From 07481b9e90df0860519043b54e07529003e74814 Mon Sep 17 00:00:00 2001
From: bohan <bohan-zhang@foxmail.com>
Date: Fri, 5 Jul 2024 00:59:58 +0800
Subject: [PATCH] use old ctx if has same expand environment during decode span

---
 compiler/rustc_span/src/hygiene.rs |  8 +++++++
 tests/incremental/decl_macro.rs    | 34 ++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+)
 create mode 100644 tests/incremental/decl_macro.rs

diff --git a/compiler/rustc_span/src/hygiene.rs b/compiler/rustc_span/src/hygiene.rs
index 483e32c645393..7e6acc0b9fba9 100644
--- a/compiler/rustc_span/src/hygiene.rs
+++ b/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,
diff --git a/tests/incremental/decl_macro.rs b/tests/incremental/decl_macro.rs
new file mode 100644
index 0000000000000..74810ae422749
--- /dev/null
+++ b/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() {}