Update SszEncoder

eth2/utils/ssz/examples/large_list_of_structs.rs

@@ -0,0 +1,32 @@
+//! Encode and decode a list many times.
+//!
+//! Useful for `cargo flamegraph`.
+
+use ssz::{Decodable, Encodable};
+use ssz_derive::{Decode, Encode};
+
+#[derive(Clone, Copy, Encode, Decode)]
+pub struct FixedLen {
+    a: u64,
+    b: u64,
+    c: u64,
+    d: u64,
+}
+
+fn main() {
+    let fixed_len = FixedLen {
+        a: 42,
+        b: 42,
+        c: 42,
+        d: 42,
+    };
+
+    let vec: Vec<FixedLen> = vec![fixed_len; 8196];
+
+    let output: Vec<Vec<u64>> = (0..40_000)
+        .into_iter()
+        .map(|_| Vec::from_ssz_bytes(&vec.as_ssz_bytes()).unwrap())
+        .collect();
+
+    println!("{}", output.len());
+}

eth2/utils/ssz/examples/struct_definition.rs

@@ -1,4 +1,4 @@
-use ssz::{Decodable, DecodeError, Encodable, SszDecoderBuilder, SszEncoder, SszStream};
+use ssz::{Decodable, DecodeError, Encodable, SszDecoderBuilder, SszEncoder};
 
 #[derive(Debug, PartialEq)]
 pub struct Foo {
@@ -23,7 +23,7 @@ impl Encodable for Foo {
         encoder.append(&self.b);
         encoder.append(&self.c);
 
-        buf.append(&mut encoder.drain());
+        encoder.drain_onto(buf);
     }
 }