auto merge of #9088 : nikomatsakis/rust/issue-6304-AST-tree-not-DAG, r=catamorphism

Ensures that each AST node has a unique id. Fixes numerous bugs in macro expansion and deriving. Add two
representative tests.

Fixes #7971
Fixes #6304
Fixes #8367
Fixes #8754
Fixes #8852
Fixes #2543
Fixes #7654

src/libsyntax/ast_util.rs

@@ -12,6 +12,7 @@ use ast::*;
 use ast;
 use ast_util;
 use codemap::{Span, dummy_sp};
+use fold;
 use opt_vec;
 use parse::token;
 use visit::{SimpleVisitor, Visitor};
@@ -370,6 +371,21 @@ pub fn empty_generics() -> Generics {
               ty_params: opt_vec::Empty}
 }
 
+///////////////////////////////////////////////////////////////////////////
+// Assigning node ids
+
+fn node_id_assigner(next_id: @fn() -> ast::NodeId) -> @fold::ast_fold {
+    let precursor = @fold::AstFoldFns {
+        new_id: |old_id| {
+            assert_eq!(old_id, ast::DUMMY_NODE_ID);
+            next_id()
+        },
+        ..*fold::default_ast_fold()
+    };
+
+    fold::make_fold(precursor)
+}
+
 // ______________________________________________________________________
 // Enumerating the IDs which appear in an AST