Require getters to return the correct type

serde/src/private/ser.rs

@@ -8,6 +8,12 @@ use self::content::{SerializeTupleVariantAsMapValue, SerializeStructVariantAsMap
 #[cfg(feature = "std")]
 use std::error;
 
+/// Used to check that serde(getter) attributes return the expected type.
+/// Not public API.
+pub fn constrain<T: ?Sized>(t: &T) -> &T {
+    t
+}
+
 /// Not public API.
 pub fn serialize_tagged_newtype<S, T>(serializer: S,
                                       type_ident: &'static str,

serde_derive/src/ser.rs

@@ -839,6 +839,9 @@ fn get_field<I>(params: &Parameters, field: &Field, ident: I) -> Tokens
             let ident = ident.into();
             quote!(&#self_var.#ident)
         }
-        Some(getter) => quote!(&#getter(#self_var)),
+        Some(getter) => {
+            let ty = field.ty;
+            quote!(_serde::private::ser::constrain::<#ty>(&#getter(#self_var)))
+        }
     }
 }

test_suite/tests/compile-fail/remote/wrong_getter.rs

@@ -0,0 +1,23 @@
+#[macro_use]
+extern crate serde_derive;
+
+mod remote {
+    pub struct S {
+        a: u8,
+    }
+
+    impl S {
+        pub fn get(&self) -> u16 {
+            self.a as u16
+        }
+    }
+}
+
+#[derive(Serialize)] //~ ERROR: mismatched types
+#[serde(remote = "remote::S")]
+struct S {
+    #[serde(getter = "remote::S::get")]
+    a: u8, //~^^^^ expected u8, found u16
+}
+
+fn main() {}