Improve error message for #![manually_drop]

Author: ssbr

compiler/rustc_passes/src/check_attr.rs

@@ -2382,7 +2382,6 @@ fn check_invalid_crate_level_attr(tcx: TyCtxt<'_>, attrs: &[Attribute]) {
         sym::rustc_main,
         sym::unix_sigpipe,
         sym::derive,
-        sym::manually_drop,
         sym::test,
         sym::test_case,
         sym::global_allocator,

src/test/ui/manually_drop_attr/manually_drop-bad-item.rs

@@ -1,5 +1,8 @@
 #![feature(manually_drop_attr)]
 #![forbid(unused_attributes)]
+#![manually_drop]
+//~^ ERROR `manually_drop` attribute cannot be used at crate level
+//~^^ ERROR attribute should be applied to a struct or enum
 
 #[manually_drop]
 //~^ ERROR attribute should be applied to a struct or enum

src/test/ui/manually_drop_attr/manually_drop-bad-item.stderr

@@ -1,5 +1,5 @@
 error: attribute should be applied to a struct or enum
-  --> $DIR/manually_drop-bad-item.rs:4:1
+  --> $DIR/manually_drop-bad-item.rs:7:1
    |
 LL | #[manually_drop]
    | ^^^^^^^^^^^^^^^^
@@ -13,5 +13,22 @@ note: the lint level is defined here
 LL | #![forbid(unused_attributes)]
    |           ^^^^^^^^^^^^^^^^^
 
-error: aborting due to previous error
+error: attribute should be applied to a struct or enum
+  --> $DIR/manually_drop-bad-item.rs:3:1
+   |
+LL | #![manually_drop]
+   | ^^^^^^^^^^^^^^^^^
+
+error: `manually_drop` attribute cannot be used at crate level
+  --> $DIR/manually_drop-bad-item.rs:3:1
+   |
+LL | #![manually_drop]
+   | ^^^^^^^^^^^^^^^^^
+   |
+help: perhaps you meant to use an outer attribute
+   |
+LL | #[manually_drop]
+   |
+
+error: aborting due to 3 previous errors