E0518 Update error format #36111

- Fixes #36111
- Part of #35233

Author: gavinb

src/librustc/hir/check_attr.rs

@@ -42,7 +42,9 @@ struct CheckAttrVisitor<'a> {
 impl<'a> CheckAttrVisitor<'a> {
     fn check_inline(&self, attr: &ast::Attribute, target: Target) {
         if target != Target::Fn {
-            span_err!(self.sess, attr.span, E0518, "attribute should be applied to function");
+            struct_span_err!(self.sess, attr.span, E0518, "attribute should be applied to function")
+                .span_label(attr.span, &format!("requires a function"))
+                .emit();
         }
     }
 

src/test/compile-fail/E0518.rs

@@ -9,9 +9,11 @@
 // except according to those terms.
 
 #[inline(always)] //~ ERROR E0518
+                  //~| requires a function
 struct Foo;
 
 #[inline(never)] //~ ERROR E0518
+                 //~| requires a function
 impl Foo {
 }