |
 LL | impl external::inner::Inner for LocalType {}

tests/ui/impl-restriction/impl-restriction-check.e2018.stderr

@@ -10,7 +10,7 @@ note: trait restricted here
 LL | pub impl(crate) trait TopLevel {}
    |     ^^^^^^^^^^^
 
-error: trait cannot be implemented outside `external::inner`
+error: trait cannot be implemented outside `external`
   --> $DIR/impl-restriction-check.rs:13:1
    |
 LL | impl external::inner::Inner for LocalType {}

tests/ui/impl-restriction/impl-restriction-check.rs

@@ -10,7 +10,7 @@ extern crate external_impl_restriction as external;
 struct LocalType; // needed to avoid orphan rule errors
 
 impl external::TopLevel for LocalType {} //~ ERROR trait cannot be implemented outside `external`
-impl external::inner::Inner for LocalType {} //~ ERROR trait cannot be implemented outside `external::inner`
+impl external::inner::Inner for LocalType {} //~ ERROR trait cannot be implemented outside `external`
 
 pub mod foo {
     pub mod bar {