feature: add the "Recursively add derive" assist

This introduces a new assist that adds the derive attributes from a struct or enum to the types of
its fields, recursively. It is recursive in the sense that the fields of field types are also
considered.

The derive attributes will be copied over if the field type meets the following criteria:
* It resides within the same crate as the "top-level" type, to avoid editing the crate registry.
* It does not already have the same derive attribute.
* It does not have a manual implementation of the trait.

The last criteria is a bit of a guess since it isn't possible to know what trait implementation(s) a
derive macro generates. If a trait has the same name as the derive macro and the top-level type
(which already has the derive attribute) implements it, it'll be used to check for manual impls.
This seems to work well in practice.

Author: IvarWithoutBones

crates/hir/src/lib.rs

@@ -409,6 +409,20 @@ impl ModuleDef {
         acc
     }
 
+    pub fn as_trait(self) -> Option<Trait> {
+        match self {
+            ModuleDef::Trait(it) => Some(it),
+            _ => None,
+        }
+    }
+
+    pub fn as_macro(self) -> Option<Macro> {
+        match self {
+            ModuleDef::Macro(it) => Some(it),
+            _ => None,
+        }
+    }
+
     pub fn as_def_with_body(self) -> Option<DefWithBody> {
         match self {
             ModuleDef::Function(it) => Some(it.into()),
@@ -2996,6 +3010,13 @@ impl ItemInNs {
         }
     }
 
+    pub fn as_macro(self) -> Option<Macro> {
+        match self {
+            ItemInNs::Types(_) | ItemInNs::Values(_) => None,
+            ItemInNs::Macros(id) => Some(id),
+        }
+    }
+
     /// Returns the crate defining this item (or `None` if `self` is built-in).
     pub fn krate(&self, db: &dyn HirDatabase) -> Option<Crate> {
         match self {

crates/hir/src/semantics.rs

@@ -99,6 +99,13 @@ impl PathResolution {
             PathResolution::SelfType(impl_def) => Some(TypeNs::SelfType((*impl_def).into())),
         }
     }
+
+    pub fn as_module_def(&self) -> Option<ModuleDef> {
+        match self {
+            PathResolution::Def(it) => Some(*it),
+            _ => None,
+        }
+    }
 }
 
 #[derive(Debug)]