Tiny-const-generics (#255)

* updates bytes to use const generics

* Add a check for a minimum rustc version in ff

* Specify min version for the compiler in README

README.md

@@ -27,7 +27,7 @@ In addition, the [`curves`](https://github.com/arkworks-rs/curves) repository co
 
 ## Build guide
 
-The library compiles on the `stable` toolchain of the Rust compiler. To install the latest version of Rust, first install `rustup` by following the instructions [here](https://rustup.rs/), or via your platform's package manager. Once `rustup` is installed, install the Rust toolchain by invoking:
+The library compiles on the `stable` toolchain of the Rust compiler (v 1.51+). To install the latest version of Rust, first install `rustup` by following the instructions [here](https://rustup.rs/), or via your platform's package manager. Once `rustup` is installed, install the Rust toolchain by invoking:
 ```bash
 rustup install stable
 ```

ff/build.rs

@@ -1,5 +1,5 @@
 extern crate rustc_version;
-use rustc_version::{version_meta, Channel};
+use rustc_version::{version, version_meta, Channel, Version};
 
 fn main() {
  println!("cargo:rerun-if-changed=build.rs");
@@ -15,4 +15,10 @@ fn main() {
  if should_use_asm {
  println!("cargo:rustc-cfg=use_asm");
  }
+
+ // TODO: remove this once RFC 2495 ships
+ if version().expect("Installed rustc version unparseable!") < Version::parse("1.51.0").unwrap()
+ {
+ panic!("This code base uses const generics and requires a Rust compiler version greater or equal to 1.51.0");
+ }
 }

ff/src/bytes.rs

@@ -14,151 +14,113 @@ pub trait FromBytes: Sized {
  fn read<R: Read>(reader: R) -> IoResult<Self>;
 }
 
-macro_rules! array_bytes {
- ($N:expr) => {
- impl ToBytes for [u8; $N] {
- #[inline]
- fn write<W: Write>(&self, mut writer: W) -> IoResult<()> {
- writer.write_all(self)
- }
- }
+impl<const N: usize> ToBytes for [u8; N] {
+ #[inline]
+ fn write<W: Write>(&self, mut writer: W) -> IoResult<()> {
+ writer.write_all(self)
+ }
+}
 
- impl FromBytes for [u8; $N] {
-  #[inline]
-  fn read<R: Read>(mut reader: R) -> IoResult<Self> {
-  let mut arr = [0u8; $N];
-  reader.read_exact(&mut arr)?;
-  Ok(arr)
-  }
- }
+impl<const N: usize> FromBytes for [u8; N] {
+ #[inline]
+ fn read<R: Read>(mut reader: R) -> IoResult<Self> {
+ let mut arr = [0u8; N];
+ reader.read_exact(&mut arr)?;
+ Ok(arr)
+ }
+}
 
- impl ToBytes for [u16; $N] {
- #[inline]
- fn write<W: Write>(&self, mut writer: W) -> IoResult<()> {
- for num in self {
- writer.write_all(&num.to_le_bytes())?;
- }
- Ok(())
- }
+impl<const N: usize> ToBytes for [u16; N] {
+ #[inline]
+ fn write<W: Write>(&self, mut writer: W) -> IoResult<()> {
+ for num in self {
+ writer.write_all(&num.to_le_bytes())?;
  }
+ Ok(())
+ }
+}
 
- impl FromBytes for [u16; $N] {
- #[inline]
- fn read<R: Read>(mut reader: R) -> IoResult<Self> {
- let mut res = [0u16; $N];
- for num in res.iter_mut() {
- let mut bytes = [0u8; 2];
- reader.read_exact(&mut bytes)?;
- *num = u16::from_le_bytes(bytes);
- }
- Ok(res)
- }
+impl<const N: usize> FromBytes for [u16; N] {
+ #[inline]
+ fn read<R: Read>(mut reader: R) -> IoResult<Self> {
+ let mut res = [0u16; N];
+ for num in res.iter_mut() {
+ let mut bytes = [0u8; 2];
+ reader.read_exact(&mut bytes)?;
+ *num = u16::from_le_bytes(bytes);
  }
+ Ok(res)
+ }
+}
 
- impl ToBytes for [u32; $N] {
- #[inline]
- fn write<W: Write>(&self, mut writer: W) -> IoResult<()> {
- for num in self {
- writer.write_all(&num.to_le_bytes())?;
- }
- Ok(())
- }
+impl<const N: usize> ToBytes for [u32; N] {
+ #[inline]
+ fn write<W: Write>(&self, mut writer: W) -> IoResult<()> {
+ for num in self {
+ writer.write_all(&num.to_le_bytes())?;
  }
+ Ok(())
+ }
+}
 
- impl FromBytes for [u32; $N] {
- #[inline]
- fn read<R: Read>(mut reader: R) -> IoResult<Self> {
- let mut res = [0u32; $N];
- for num in res.iter_mut() {
- let mut bytes = [0u8; 4];
- reader.read_exact(&mut bytes)?;
- *num = u32::from_le_bytes(bytes);
- }
- Ok(res)
- }
+impl<const N: usize> FromBytes for [u32; N] {
+ #[inline]
+ fn read<R: Read>(mut reader: R) -> IoResult<Self> {
+ let mut res = [0u32; N];
+ for num in res.iter_mut() {
+ let mut bytes = [0u8; 4];
+ reader.read_exact(&mut bytes)?;
+ *num = u32::from_le_bytes(bytes);
  }
+ Ok(res)
+ }
+}
 
- impl ToBytes for [u64; $N] {
- #[inline]
- fn write<W: Write>(&self, mut writer: W) -> IoResult<()> {
- for num in self {
- writer.write_all(&num.to_le_bytes())?;
- }
- Ok(())
- }
+impl<const N: usize> ToBytes for [u64; N] {
+ #[inline]
+ fn write<W: Write>(&self, mut writer: W) -> IoResult<()> {
+ for num in self {
+ writer.write_all(&num.to_le_bytes())?;
  }
+ Ok(())
+ }
+}
 
- impl FromBytes for [u64; $N] {
- #[inline]
- fn read<R: Read>(mut reader: R) -> IoResult<Self> {
- let mut res = [0u64; $N];
- for num in res.iter_mut() {
- let mut bytes = [0u8; 8];
- reader.read_exact(&mut bytes)?;
- *num = u64::from_le_bytes(bytes);
- }
- Ok(res)
- }
+impl<const N: usize> FromBytes for [u64; N] {
+ #[inline]
+ fn read<R: Read>(mut reader: R) -> IoResult<Self> {
+ let mut res = [0u64; N];
+ for num in res.iter_mut() {
+ let mut bytes = [0u8; 8];
+ reader.read_exact(&mut bytes)?;
+ *num = u64::from_le_bytes(bytes);
  }
+ Ok(res)
+ }
+}
 
- impl ToBytes for [u128; $N] {
- #[inline]
- fn write<W: Write>(&self, mut writer: W) -> IoResult<()> {
- for num in self {
- writer.write_all(&num.to_le_bytes())?;
- }
- Ok(())
- }
+impl<const N: usize> ToBytes for [u128; N] {
+ #[inline]
+ fn write<W: Write>(&self, mut writer: W) -> IoResult<()> {
+ for num in self {
+ writer.write_all(&num.to_le_bytes())?;
  }
+ Ok(())
+ }
+}
 
- impl FromBytes for [u128; $N] {
- #[inline]
- fn read<R: Read>(mut reader: R) -> IoResult<Self> {
- let mut res = [0u128; $N];
- for num in res.iter_mut() {
- let mut bytes = [0u8; 16];
- reader.read_exact(&mut bytes)?;
- *num = u128::from_le_bytes(bytes);
- }
- Ok(res)
- }
+impl<const N: usize> FromBytes for [u128; N] {
+ #[inline]
+ fn read<R: Read>(mut reader: R) -> IoResult<Self> {
+ let mut res = [0u128; N];
+ for num in res.iter_mut() {
+ let mut bytes = [0u8; 16];
+ reader.read_exact(&mut bytes)?;
+ *num = u128::from_le_bytes(bytes);
  }
- };
-}
-
-array_bytes!(0);
-array_bytes!(1);
-array_bytes!(2);
-array_bytes!(3);
-array_bytes!(4);
-array_bytes!(5);
-array_bytes!(6);
-array_bytes!(7);
-array_bytes!(8);
-array_bytes!(9);
-array_bytes!(10);
-array_bytes!(11);
-array_bytes!(12);
-array_bytes!(13);
-array_bytes!(14);
-array_bytes!(15);
-array_bytes!(16);
-array_bytes!(17);
-array_bytes!(18);
-array_bytes!(19);
-array_bytes!(20);
-array_bytes!(21);
-array_bytes!(22);
-array_bytes!(23);
-array_bytes!(24);
-array_bytes!(25);
-array_bytes!(26);
-array_bytes!(27);
-array_bytes!(28);
-array_bytes!(29);
-array_bytes!(30);
-array_bytes!(31);
-array_bytes!(32);
+ Ok(res)
+ }
+}
 
 /// Takes as input a sequence of structs, and converts them to a series of
 /// bytes. All traits that implement `Bytes` can be automatically converted to