Add HIR for impl blocks

Since we need to be able to go from def to containing impl block, as well as the
other direction, and to find all impls for a certain type, a design similar to
the one for modules, where we collect all impls for the whole crate and keep
them in an arena, seemed fitting. The ImplBlock type, which provides the public
interface, then consists only of an Arc to the arena containing all impls, and
the index into it.

crates/ra_syntax/src/ast/generated.rs

@@ -1442,7 +1442,39 @@ impl<R: TreeRoot<RaTypes>> ImplBlockNode<R> {
 }
 
 
-impl<'a> ImplBlock<'a> {}
+impl<'a> ImplBlock<'a> {
+    pub fn item_list(self) -> Option<ItemList<'a>> {
+        super::child_opt(self)
+    }
+}
+
+// ImplItem
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ImplItem<'a> {
+    FnDef(FnDef<'a>),
+    TypeDef(TypeDef<'a>),
+    ConstDef(ConstDef<'a>),
+}
+
+impl<'a> AstNode<'a> for ImplItem<'a> {
+    fn cast(syntax: SyntaxNodeRef<'a>) -> Option<Self> {
+        match syntax.kind() {
+            FN_DEF => Some(ImplItem::FnDef(FnDef { syntax })),
+            TYPE_DEF => Some(ImplItem::TypeDef(TypeDef { syntax })),
+            CONST_DEF => Some(ImplItem::ConstDef(ConstDef { syntax })),
+            _ => None,
+        }
+    }
+    fn syntax(self) -> SyntaxNodeRef<'a> {
+        match self {
+            ImplItem::FnDef(inner) => inner.syntax(),
+            ImplItem::TypeDef(inner) => inner.syntax(),
+            ImplItem::ConstDef(inner) => inner.syntax(),
+        }
+    }
+}
+
+impl<'a> ImplItem<'a> {}
 
 // ImplTraitType
 #[derive(Debug, Clone, Copy,)]
@@ -1555,7 +1587,11 @@ impl<R: TreeRoot<RaTypes>> ItemListNode<R> {
 
 impl<'a> ast::FnDefOwner<'a> for ItemList<'a> {}
 impl<'a> ast::ModuleItemOwner<'a> for ItemList<'a> {}
-impl<'a> ItemList<'a> {}
+impl<'a> ItemList<'a> {
+    pub fn impl_items(self) -> impl Iterator<Item = ImplItem<'a>> + 'a {
+        super::children(self)
+    }
+}
 
 // Label
 #[derive(Debug, Clone, Copy,)]