fix: better CUDA detection (#61)

# What ❔

This PR improves the detection of the CUDA toolkit under Linux, and the
detection of its version under all OSes.

## Why ❔

Under Linux, the CUDA detection is hardcoded to look into
`/usr/local/cuda`, which is not guaranteed to be the default
installation folder. The new method first looks for the `CUDA_PATH` env.
var., then default to `/usr/local/cuda` if it is empty.

The current way of finding out the installed CUDA version is to look for
a `version.json` file in the CUDA installation path. This file is not
guaranteed to exist. However, `nvcc` **is** guaranteed to be installed
with the toolkit; so the new method is to parse its output to find the
exact version there.

## Checklist
- [X] PR title corresponds to the body of PR (we generate changelog
entries from PRs).
- [X] Tests for the changes have been added / updated.
- [X] Documentation comments have been added / updated.
- [X] Code has been formatted via `zk fmt` and `zk lint`.

crates/cudart-sys/Cargo.toml

@@ -11,7 +11,7 @@ name = "era_cudart_sys"
 description = "Raw CUDA bindings for ZKsync"
 
 [dependencies]
-serde_json = "1.0"
+regex-lite = "0.1"
 
 [build-dependencies]
-serde_json = "1.0"
+regex-lite = "0.1"

crates/cudart-sys/src/utils.rs

@@ -4,12 +4,16 @@ use std::path::{Path, PathBuf};
 pub fn get_cuda_path() -> Option<&'static Path> {
     #[cfg(target_os = "linux")]
     {
-        let path = Path::new("/usr/local/cuda");
-        if path.exists() {
-            Some(path)
-        } else {
-            None
+        for path_name in [option_env!("CUDA_PATH"), Some("/usr/local/cuda")]
+            .iter()
+            .flatten()
+        {
+            let path = Path::new(path_name);
+            if path.exists() {
+                return Some(path);
+            }
         }
+        None
     }
     #[cfg(target_os = "windows")]
     {
@@ -44,10 +48,19 @@ pub fn get_cuda_version() -> Option<String> {
     if let Some(version) = option_env!("CUDA_VERSION") {
         Some(version.to_string())
     } else if let Some(path) = get_cuda_path() {
-        let file = File::open(path.join("version.json")).expect("CUDA Toolkit should be installed");
-        let reader = std::io::BufReader::new(file);
-        let value: serde_json::Value = serde_json::from_reader(reader).unwrap();
-        Some(value["cuda"]["version"].as_str().unwrap().to_string())
+        let re = regex_lite::Regex::new(r"V(?<version>\d{2}\.\d+\.\d+)").unwrap();
+        let nvcc_out = std::process::Command::new("nvcc")
+            .arg("--version")
+            .output()
+            .expect("failed to start `nvcc`");
+        let nvcc_str = std::str::from_utf8(&nvcc_out.stdout).expect("`nvcc` output is not UTF8");
+        let captures = re.captures(&nvcc_str).unwrap();
+        let version = captures
+            .get(0)
+            .expect("unable to find nvcc version in the form VMM.mm.pp in the output of `nvcc --version`:\n{nvcc_str}")
+            .as_str()
+            .to_string();
+        Some(version)
     } else {
         None
     }