Set RISC0_FEATURE_bigint2 in risc0-build to signal zkVM features and …

…fix usage of `stability::unstable` with `no_mangle` (risc0#2567)

This is a proposal for how we might improve build debugging when someone
tries to use bigint2, but it is not available. In this PR, an env var is
added to invocations of `cargo` in `risc0-build` that is a positive
signal to downstream crates that that a new "hardware" feature is
available.

In the case of upcoming cryptography patches that only work with
`bigint2`, we'd add the following snippet to the `lib.rs` file. The
advantage being that we give an error and link to our docs rather than a
linker error at the end of the build process. From there, we can provide
better guidance on either upgrading the `risc0-zkvm` version or
downgrading the cryptography patch version.

```rust
#[cfg(all(target_os = "zkvm", target_arch = "riscv32"))]
const _: () = {
    assert!(
        core::option_env!("RISC0_FEATURE_bigint2").is_some(),
        r#"
RISC Zero zkVM feature bigint2 is not available, and is required by this crate.

If you'd like to use bigint2, please upgrade to risc0-zkvm and risc0-build and ensure the required
feature flags are enabled. See the RISC Zero dev docs for more information.
https://dev.risczero.com/api/zkvm/acceleration
"#
    );
};
```

This is written with bigint2 in mind. It could also be used when
transitioning to the new ABI with zkVM revision 2.

When I went to test this I found that my ECDSA program linked and built
without setting `unstable` on `risc0-zkvm`, which was unexpected.
Digging into the `stability` crate I found that the way the implement
their macro [1], it doesn't play nice with `no_mangle` and the symbol
will still be included.

https://docs.rs/stability/latest/src/stability/unstable.rs.html#85

examples/jwt-validator/methods/Cargo.toml

@@ -4,7 +4,7 @@ version = "0.1.0"
 edition = "2021"
 
 [build-dependencies]
-risc0-build = { path = "../../../risc0/build" }
+risc0-build = { path = "../../../risc0/build", features = ["unstable"] }
 
 [package.metadata.risc0]
 methods = ["guest"]

examples/jwt-validator/methods/guest/Cargo.toml

@@ -9,6 +9,7 @@ edition = "2021"
 jwt-core = { path = "../../core/" }
 risc0-zkvm = { path = "../../../../risc0/zkvm", default-features = false, features = [
   "std",
+  "unstable",
 ] }
 
 [patch.crates-io]

risc0/bigint2/methods/Cargo.toml

@@ -4,7 +4,7 @@ version = { workspace = true }
 edition = { workspace = true }
 
 [build-dependencies]
-risc0-build = { workspace = true }
+risc0-build = { workspace = true, features = ["unstable"] }
 
 [package.metadata.release]
 release = false

risc0/bigint2/src/lib.rs

@@ -32,6 +32,20 @@ pub trait ToBigInt2Buffer<const WIDTH: usize> {
     fn from_u32_array(array: [u32; WIDTH]) -> Self;
 }
 
+#[cfg(all(target_os = "zkvm", target_arch = "riscv32"))]
+const _: () = {
+    assert!(
+        core::option_env!("RISC0_FEATURE_bigint2").is_some(),
+        r#"
+RISC Zero zkVM feature bigint2 is not available, and is required by this crate.
+
+If you'd like to use bigint2, please upgrade to risc0-zkvm and risc0-build and ensure the required
+feature flags are enabled. See the RISC Zero dev docs for more information.
+https://dev.risczero.com/api/zkvm/acceleration
+"#
+    );
+};
+
 #[cfg(feature = "num-bigint")]
 impl<const WIDTH: usize> ToBigInt2Buffer<WIDTH> for num_bigint::BigUint {
     fn to_u32_array(&self) -> [u32; WIDTH] {

risc0/build/Cargo.toml

@@ -27,3 +27,4 @@ all-features = true
 [features]
 docker = []
 guest-list = []
+unstable = []

risc0/build/src/lib.rs

@@ -443,6 +443,12 @@ pub fn cargo_command(subcmd: &str, rust_flags: &[&str]) -> Command {
             .join("cpp/bin/riscv32-unknown-elf-gcc");
         cmd.env("CC", cc_path)
             .env("CFLAGS_riscv32im_risc0_zkvm_elf", "-march=rv32im -nostdlib");
+        // Signal to dependencies, cryptography patches in particular, that the bigint2 zkVM
+        // feature is available. Gated behind unstable to match risc0-zkvm-platform. Note that this
+        // would be seamless if there was a reliable way to tell whether it is enabled in
+        // risc0-zkvm-platform, however, this problem is also temporary.
+        #[cfg(feature = "unstable")]
+        cmd.env("RISC0_FEATURE_bigint2", "");
     }
 
     cmd.env("RUSTC", rustc)

risc0/zkvm/platform/src/syscall.rs

@@ -895,8 +895,7 @@ pub extern "C" fn sys_exit(status: i32) -> ! {
 /// `control_id` must be aligned and dereferenceable.
 ///
 /// `input` must be aligned and have `input_len` u32s dereferenceable
-#[cfg(feature = "export-syscalls")]
-#[no_mangle]
+#[cfg_attr(all(feature = "export-syscalls", feature = "unstable"), no_mangle)]
 #[stability::unstable]
 pub unsafe extern "C" fn sys_prove_zkr(
     claim_digest: *const [u32; DIGEST_WORDS],
@@ -931,8 +930,7 @@ pub unsafe extern "C" fn sys_prove_zkr(
 /// invoked during `hasher.finalize(...)`
 ///
 /// # Safety
-#[cfg(feature = "export-syscalls")]
-#[no_mangle]
+#[cfg_attr(all(feature = "export-syscalls", feature = "unstable"), no_mangle)]
 #[stability::unstable]
 pub unsafe extern "C" fn sys_keccak(
     input_ptr: *const u8,
@@ -961,8 +959,7 @@ pub unsafe extern "C" fn sys_keccak(
 /// `control_root` must be aligned and dereferenceable.
 ///
 /// `input` must be aligned and have `input_len` u32s dereferenceable
-#[cfg(feature = "export-syscalls")]
-#[no_mangle]
+#[cfg_attr(all(feature = "export-syscalls", feature = "unstable"), no_mangle)]
 #[stability::unstable]
 pub unsafe extern "C" fn sys_prove_keccak(
     po2: usize,
@@ -1017,7 +1014,7 @@ macro_rules! impl_sys_bigint2 {
         /// # Safety
         ///
         /// `blob_ptr` and all arguments must be aligned and dereferenceable.
-        #[cfg_attr(feature = "export-syscalls", no_mangle)]
+        #[cfg_attr(all(feature = "export-syscalls", feature = "unstable"), no_mangle)]
         #[stability::unstable]
         pub unsafe extern "C" fn $func_name(blob_ptr: *const u8, a1: *const u32
             $(, $a2: *const u32