Add codegen option instead of derive

Cargo.toml

@@ -1,3 +1,10 @@
+[workspace]
+members = [
+    ".",
+    "ff_codegen",
+    "ff_derive",
+]
+
 [package]
 name = "ff"
 version = "0.12.0"
@@ -18,7 +25,7 @@ bitvec = { version = "1", default-features = false, optional = true }
 byteorder = { version = "1", default-features = false, optional = true }
 ff_derive = { version = "0.12", path = "ff_derive", optional = true }
 rand_core = { version = "0.6", default-features = false }
-subtle = { version = "2.2.1", default-features = false, features = ["i128"] }
+subtle = { version = ">=2.6.0", default-features = false, features = ["i128"] }
 
 [dev-dependencies]
 rand = "0.8"

README.md

@@ -38,8 +38,7 @@ ff = { version = "0.12", features = ["derive"] }
 And then use the macro like so:
 
 ```rust
-#[macro_use]
-extern crate ff;
+use ff::PrimeField;
 
 #[derive(PrimeField)]
 #[PrimeFieldModulus = "52435875175126190479447740508185965837690552500527637822603658699938581184513"]
@@ -50,6 +49,39 @@ struct Fp([u64; 4]);
 
 And that's it! `Fp` now implements `Field` and `PrimeField`.
 
+### `build.rs`
+Using the `derive(PrimeField)` functionality can slow down compile times. As an alternative, the
+`ff` library's code generation functionality can be invoked via a build script.
+
+First, add the dependencies:
+```toml
+[dependencies]
+ff = { version = "0.12", features = ["derive"] }
+
+[build-dependencies]
+ff_codegen = "0.12"
+```
+
+Then write the `build.rs` file:
+```rust
+fn main() {
+    let path = std::path::Path::new(&std::env::var("OUT_DIR").unwrap()).join("codegen.rs");
+    std::fs::write(&path, ff_codegen::PrimeFieldCodegen {
+        ident: "Fp",
+        is_pub: false,
+        modulus: "52435875175126190479447740508185965837690552500527637822603658699938581184513",
+        generator: "7",
+        endianness: ff_codegen::ReprEndianness::Little,
+    }.to_string()).unwrap();
+    println!("cargo:rerun-if-changed=build.rs");
+}
+```
+
+And, finally, in the module you want to include the code in:
+```rust
+include!(concat!(env!("OUT_DIR"), "/codegen.rs"));
+```
+
 ## Minimum Supported Rust Version
 
 Requires Rust **1.56** or higher.

ff_codegen/Cargo.toml

@@ -0,0 +1,30 @@
+[package]
+name = "ff_codegen"
+version = "0.12.0"
+authors = [
+    "Sean Bowe <[email protected]>",
+    "Jack Grigg <[email protected]>",
+]
+description = "Code generator library used to build custom prime field implementations"
+documentation = "https://docs.rs/ff-common/"
+homepage = "https://github.com/zkcrypto/ff"
+license = "MIT/Apache-2.0"
+repository = "https://github.com/zkcrypto/ff"
+edition = "2021"
+
+[features]
+# enabled when generating bitvec code utilizing the version of ff's bitvec
+bits = []
+
+[dependencies]
+addchain = "0.2"
+num-bigint = "0.3"
+num-traits = "0.2"
+num-integer = "0.1"
+proc-macro2 = "1"
+quote = "1"
+syn = { version = "1", features = ["full"] }
+cfg-if = "1"
+
+[badges]
+maintenance = { status = "passively-maintained" }