Add negative impl for Ord, PartialOrd on LocalDefId

Add comment about why `LocalDefId` should not be `Ord`

Also fix some formatting in the doc comment.

compiler/rustc_span/src/def_id.rs

@@ -316,17 +316,23 @@ impl fmt::Debug for DefId {
 
 rustc_data_structures::define_id_collections!(DefIdMap, DefIdSet, DefId);
 
-/// A LocalDefId is equivalent to a DefId with `krate == LOCAL_CRATE`. Since
+/// A `LocalDefId` is equivalent to a `DefId` with `krate == LOCAL_CRATE`. Since
 /// we encode this information in the type, we can ensure at compile time that
-/// no DefIds from upstream crates get thrown into the mix. There are quite a
-/// few cases where we know that only DefIds from the local crate are expected
-/// and a DefId from a different crate would signify a bug somewhere. This
-/// is when LocalDefId comes in handy.
+/// no `DefId`s from upstream crates get thrown into the mix. There are quite a
+/// few cases where we know that only `DefId`s from the local crate are expected;
+/// a `DefId` from a different crate would signify a bug somewhere. This
+/// is when `LocalDefId` comes in handy.
 #[derive(Clone, Copy, PartialEq, Eq, Hash)]
 pub struct LocalDefId {
     pub local_def_index: DefIndex,
 }
 
+// To ensure correctness of incremental compilation,
+// `LocalDefId` must not implement `Ord` or `PartialOrd`.
+// See https://github.com/rust-lang/rust/issues/90317.
+impl !Ord for LocalDefId {}
+impl !PartialOrd for LocalDefId {}
+
 pub const CRATE_DEF_ID: LocalDefId = LocalDefId { local_def_index: CRATE_DEF_INDEX };
 
 impl Idx for LocalDefId {