Loading dependencies from an env var.

README.md

@@ -0,0 +1,16 @@
+This crate uses the external submodules
+[pil-stark]([email protected]:powdr-labs/pil-stark.git) and
+[zkevm-prover](https://github.com/powdr-labs/zkevm-prover) to generate EStark ZK
+proofs from a rust friendly interface.
+
+At runtime, it needs to find the contents of the cargo's `OUT_DIR` (which is
+populated by `build.rs` at build time) otherwise execution will panic. This
+means that, if used as a cargo dependency from the same machine it is built,
+e.g. with `cargo run` or `cargo test`, it will work out of the box. But if the
+binaries are executed from another machine, e.g. from a `nextest` archive, it
+will fail unless the original `OUT_DIR` contents are manually provided.
+
+Cargo's `OUT_DIR` will typically be
+`target/<profile>/build/pil-stark-prover-<hash>/out`. If the original directory
+is not found at runtime, the library will search for its contents in the path
+pointed by environment variable `PIL_STARK_PROVER_DEPS`.

src/lib.rs

@@ -1,5 +1,6 @@
 use std::{
-    fs,
+    borrow::Cow,
+    env, fs,
     io::{BufRead, BufReader},
     path::{Path, PathBuf},
     process::{Command, ExitStatus, Stdio},
@@ -14,7 +15,20 @@ pub struct OutputFiles {
 }
 
 const MAX_NODE_MEM: u32 = 1024 * 16;
-const EXTERNALS_DIR: &str = env!("OUT_DIR");
+
+fn deps_dir() -> Cow<'static, Path> {
+    const DEFAULT_DEPS_DIR: &str = env!("OUT_DIR");
+
+    let default_deps_dir = Path::new(DEFAULT_DEPS_DIR);
+    if default_deps_dir.is_dir() {
+        Cow::Borrowed(default_deps_dir)
+    } else {
+        Cow::Owned(env::var("PIL_STARK_PROVER_DEPS").map_or_else(|err| match err {
+            env::VarError::NotPresent => panic!("pil-stark-prover dependencies directory not found!\nEither set PIL_STARK_PROVER_DEPS environment variable, or build the project locally from source.\nSee README.md for more information."),
+            env::VarError::NotUnicode(os_str) => os_str.into(),
+        }, PathBuf::from))
+    }
+}
 
 #[derive(thiserror::Error, Debug)]
 pub enum Error {
@@ -72,7 +86,7 @@ pub fn generate_proof(
     commits_bin: &Path,
     output_dir: &Path,
 ) -> Result<OutputFiles, Error> {
-    let externals_dir = Path::new(EXTERNALS_DIR);
+    let externals_dir = deps_dir();
     let pil_stark_root = externals_dir.join("pil-stark");
     let pil_stark_src = pil_stark_root.join("src");
 
@@ -217,7 +231,7 @@ pub fn verify_proof(
     proof_json: &Path,
     publics_json: &Path,
 ) -> Result<(), Error> {
-    let externals_dir = Path::new(EXTERNALS_DIR);
+    let externals_dir = deps_dir();
     let pil_stark_root = externals_dir.join("pil-stark");
     let pil_stark_src = pil_stark_root.join("src");