Give spans their parent item during lowering.

We only do this operation when incremental compilation is enabled. This
avoids pessimizing the span handling for non-incremental compilation.

compiler/rustc_ast_lowering/src/expr.rs

@@ -422,7 +422,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let if_kind = hir::ExprKind::If(new_cond, self.arena.alloc(then), Some(else_expr));
         let if_expr = self.expr(span, if_kind, ThinVec::new());
         let block = self.block_expr(self.arena.alloc(if_expr));
-        hir::ExprKind::Loop(block, opt_label, hir::LoopSource::While, span.with_hi(cond.span.hi()))
+        let span = self.lower_span(span.with_hi(cond.span.hi()));
+        hir::ExprKind::Loop(block, opt_label, hir::LoopSource::While, span)
     }
 
     /// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_output(<expr>) }`,

compiler/rustc_ast_lowering/src/lib.rs

@@ -718,9 +718,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     }
 
     /// Intercept all spans entering HIR.
-    /// For now we are not doing anything with the intercepted spans.
+    /// Mark a span as relative to the current owning item.
     fn lower_span(&self, span: Span) -> Span {
-        span
+        if self.sess.opts.debugging_opts.incremental_relative_spans {
+            span.with_parent(Some(self.current_hir_id_owner.0))
+        } else {
+            // Do not make spans relative when not using incremental compilation.
+            span
+        }
     }
 
     fn lower_ident(&self, ident: Ident) -> Ident {

compiler/rustc_session/src/options.rs

@@ -1106,6 +1106,8 @@ options! {
     incremental_info: bool = (false, parse_bool, [UNTRACKED],
         "print high-level information about incremental reuse (or the lack thereof) \
         (default: no)"),
+    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_mir: Option<bool> = (None, parse_opt_bool, [TRACKED],