remove unused return types such as empty Results or Options that would always be Some(..)

remove unused return type of dropck::check_drop_obligations()
don't wrap return type in Option in get_macro_by_def_id() since we would always return Some(..)
remove redundant return type of back::write::optimize()
don't Option-wrap return type of compute_type_parameters() since we always return Some(..)
don't return empty Result in assemble_generator_candidates()
don't return empty Result in assemble_closure_candidates()
don't return empty result in assemble_fn_pointer_candidates()
don't return empty result in assemble_candidates_from_impls()
don't return empty result in assemble_candidates_from_auto_impls()
don't return emtpy result in assemble_candidates_for_trait_alias()
don't return empty result in assemble_builtin_bound_candidates()
don't return empty results in assemble_extension_candidates_for_traits_in_scope() and assemble_extension_candidates_for_trait()
remove redundant wrapping of return type of StripItem::strip() since it always returns Some(..)
remove unused return type of assemble_extension_candidates_for_all_traits()

src/librustdoc/fold.rs

@@ -3,12 +3,12 @@ use crate::clean::*;
 crate struct StripItem(pub Item);
 
 impl StripItem {
-    crate fn strip(self) -> Option<Item> {
+    crate fn strip(self) -> Item {
         match self.0 {
-            Item { kind: box StrippedItem(..), .. } => Some(self.0),
+            Item { kind: box StrippedItem(..), .. } => self.0,
             mut i => {
                 i.kind = box StrippedItem(i.kind);
-                Some(i)
+                i
             }
         }
     }

src/librustdoc/passes/strip_hidden.rs

@@ -49,7 +49,7 @@ impl<'a> DocFolder for Stripper<'a> {
                     let old = mem::replace(&mut self.update_retained, false);
                     let ret = StripItem(self.fold_item_recur(i)).strip();
                     self.update_retained = old;
-                    return ret;
+                    return Some(ret);
                 }
                 _ => return None,
             }

src/librustdoc/passes/stripper.rs

@@ -51,7 +51,7 @@ impl<'a> DocFolder for Stripper<'a> {
 
             clean::StructFieldItem(..) => {
                 if !i.visibility.is_public() {
-                    return StripItem(i).strip();
+                    return Some(StripItem(i).strip());
                 }
             }
 
@@ -61,7 +61,7 @@ impl<'a> DocFolder for Stripper<'a> {
                     let old = mem::replace(&mut self.update_retained, false);
                     let ret = StripItem(self.fold_item_recur(i)).strip();
                     self.update_retained = old;
-                    return ret;
+                    return Some(ret);
                 }
             }