diff --git a/capnp/src/lib.rs b/capnp/src/lib.rs
index 22aa8aef0..daafc23c1 100644
--- a/capnp/src/lib.rs
+++ b/capnp/src/lib.rs
@@ -401,6 +401,7 @@ pub enum ErrorKind {
 
 impl Error {
     /// Writes to the `extra` field. Does nothing if the "alloc" feature is not enabled.
+    /// This is intended to be used with the `write!()` macro from core.
     pub fn write_fmt(&mut self, fmt: core::fmt::Arguments<'_>) {
         #[cfg(feature = "alloc")]
         {
diff --git a/capnp/src/schema.rs b/capnp/src/schema.rs
index 2ed5f5c9a..8750a0a66 100644
--- a/capnp/src/schema.rs
+++ b/capnp/src/schema.rs
@@ -70,8 +70,6 @@ impl StructSchema {
         if let Some(field) = self.find_field_by_name(name)? {
             Ok(field)
         } else {
-            // error needs to be mutable only when the alloc feature is enabled
-            #[allow(unused_mut)]
             let mut error = crate::Error::from_kind(crate::ErrorKind::FieldNotFound);
             write!(error, "{}", name);
             Err(error)