diff --git a/CHANGELOG.md b/CHANGELOG.md
index c076b26..4daddbb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,10 @@ All notable changes to this project will be documented in this file.
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/), and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## 0.5.13
+
+Added support for specifying `let mut <param>: <Type>` when using the `#[substreams::handlers::map]` macros, this enables in-place trimming of received data.
+
 ## 0.5.12
 
 Added back some lost binary operations on `BigInt` like `BigInt - BigInt` for example.
diff --git a/substreams-macro/src/handler.rs b/substreams-macro/src/handler.rs
index 7562782..6dcd299 100644
--- a/substreams-macro/src/handler.rs
+++ b/substreams-macro/src/handler.rs
@@ -99,7 +99,13 @@ pub fn main(item: TokenStream, module_type: ModuleType) -> TokenStream {
                     } else if input_obj.is_string {
                         proto_decodings.push(quote! { let #var_name: String = std::mem::ManuallyDrop::new(unsafe {String::from_raw_parts(#var_ptr, #var_len, #var_len)}).to_string(); });
                     } else {
-                        proto_decodings.push(quote! { let #var_name: #argument_type = substreams::proto::decode_ptr(#var_ptr, #var_len).unwrap_or_else(|_| panic!("Unable to decode Protobuf data ({} bytes) to '{}' message's struct", #var_len, stringify!(#argument_type))); })
+                        let mutability = if v.mutability.is_some() {
+                            quote! { mut }
+                        } else {
+                            quote! {}
+                        };
+
+                        proto_decodings.push(quote! { let #mutability #var_name: #argument_type = substreams::proto::decode_ptr(#var_ptr, #var_len).unwrap_or_else(|_| panic!("Unable to decode Protobuf data ({} bytes) to '{}' message's struct", #var_len, stringify!(#argument_type))); })
                     }
                 }
                 _ => {
diff --git a/substreams-macro/src/lib.rs b/substreams-macro/src/lib.rs
index 0f7a84d..3fdd3ee 100644
--- a/substreams-macro/src/lib.rs
+++ b/substreams-macro/src/lib.rs
@@ -56,6 +56,35 @@ mod test {
         );
     }
 
+    #[test]
+    fn test_map_mut() {
+        let item = quote! {
+            fn map_transfers(mut blk: eth::Block) -> pb::Custom {
+                unimplemented!("do something");
+            }
+        };
+
+        assert_ast_eq(
+            main(item, ModuleType::Map).into(),
+            quote! {
+                #[no_mangle]
+                pub extern "C" fn map_transfers(blk_ptr: *mut u8, blk_len: usize) {
+                    substreams::register_panic_hook();
+                    let func = || -> pb::Custom {
+                        let mut blk: eth::Block = substreams::proto::decode_ptr(blk_ptr, blk_len)
+                            .unwrap_or_else(|_| panic!("Unable to decode Protobuf data ({} bytes) to '{}' message's struct", blk_len, stringify!(eth::Block)));
+                        let result = {
+                            unimplemented!("do something");
+                        };
+                        result
+                    };
+                    let result = func();
+                    substreams::output(result);
+                }
+            },
+        );
+    }
+
     #[test]
     fn test_map_option() {
         let item = quote! {