zb: allow direct access to message properties without allocating

zbus/src/message/builder.rs

@@ -231,7 +231,7 @@ impl<'a> Builder<'a> {
         let ctxt = dbus_context!(self, 0);
         let mut header = self.header;
 
-        header.fields_mut().signature = signature;
+        header.fields_mut().signature = std::borrow::Cow::Owned(signature);
 
         let body_len_u32 = body_size.size().try_into().map_err(|_| Error::ExcessData)?;
         header.primary_mut().set_body_len(body_len_u32);
@@ -285,7 +285,7 @@ impl<'m> From<Header<'m>> for Builder<'m> {
     fn from(mut header: Header<'m>) -> Self {
         // Signature and Fds are added by body* methods.
         let fields = header.fields_mut();
-        fields.signature = Signature::Unit;
+        fields.signature = std::borrow::Cow::Owned(Signature::Unit);
         fields.unix_fds = None;
 
         Self { header }

zbus/src/message/fields.rs

@@ -5,6 +5,7 @@ use serde::{
 };
 use static_assertions::assert_impl_all;
 use std::num::NonZeroU32;
+use std::borrow::Cow;
 use zbus_names::{BusName, ErrorName, InterfaceName, MemberName, UniqueName};
 use zvariant::{ObjectPath, Signature, Type, Value};
 
@@ -23,7 +24,7 @@ pub(crate) struct Fields<'f> {
     pub reply_serial: Option<NonZeroU32>,
     pub destination: Option<BusName<'f>>,
     pub sender: Option<UniqueName<'f>>,
-    pub signature: Signature,
+    pub signature: Cow<'f, Signature>,
     pub unix_fds: Option<u32>,
 }
 
@@ -63,7 +64,7 @@ impl<'f> Serialize for Fields<'f> {
         if let Some(sender) = &self.sender {
             seq.serialize_element(&(FieldCode::Sender, Value::from(sender.as_str())))?;
         }
-        if !matches!(&self.signature, Signature::Unit) {
+        if self.signature != Cow::Borrowed(&Signature::Unit) {
             seq.serialize_element(&(FieldCode::Signature, SignatureSerializer(&self.signature)))?;
         }
         if let Some(unix_fds) = self.unix_fds {
@@ -149,7 +150,7 @@ impl<'de> Visitor<'de> for FieldsVisitor {
                     fields.sender = Some(UniqueName::try_from(value).map_err(V::Error::custom)?)
                 }
                 FieldCode::Signature => {
-                    fields.signature = Signature::try_from(value).map_err(V::Error::custom)?
+                    fields.signature = Cow::Owned(Signature::try_from(value).map_err(V::Error::custom)?)
                 }
                 FieldCode::UnixFDs => {
                     fields.unix_fds = Some(u32::try_from(value).map_err(V::Error::custom)?)

zbus/src/message/mod.rs

@@ -68,6 +68,27 @@ pub(super) struct Inner {
 
 assert_impl_all!(Message: Send, Sync, Unpin);
 
+impl Message {
+    pub fn into_body(self) -> Body {
+        Body::new(self.inner.bytes.slice(self.inner.body_offset..), self)
+    }
+    pub fn signature(&self) -> &zvariant::Signature {
+        self.quick_fields().signature()
+    }
+    pub fn interface(&self) -> Option<InterfaceName<'_>> {
+        self.quick_fields().interface(self)
+    }
+    pub fn member(&self) -> Option<MemberName<'_>> {
+        self.quick_fields().member(self)
+    }
+    pub fn path(&self) -> Option<zvariant::ObjectPath<'_>> {
+        self.quick_fields().path(self)
+    }
+    pub fn sender(&self) -> Option<zbus_names::UniqueName<'_>> {
+        self.quick_fields().sender(self)
+    }
+}
+
 impl Message {
     /// Create a builder for a message of type [`Type::MethodCall`].
     pub fn method_call<'b, 'p: 'b, 'm: 'b, P, M>(path: P, method_name: M) -> Result<Builder<'b>>
@@ -176,7 +197,7 @@ impl Message {
             reply_serial: quick_fields.reply_serial(),
             destination: quick_fields.destination(self),
             sender: quick_fields.sender(self),
-            signature: quick_fields.signature().clone(),
+            signature: std::borrow::Cow::Borrowed(quick_fields.signature()),
             unix_fds: quick_fields.unix_fds(),
         };