Warnings-as-errors in check-builtin-attr-ice.rs.

This adds two new warnings, both of which print the attribute
incorrectly as `#[]`. The next commit will fix this.

Author: nnethercote

tests/ui/attributes/check-builtin-attr-ice.rs

@@ -39,13 +39,17 @@
 
 // Notably, `should_panic` is a `AttributeType::Normal` attribute that is checked separately.
 
+#![deny(unused_attributes)]
+
 struct Foo {
     #[should_panic::skip]
     //~^ ERROR failed to resolve
+    //~| ERROR `#[]` 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
     pub field2: u8,
 }
 

tests/ui/attributes/check-builtin-attr-ice.stderr

@@ -1,21 +1,39 @@
 error[E0433]: failed to resolve: use of unresolved module or unlinked crate `should_panic`
-  --> $DIR/check-builtin-attr-ice.rs:43:7
+  --> $DIR/check-builtin-attr-ice.rs:45:7
    |
 LL |     #[should_panic::skip]
    |       ^^^^^^^^^^^^ use of unresolved module or unlinked crate `should_panic`
 
 error[E0433]: failed to resolve: use of unresolved module or unlinked crate `should_panic`
-  --> $DIR/check-builtin-attr-ice.rs:47:7
+  --> $DIR/check-builtin-attr-ice.rs:50:7
    |
 LL |     #[should_panic::a::b::c]
    |       ^^^^^^^^^^^^ use of unresolved module or unlinked crate `should_panic`
 
 error[E0433]: failed to resolve: use of unresolved module or unlinked crate `deny`
-  --> $DIR/check-builtin-attr-ice.rs:55:7
+  --> $DIR/check-builtin-attr-ice.rs:59:7
    |
 LL |     #[deny::skip]
    |       ^^^^ use of unresolved module or unlinked crate `deny`
 
-error: aborting due to 3 previous errors
+error: `#[]` only has an effect on functions
+  --> $DIR/check-builtin-attr-ice.rs:45:5
+   |
+LL |     #[should_panic::skip]
+   |     ^^^^^^^^^^^^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/check-builtin-attr-ice.rs:42:9
+   |
+LL | #![deny(unused_attributes)]
+   |         ^^^^^^^^^^^^^^^^^
+
+error: `#[]` only has an effect on functions
+  --> $DIR/check-builtin-attr-ice.rs:50:5
+   |
+LL |     #[should_panic::a::b::c]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 5 previous errors
 
 For more information about this error, try `rustc --explain E0433`.