Only enable relative span hashing on nightly.

compiler/rustc_ast_lowering/src/lib.rs

@@ -776,7 +776,7 @@ fn mark_span_with_reason(
     /// Intercept all spans entering HIR.
     /// Mark a span as relative to the current owning item.
     fn lower_span(&self, span: Span) -> Span {
-        if self.tcx.sess.opts.incremental.is_some() {
+        if self.tcx.sess.opts.incremental_relative_spans() {
             span.with_parent(Some(self.current_hir_id_owner.def_id))
         } else {
             // Do not make spans relative when not using incremental compilation.

compiler/rustc_expand/src/expand.rs

@@ -587,7 +587,7 @@ fn collect_invocations(
                 .resolver
                 .visit_ast_fragment_with_placeholders(self.cx.current_expansion.id, &fragment);
 
-            if self.cx.sess.opts.incremental.is_some() {
+            if self.cx.sess.opts.incremental_relative_spans() {
                 for (invoc, _) in invocations.iter_mut() {
                     let expn_id = invoc.expansion_data.id;
                     let parent_def = self.cx.resolver.invocation_parent(expn_id);

compiler/rustc_middle/src/hir/map/mod.rs

@@ -1162,7 +1162,7 @@ pub(super) fn crate_hash(tcx: TyCtxt<'_>, crate_num: CrateNum) -> Svh {
         hir_body_hash.hash_stable(&mut hcx, &mut stable_hasher);
         upstream_crates.hash_stable(&mut hcx, &mut stable_hasher);
         source_file_names.hash_stable(&mut hcx, &mut stable_hasher);
-        if tcx.sess.opts.incremental.is_some() {
+        if tcx.sess.opts.incremental_relative_spans() {
             let definitions = tcx.definitions_untracked();
             let mut owner_spans: Vec<_> = krate
                 .owners

compiler/rustc_session/src/config.rs

@@ -787,6 +787,12 @@ pub fn share_generics(&self) -> bool {
     pub fn get_symbol_mangling_version(&self) -> SymbolManglingVersion {
         self.cg.symbol_mangling_version.unwrap_or(SymbolManglingVersion::Legacy)
     }
+
+    #[allow(rustc::bad_opt_access)]
+    pub fn incremental_relative_spans(&self) -> bool {
+        self.unstable_opts.incremental_relative_spans
+            || (self.unstable_features.is_nightly_build() && self.incremental.is_some())
+    }
 }
 
 impl UnstableOptions {

compiler/rustc_session/src/options.rs

@@ -1338,6 +1338,9 @@ pub(crate) fn parse_proc_macro_execution_strategy(
     incremental_info: bool = (false, parse_bool, [UNTRACKED],
         "print high-level information about incremental reuse (or the lack thereof) \
         (default: no)"),
+    #[rustc_lint_opt_deny_field_access("use `Session::incremental_relative_spans` instead of this field")]
+    incremental_relative_spans: bool = (false, parse_bool, [TRACKED],
+        "hash spans relative to their parent item for incr. comp. (default: no)"),
     incremental_verify_ich: bool = (false, parse_bool, [UNTRACKED],
         "verify incr. comp. hashes of green query instances (default: no)"),
     inline_in_all_cgus: Option<bool> = (None, parse_opt_bool, [TRACKED],