1 files changed, 17 insertions, 1 deletions

diff --git a/crates/ra_tt/src/lib.rs b/crates/ra_tt/src/lib.rs
index df31f72f3..c1f37b889 100644
--- a/crates/ra_tt/src/lib.rs
+++ b/crates/ra_tt/src/lib.rs
@@ -1,4 +1,4 @@
-/// `tt` crate defines a `TokenTree` datastructure: this is the interface (both
+/// `tt` crate defines a `TokenTree` data structure: this is the interface (both
 /// input and output) of macros. It closely mirrors `proc_macro` crate's
 /// `TokenTree`.
 
@@ -18,6 +18,21 @@ use std::fmt;
 
 use smol_str::SmolStr;
 
+/// Represents identity of the token.
+///
+/// For hygiene purposes, we need to track which expanded tokens originated from
+/// which source tokens. We do it by assigning an distinct identity to each
+/// source token and making sure that identities are preserved during macro
+/// expansion.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct TokenId(pub u32);
+
+impl TokenId {
+    pub const fn unspecified() -> TokenId {
+        TokenId(!0)
+    }
+}
+
 #[derive(Debug, Clone)]
 pub enum TokenTree {
     Leaf(Leaf),
@@ -67,6 +82,7 @@ pub enum Spacing {
 #[derive(Debug, Clone)]
 pub struct Ident {
     pub text: SmolStr,
+    pub id: TokenId,
 }
 
 impl fmt::Display for TokenTree {