Add regression test for issue 2846

    error[E0425]: cannot find value `__e` in this scope
      --> test_suite/tests/regression/issue2846.rs:12:19
       |
    12 | declare_in_macro!("with");
       |                   ^^^^^^ not found in this scope

Author: dtolnay

test_suite/tests/regression/issue2846.rs

@@ -0,0 +1,23 @@
+#![allow(clippy::trivially_copy_pass_by_ref)]
+
+use serde_derive::Deserialize;
+
+macro_rules! declare_in_macro {
+    ($with:literal) => {
+        #[derive(Deserialize)]
+        pub struct S(#[serde(with = $with)] i32);
+    };
+}
+
+declare_in_macro!("with");
+
+mod with {
+    use serde::Deserializer;
+
+    pub fn deserialize<'de, D>(_: D) -> Result<i32, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        unimplemented!()
+    }
+}