Add regression test for issue 2844

    error[E0424]: expected value, found module `self`
      --> test_suite/tests/regression/issue2844.rs:13:19
       |
    5  |         #[derive(Serialize, Deserialize)]
       |                  --------- this function has a `self` parameter, but a macro invocation can only access identifiers it receives from parameters
    ...
    13 | declare_in_macro!("with");
       |                   ^^^^^^ `self` value is a keyword only available in methods with a `self` parameter

    error[E0425]: cannot find value `__s` in this scope
      --> test_suite/tests/regression/issue2844.rs:13:19
       |
    13 | declare_in_macro!("with");
       |                   ^^^^^^ not found in this scope

    error[E0425]: cannot find value `__deserializer` in this scope
      --> test_suite/tests/regression/issue2844.rs:13:19
       |
    13 | declare_in_macro!("with");
       |                   ^^^^^^ not found in this scope

Author: dtolnay

test_suite/tests/regression/issue2844.rs

@@ -0,0 +1,31 @@
+use serde_derive::{Deserialize, Serialize};
+
+macro_rules! declare_in_macro {
+    ($with:literal) => {
+        #[derive(Serialize, Deserialize)]
+        pub struct S {
+            #[serde(with = $with)]
+            f: i32,
+        }
+    };
+}
+
+declare_in_macro!("with");
+
+mod with {
+    use serde::{Deserializer, Serializer};
+
+    pub fn serialize<S>(_: &i32, _: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        unimplemented!()
+    }
+
+    pub fn deserialize<'de, D>(_: D) -> Result<i32, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        unimplemented!()
+    }
+}