Next gen IDs for functions

The current system with AstIds has two primaraly drawbacks:

* It is possible to manufacture IDs out of thin air.
  For example, it's possible to create IDs for items which are not
  considered in CrateDefMap due to cfg. Or it is possible to mixup
  structs and unions, because they share ID space.

* Getting the ID of a parent requires a secondary index.

Instead, the plan is to pursue the more traditional approach, where
each items stores the id of the parent declaration. This makes
`FromSource` more awkward, but also more correct: now, to get from an
AST to HIR, we first do this recursively for the parent item, and the
just search the children of the parent for the matching def

crates/ra_syntax/src/ptr.rs

@@ -43,7 +43,7 @@ impl SyntaxNodePtr {
 }
 
 /// Like `SyntaxNodePtr`, but remembers the type of node
-#[derive(Debug, PartialEq, Eq, Hash)]
+#[derive(Debug, Hash)]
 pub struct AstPtr<N: AstNode> {
     raw: SyntaxNodePtr,
     _ty: PhantomData<fn() -> N>,
@@ -56,6 +56,14 @@ impl<N: AstNode> Clone for AstPtr<N> {
     }
 }
 
+impl<N: AstNode> Eq for AstPtr<N> {}
+
+impl<N: AstNode> PartialEq for AstPtr<N> {
+    fn eq(&self, other: &AstPtr<N>) -> bool {
+        self.raw == other.raw
+    }
+}
+
 impl<N: AstNode> AstPtr<N> {
     pub fn new(node: &N) -> AstPtr<N> {
         AstPtr { raw: SyntaxNodePtr::new(node.syntax()), _ty: PhantomData }