Make UncheckedExtrinsic encode more readable (#9531)

Actually this will cost us another allocation, but before this wasn't
really safe. Assuming that we only need `size_of` bytes for the encoding
of the tx could have ended with an invalid encoding.

substrate/primitives/runtime/src/generic/mod.rs

@@ -36,28 +36,3 @@ pub use self::{
 	header::Header,
 	unchecked_extrinsic::{SignedPayload, UncheckedExtrinsic},
 };
-
-use crate::codec::Encode;
-use sp_std::prelude::*;
-
-fn encode_with_vec_prefix<T: Encode, F: Fn(&mut Vec<u8>)>(encoder: F) -> Vec<u8> {
-	let size = ::sp_std::mem::size_of::<T>();
-	let reserve = match size {
-		0..=0b00111111 => 1,
-		0b01000000..=0b00111111_11111111 => 2,
-		_ => 4,
-	};
-	let mut v = Vec::with_capacity(reserve + size);
-	v.resize(reserve, 0);
-	encoder(&mut v);
-
-	// need to prefix with the total length to ensure it's binary compatible with
-	// Vec<u8>.
-	let mut length: Vec<()> = Vec::new();
-	length.resize(v.len() - reserve, ());
-	length.using_encoded(|s| {
-		v.splice(0..reserve, s.iter().cloned());
-	});
-
-	v
-}

substrate/primitives/runtime/src/generic/unchecked_extrinsic.rs

@@ -228,19 +228,29 @@ where
 	Extra: SignedExtension,
 {
 	fn encode(&self) -> Vec<u8> {
-		super::encode_with_vec_prefix::<Self, _>(|v| {
-			// 1 byte version id.
-			match self.signature.as_ref() {
-				Some(s) => {
-					v.push(EXTRINSIC_VERSION | 0b1000_0000);
-					s.encode_to(v);
-				},
-				None => {
-					v.push(EXTRINSIC_VERSION & 0b0111_1111);
-				},
-			}
-			self.function.encode_to(v);
-		})
+		let mut tmp = Vec::with_capacity(sp_std::mem::size_of::<Self>());
+
+		// 1 byte version id.
+		match self.signature.as_ref() {
+			Some(s) => {
+				tmp.push(EXTRINSIC_VERSION | 0b1000_0000);
+				s.encode_to(&mut tmp);
+			},
+			None => {
+				tmp.push(EXTRINSIC_VERSION & 0b0111_1111);
+			},
+		}
+		self.function.encode_to(&mut tmp);
+
+		let compact_len = codec::Compact::<u32>(tmp.len() as u32);
+
+		// Allocate the output buffer with the correct length
+		let mut output = Vec::with_capacity(compact_len.size_hint() + tmp.len());
+
+		compact_len.encode_to(&mut output);
+		output.extend(tmp);
+
+		output
 	}
 }