change some statics to constants

src/libsyntax/diagnostics/plugin.rs

@@ -200,9 +200,8 @@ pub fn expand_build_diagnostic_array<'cx>(ecx: &'cx mut ExtCtxt,
             ident: name.clone(),
             attrs: Vec::new(),
             id: ast::DUMMY_NODE_ID,
-            node: ast::ItemStatic(
+            node: ast::ItemConst(
                 ty,
-                ast::MutImmutable,
                 expr,
             ),
             vis: ast::Public,