use old ctx if has same expand environment during decode span

Author: bvanjoi

compiler/rustc_span/src/hygiene.rs

@@ -1413,6 +1413,14 @@ pub fn decode_syntax_context<D: Decoder, F: FnOnce(&mut D, u32) -> SyntaxContext
 
     // Overwrite the dummy data with our decoded SyntaxContextData
     HygieneData::with(|hygiene_data| {
+        if let Some(old) = hygiene_data.syntax_context_data.get(raw_id as usize)
+            && old.outer_expn == ctxt_data.outer_expn
+            && old.outer_transparency == ctxt_data.outer_transparency
+            && old.parent == ctxt_data.parent
+        {
+            ctxt_data = old.clone();
+        }
+
         let dummy = std::mem::replace(
             &mut hygiene_data.syntax_context_data[ctxt.as_u32() as usize],
             ctxt_data,

tests/incremental/decl_macro.rs

@@ -0,0 +1,34 @@
+//@ revisions: rpass1 rpass2
+
+// issue#112680
+
+#![feature(decl_macro)]
+
+pub trait T {
+    type Key;
+    fn index_from_key(key: Self::Key) -> usize;
+}
+
+pub macro m($key_ty:ident, $val_ty:ident) {
+    struct $key_ty {
+        inner: usize,
+    }
+
+    impl T for $val_ty {
+        type Key = $key_ty;
+
+        fn index_from_key(key: Self::Key) -> usize {
+            key.inner
+        }
+    }
+}
+
+m!(TestId, Test);
+
+#[cfg(rpass1)]
+struct Test(u32);
+
+#[cfg(rpass2)]
+struct Test;
+
+fn main() {}