Evaluate `const`s via evaluation

[tgross35]

It seems like cbindgen unfortunately hits a wall in converting consts to #defines when it involves expansion or function evaluation. But would it be possible for bindgen to compile a shim and extract those values? For example, I have a crate with this:

pub const RC_AEAD_AES128GCM_NONCEBYTES: usize = <<Aes128Gcm as aead::AeadCore>::NonceSize>::USIZE; // 12usize

It seems like Bindgen could turn this into:

fn main() {
    println!("RC_AEAD_AES128GCM_NONCEBYTES={}", maincrate::RC_AEAD_AES128GCM_NONCEBYTES);
}

compile it, and read the result. It's a bit tricky but seems not impossible?

There is of course a bootstrapping problem when using the library, i.e. the build script can't complete without already having a built library. But it would be usable via CLI or from a separate crate.

[tgross35]

Actually, maybe we could use the rustdoc JSON output? It is unstable, but it includes the following:

    "0:3:1735": {
      "id": "0:3:1735",
      "crate_id": 0,
      "name": "I_AM_A_CONST",
      "span": {
        "filename": "rcrypto/src/lib.rs",
        "begin": [
          1,
          0
        ],
        "end": [
          1,
          38
        ]
      },
      "visibility": "public",
      "docs": null,
      "links": {},
      "attrs": [],
      "deprecation": null,
      "inner": {
        "constant": {
          "type": {
            "primitive": "usize"
          },
          "expr": "_",
          "value": "63_737usize",
          "is_literal": false
        }
      }
    },

from this

pub const I_AM_A_CONST: usize = foo();

/// export thing
pub const fn foo() -> usize {
    63737
}

[tgross35]

Yes, rustdoc can do it! I needed to move bindings generation to a new binary crate (Cargo deadlocks if you call cargo rustdoc from within build.rs...) and this only works for integers, but the recipe is here

My consts are a mess, generated in macros

        /// Length of the nonce (initialization vector) for
        #[doc = $name]
        pub const $noncebytes: usize = <$alg as aead::AeadCore>::NonceSize::USIZE;

        /// Length of the key for
        #[doc = $name]
        pub const $keybytes: usize = <$alg as aead::KeySizeUser>::KeySize::USIZE;

        /// Length of the MAC (tag) for
        #[doc = $name]
        pub const $macbytes: usize = <$alg as aead::AeadCore>::TagSize::USIZE;

But using this strategy I get the full correct thing:

#define RC_AEAD_AES128GCM_NONCEBYTES 12
#define RC_AEAD_AES128GCM_KEYBYTES 16
#define RC_AEAD_AES128GCM_MACBYTES 16
#define RC_AEAD_AES256GCM_NONCEBYTES 12
#define RC_AEAD_AES256GCM_KEYBYTES 32
#define RC_AEAD_AES256GCM_MACBYTES 16
#define RC_AEAD_CHACHA20POLY1305_NONCEBYTES 12
#define RC_AEAD_CHACHA20POLY1305_KEYBYTES 32
#define RC_AEAD_CHACHA20POLY1305_MACBYTES 16
#define RC_AEAD_XCHACHA20POLY1305_NONCEBYTES 24
#define RC_AEAD_XCHACHA20POLY1305_KEYBYTES 32
#define RC_AEAD_XCHACHA20POLY1305_MACBYTES 16
#define RC_SECRETBOX_KEYBYTES 24
#define RC_SECRETBOX_NONCEBYTES 32
#define RC_SECRETBOX_MACBYTES 16

main.rs for the crate that does this (requires serde_json and rustdoc_json):

use serde_json::Value;
use std::fmt::Write;
use std::fs::File;
use std::{env, path::Path};

const HEADER_BASE: &str = "/*
* This file is automatically generated upon running
* `cargo +nightly build`. Do not modify by hand.
*/";

fn main() {
    let this_dir = env::var("CARGO_MANIFEST_DIR").unwrap();
    let target_lib_dir = Path::new(&this_dir).parent().unwrap().join("rcrypto-lib");

    let json_path = rustdoc_json::Builder::default()
        .toolchain("nightly")
        .manifest_path(target_lib_dir.join("Cargo.toml"))
        .build()
        .unwrap();

    let reader = File::open(json_path).unwrap();
    let json: Value = serde_json::from_reader(reader).unwrap();

    let index = json.as_object().unwrap()["index"].as_object().unwrap();
    let consts = index
        .values()
        .filter(|v| v.get("visibility").map(|v| v.as_str().unwrap()) == Some("public"))
        .filter_map(|v| {
            // Messy, but we first check if `entry.inner.constant` exists (indicating a constant)
            // then pair that constant's value with its name
            v.get("inner")
                .and_then(|v| v.as_object().unwrap().get("constant"))
                .map(|c| {
                    (
                        v.get("name").unwrap().as_str().unwrap(),
                        c.get("value").unwrap().as_str().unwrap(),
                    )
                })
        });

    let mut header = HEADER_BASE.to_owned();
    header.push('\n');

    for (cname, cval) in consts {
        // We only focus on integers, so trim the suffix from e.g. `0usize`;
        let new_cval = cval.trim_end_matches(char::is_alphabetic);

        writeln!(header, "#define {cname} {new_cval}").unwrap();
    }

    cbindgen::Builder::new()
        .with_crate(target_lib_dir)
        .with_no_includes()
        .with_sys_include("stdint.h")
        .with_parse_expand(&["rcrypto-lib"])
        .with_language(cbindgen::Language::C)
        .with_cpp_compat(true)
        .with_header(header)
        .generate()
        .expect("failed to generate C bindings")
        .write_to_file("../rcrypto.h");
}