1 files changed, 49 insertions, 0 deletions
diff --git a/compiler/rustc_hir/src/hir_id.rs b/compiler/rustc_hir/src/hir_id.rs
new file mode 100644
index 00000000000..fea850c12d9
--- /dev/null
+++ b/compiler/rustc_hir/src/hir_id.rs
@@ -0,0 +1,49 @@
+use crate::def_id::{LocalDefId, CRATE_DEF_INDEX};
+use std::fmt;
+
+/// Uniquely identifies a node in the HIR of the current crate. It is
+/// composed of the `owner`, which is the `LocalDefId` of the directly enclosing
+/// `hir::Item`, `hir::TraitItem`, or `hir::ImplItem` (i.e., the closest "item-like"),
+/// and the `local_id` which is unique within the given owner.
+///
+/// This two-level structure makes for more stable values: One can move an item
+/// around within the source code, or add or remove stuff before it, without
+/// the `local_id` part of the `HirId` changing, which is a very useful property in
+/// incremental compilation where we have to persist things through changes to
+/// the code base.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, PartialOrd, Ord)]
+#[derive(Encodable, Decodable)]
+pub struct HirId {
+    pub owner: LocalDefId,
+    pub local_id: ItemLocalId,
+}
+
+impl fmt::Display for HirId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{:?}", self)
+    }
+}
+
+rustc_data_structures::define_id_collections!(HirIdMap, HirIdSet, HirId);
+rustc_data_structures::define_id_collections!(ItemLocalMap, ItemLocalSet, ItemLocalId);
+
+rustc_index::newtype_index! {
+    /// An `ItemLocalId` uniquely identifies something within a given "item-like";
+    /// that is, within a `hir::Item`, `hir::TraitItem`, or `hir::ImplItem`. There is no
+    /// guarantee that the numerical value of a given `ItemLocalId` corresponds to
+    /// the node's position within the owning item in any way, but there is a
+    /// guarantee that the `LocalItemId`s within an owner occupy a dense range of
+    /// integers starting at zero, so a mapping that maps all or most nodes within
+    /// an "item-like" to something else can be implemented by a `Vec` instead of a
+    /// tree or hash map.
+    pub struct ItemLocalId { .. }
+}
+rustc_data_structures::impl_stable_hash_via_hash!(ItemLocalId);
+
+/// The `HirId` corresponding to `CRATE_NODE_ID` and `CRATE_DEF_INDEX`.
+pub const CRATE_HIR_ID: HirId = HirId {
+    owner: LocalDefId { local_def_index: CRATE_DEF_INDEX },
+    local_id: ItemLocalId::from_u32(0),
+};
+
+pub const DUMMY_ITEM_LOCAL_ID: ItemLocalId = ItemLocalId::MAX;