Add walk_chain.

This combines multiple `HygieneData::with` calls on a hot path.

src/librustc_codegen_ssa/mir/mod.rs

@@ -128,14 +128,7 @@ pub fn debug_loc(&self, source_info: mir::SourceInfo) -> (Option<Bx::DIScope>, S
             // Walk up the macro expansion chain until we reach a non-expanded span.
             // We also stop at the function body level because no line stepping can occur
             // at the level above that.
-            let mut span = source_info.span;
-            while span.ctxt() != NO_EXPANSION && span.ctxt() != self.mir.span.ctxt() {
-                if let Some(info) = span.ctxt().outer_expn_info() {
-                    span = info.call_site;
-                } else {
-                    break;
-                }
-            }
+            let span = syntax_pos::hygiene::walk_chain(source_info.span, self.mir.span.ctxt());
             let scope = self.scope_metadata_for_loc(source_info.scope, span.lo());
             // Use span of the outermost expansion site, while keeping the original lexical scope.
             (scope, span)

src/libsyntax_pos/hygiene.rs

@@ -257,6 +257,17 @@ fn marks(&self, mut ctxt: SyntaxContext) -> Vec<(Mark, Transparency)> {
         marks
     }
 
+    fn walk_chain(&self, mut span: Span, to: SyntaxContext) -> Span {
+        while span.ctxt() != crate::NO_EXPANSION && span.ctxt() != to {
+            if let Some(info) = self.expn_info(self.outer(span.ctxt())) {
+                span = info.call_site;
+            } else {
+                break;
+            }
+        }
+        span
+    }
+
     fn adjust(&self, ctxt: &mut SyntaxContext, expansion: Mark) -> Option<Mark> {
         let mut scope = None;
         while !self.is_descendant_of(expansion, self.outer(*ctxt)) {
@@ -366,6 +377,10 @@ pub fn clear_markings() {
     HygieneData::with(|data| data.markings = FxHashMap::default());
 }
 
+pub fn walk_chain(span: Span, to: SyntaxContext) -> Span {
+    HygieneData::with(|data| data.walk_chain(span, to))
+}
+
 impl SyntaxContext {
     #[inline]
     pub const fn empty() -> Self {