Improve allocations for SszEncoder

eth2/utils/ssz/examples/struct_definition.rs

@@ -17,13 +17,13 @@ impl Encodable for Foo {
             + <Vec<u16> as Encodable>::ssz_fixed_len()
             + <u16 as Encodable>::ssz_fixed_len();
 
-        let mut encoder = SszEncoder::container(offset);
+        let mut encoder = SszEncoder::container(buf, offset);
 
         encoder.append(&self.a);
         encoder.append(&self.b);
         encoder.append(&self.c);
 
-        encoder.drain_onto(buf);
+        encoder.finalize();
     }
 }
 

eth2/utils/ssz/src/encode.rs

@@ -25,48 +25,51 @@ pub trait Encodable {
     }
 }
 
-pub struct SszEncoder {
+pub struct SszEncoder<'a> {
     offset: usize,
-    fixed_bytes: Vec<u8>,
+    buf: &'a mut Vec<u8>,
     variable_bytes: Vec<u8>,
 }
 
-impl SszEncoder {
-    pub fn list(num_fixed_bytes: usize) -> Self {
-        Self::container(num_fixed_bytes)
+impl<'a> SszEncoder<'a> {
+    pub fn list(buf: &'a mut Vec<u8>, num_fixed_bytes: usize) -> Self {
+        Self::container(buf, num_fixed_bytes)
     }
 
-    pub fn container(num_fixed_bytes: usize) -> Self {
+    pub fn container(buf: &'a mut Vec<u8>, num_fixed_bytes: usize) -> Self {
+        buf.reserve(num_fixed_bytes);
+
         Self {
             offset: num_fixed_bytes,
-            fixed_bytes: Vec::with_capacity(num_fixed_bytes),
+            buf,
             variable_bytes: vec![],
         }
     }
 
     pub fn append<T: Encodable>(&mut self, item: &T) {
         if T::is_ssz_fixed_len() {
-            item.ssz_append(&mut self.fixed_bytes);
+            item.ssz_append(&mut self.buf);
         } else {
-            self.fixed_bytes
+            self.buf
                 .append(&mut encode_length(self.offset + self.variable_bytes.len()));
 
             item.ssz_append(&mut self.variable_bytes);
         }
     }
 
-    pub fn drain_onto(mut self, buf: &mut Vec<u8>) {
-        buf.append(&mut self.fixed_bytes);
-        buf.append(&mut self.variable_bytes);
+    pub fn finalize(&mut self) -> &mut Vec<u8> {
+        self.buf.append(&mut self.variable_bytes);
+
+        &mut self.buf
     }
 }
 
+/*
 pub struct VariableLengths {
     pub fixed_bytes_position: usize,
     pub variable_bytes_length: usize,
 }
 
-/*
 /// Provides a buffer for appending SSZ values.
 #[derive(Default)]
 pub struct SszStream {

eth2/utils/ssz/src/encode/impls.rs

@@ -38,13 +38,13 @@ impl<T: Encodable> Encodable for Vec<T> {
                 item.ssz_append(buf);
             }
         } else {
-            let mut encoder = SszEncoder::list(self.len() * BYTES_PER_LENGTH_OFFSET);
+            let mut encoder = SszEncoder::list(buf, self.len() * BYTES_PER_LENGTH_OFFSET);
 
             for item in self {
                 encoder.append(item);
             }
 
-            encoder.drain_onto(buf);
+            encoder.finalize();
         }
     }
 }

eth2/utils/ssz_derive/src/lib.rs

@@ -109,13 +109,13 @@ pub fn ssz_encode_derive(input: TokenStream) -> TokenStream {
                     )*
                         0;
 
-                let mut encoder = ssz::SszEncoder::container(offset);
+                let mut encoder = ssz::SszEncoder::container(buf, offset);
 
                 #(
                     encoder.append(&self.#field_idents);
                 )*
 
-                encoder.drain_onto(buf);
+                encoder.finalize();
             }
         }
     };