Add sanity check.

We force the relative span's parent to be absolute. This avoids having to
handle long dependency chains.

Author: cjgillot

compiler/rustc_ast_lowering/src/lib.rs

@@ -788,7 +788,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             node_id,
             DefPathData::LifetimeNs(str_name),
             ExpnId::root(),
-            span,
+            span.with_parent(None),
         );
 
         hir::GenericParam {
@@ -1520,7 +1520,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         def_node_id,
                         DefPathData::LifetimeNs(name.ident().name),
                         ExpnId::root(),
-                        span,
+                        span.with_parent(None),
                     );
 
                     let (name, kind) = match name {

compiler/rustc_hir/src/definitions.rs

@@ -345,6 +345,8 @@ impl Definitions {
         assert_eq!(root.local_def_index, CRATE_DEF_INDEX);
 
         let mut def_id_to_span = IndexVec::new();
+        // A relative span's parent must be an absolute span.
+        debug_assert_eq!(crate_span.data_untracked().parent, None);
         let _root = def_id_to_span.push(crate_span);
         debug_assert_eq!(_root, root);
 
@@ -394,6 +396,8 @@ impl Definitions {
             self.expansions_that_defined.insert(def_id, expn_id);
         }
 
+        // A relative span's parent must be an absolute span.
+        debug_assert_eq!(span.data_untracked().parent, None);
         let _id = self.def_id_to_span.push(span);
         debug_assert_eq!(_id, def_id);
 

compiler/rustc_interface/src/callbacks.rs

@@ -28,7 +28,9 @@ fn span_debug(span: rustc_span::Span, f: &mut fmt::Formatter<'_>) -> fmt::Result
 fn track_span_parent(def_id: rustc_span::def_id::LocalDefId) {
     tls::with_opt(|tcx| {
         if let Some(tcx) = tcx {
-            let _ = tcx.source_span(def_id);
+            let _span = tcx.source_span(def_id);
+            // Sanity check: relative span's parent must be an absolute span.
+            debug_assert_eq!(_span.data_untracked().parent, None);
         }
     })
 }