Preserve span parent in try_metavars.

Author: cjgillot

compiler/rustc_span/src/lib.rs

@@ -916,35 +916,43 @@ impl Span {
     }
 
     /// Check if you can select metavar spans for the given spans to get matching contexts.
-    fn try_metavars(a: SpanData, b: SpanData, a_orig: Span, b_orig: Span) -> (SpanData, SpanData) {
-        match with_metavar_spans(|mspans| (mspans.get(a_orig), mspans.get(b_orig))) {
-            (None, None) => {}
-            (Some(meta_a), None) => {
-                let meta_a = meta_a.data();
-                if meta_a.ctxt == b.ctxt {
-                    return (meta_a, b);
-                }
-            }
-            (None, Some(meta_b)) => {
-                let meta_b = meta_b.data();
-                if a.ctxt == meta_b.ctxt {
-                    return (a, meta_b);
-                }
-            }
-            (Some(meta_a), Some(meta_b)) => {
-                let meta_b = meta_b.data();
-                if a.ctxt == meta_b.ctxt {
-                    return (a, meta_b);
-                }
-                let meta_a = meta_a.data();
-                if meta_a.ctxt == b.ctxt {
-                    return (meta_a, b);
-                } else if meta_a.ctxt == meta_b.ctxt {
-                    return (meta_a, meta_b);
-                }
-            }
+    #[tracing::instrument(level = "trace", ret)]
+    fn try_metavars(
+        mut a: SpanData,
+        mut b: SpanData,
+        a_orig: Span,
+        b_orig: Span,
+    ) -> (SpanData, SpanData) {
+        let a_parent = a.parent;
+        let b_parent = b.parent;
+
+        let (meta_a, meta_b) = with_metavar_spans(|mspans| {
+            (mspans.get(a_orig.with_parent(None)), mspans.get(b_orig.with_parent(None)))
+        });
+
+        let meta_a = meta_a.map(Span::data);
+        let meta_b = meta_b.map(Span::data);
+
+        if let Some(meta_b) = meta_b
+            && meta_b.ctxt == a.ctxt
+        {
+            b = meta_b;
+        }
+        if let Some(meta_a) = meta_a
+            && meta_a.ctxt == b.ctxt
+        {
+            a = meta_a;
+        }
+        if let Some(meta_a) = meta_a
+            && let Some(meta_b) = meta_b
+            && meta_a.ctxt == meta_b.ctxt
+        {
+            a = meta_a;
+            b = meta_b;
         }
 
+        a.parent = a_parent;
+        b.parent = b_parent;
         (a, b)
     }