Fix attribute printing in an error.

The current code assumes that the attribute is just an identifier, and
so misprints paths.

Author: nnethercote

Diff for: compiler/rustc_passes/src/check_attr.rs

@@ -561,12 +561,15 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
         allowed_target: Target,
     ) {
         if target != allowed_target {
+            let path = attr.path();
+            let path: Vec<_> = path.iter().map(|s| s.as_str()).collect();
+            let attr_name = path.join("::");
             self.tcx.emit_node_span_lint(
                 UNUSED_ATTRIBUTES,
                 hir_id,
                 attr.span(),
                 errors::OnlyHasEffectOn {
-                    attr_name: attr.name_or_empty(),
+                    attr_name,
                     target_name: allowed_target.name().replace(' ', "_"),
                 },
             );

Diff for: compiler/rustc_passes/src/errors.rs

@@ -1433,7 +1433,7 @@ pub(crate) struct UselessAssignment<'a> {
 #[derive(LintDiagnostic)]
 #[diag(passes_only_has_effect_on)]
 pub(crate) struct OnlyHasEffectOn {
-    pub attr_name: Symbol,
+    pub attr_name: String,
     pub target_name: String,
 }
 

Diff for: tests/ui/attributes/check-builtin-attr-ice.rs

@@ -44,12 +44,12 @@
 struct Foo {
     #[should_panic::skip]
     //~^ ERROR failed to resolve
-    //~| ERROR `#[]` only has an effect on functions
+    //~| ERROR `#[should_panic::skip]` only has an effect on functions
     pub field: u8,
 
     #[should_panic::a::b::c]
     //~^ ERROR failed to resolve
-    //~| ERROR `#[]` only has an effect on functions
+    //~| ERROR `#[should_panic::a::b::c]` only has an effect on functions
     pub field2: u8,
 }
 

Diff for: tests/ui/attributes/check-builtin-attr-ice.stderr

@@ -16,7 +16,7 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `den
 LL |     #[deny::skip]
    |       ^^^^ use of unresolved module or unlinked crate `deny`
 
-error: `#[]` only has an effect on functions
+error: `#[should_panic::skip]` only has an effect on functions
   --> $DIR/check-builtin-attr-ice.rs:45:5
    |
 LL |     #[should_panic::skip]
@@ -28,7 +28,7 @@ note: the lint level is defined here
 LL | #![deny(unused_attributes)]
    |         ^^^^^^^^^^^^^^^^^
 
-error: `#[]` only has an effect on functions
+error: `#[should_panic::a::b::c]` only has an effect on functions
   --> $DIR/check-builtin-attr-ice.rs:50:5
    |
 LL |     #[should_panic::a::b::c]