feat: enable compilation with CUDA stubs via the `ZKSYNC_USE_CUDA_STU…

…BS` environment variable (#29)

# What ❔

This PR is a follow-up to

matter-labs/era-cuda#9
matter-labs/era-boojum-cuda#36

making it possible to enable the compilation with CUDA stubs by setting
the environment variable `ZKSYNC_USE_CUDA_STUBS` to `1` or `true` or
`yes` in any capitalization in addition to the `no_cuda` configuration
flag.

## Why ❔

Other way to enable the compilation with stubs besides a configuration
flag is desirable.

## Checklist

- [x] PR title corresponds to the body of PR (we generate changelog
entries from PRs).
- [x] Code has been formatted via `cargo fmt` and linted via `cargo
clippy`.

README.md

@@ -25,6 +25,10 @@ Then add the following variable to your config (`.bashrc`/`.zshrc`):
 export BELLMAN_CUDA_DIR=<PATH_TO>/era-bellman-cuda
 ```
 
+Alternatively, if you can't or don't want to install the CUDA toolkit, you can compile the crates `cudart-sys` and `boojum-cuda` and crates that depend on them without CUDA toolkit.
+Doing so will result in stubs replacing calls to CUDA API and any GPU device code. The code will compile but any call to one of the stubs will result in an error during runtime.
+To compile in this mode, either include the rucstc `cfg` flag named `no_cuda`, for example by setting the `RUSTFLAGS` environment variable to  `--cfg no_cuda`, or by setting the environment variable `ZKSYNC_USE_CUDA_STUBS` to `1` or `true` or `yes` in any capitalization.
+
 ## Crates
 
 - [bindings-generator](./crates/bindings-generator/)

crates/boojum-cuda/build/main.rs

@@ -10,12 +10,10 @@ fn main() {
     gates::generate();
     poseidon_constants::generate();
     println!("cargo::rustc-check-cfg=cfg(no_cuda)");
-    #[cfg(no_cuda)]
-    {
+    if era_cudart_sys::is_no_cuda() {
         println!("cargo::warning={}", era_cudart_sys::no_cuda_message!());
-    }
-    #[cfg(not(no_cuda))]
-    {
+        println!("cargo::rustc-cfg=no_cuda");
+    } else {
         use era_cudart_sys::{get_cuda_lib_path, get_cuda_version};
         use std::env::var;
         let cuda_version =

crates/cudart-sys/build.rs

@@ -4,12 +4,10 @@ include!("src/utils.rs");
 
 fn main() {
     println!("cargo::rustc-check-cfg=cfg(no_cuda)");
-    #[cfg(no_cuda)]
-    {
+    if is_no_cuda() {
         println!("cargo::warning={}", no_cuda_message!());
-    }
-    #[cfg(not(no_cuda))]
-    {
+        println!("cargo::rustc-cfg=no_cuda");
+    } else {
         let cuda_version =
             get_cuda_version().expect("Failed to determine the CUDA Toolkit version.");
         if !cuda_version.starts_with("12.") {

crates/cudart-sys/src/utils.rs

@@ -53,7 +53,15 @@ pub fn get_cuda_version() -> Option<String> {
     }
 }
 
-#[cfg(no_cuda)]
+pub fn is_no_cuda() -> bool {
+    if cfg!(no_cuda) {
+        true
+    } else {
+        let no_cuda = option_env!("ZKSYNC_USE_CUDA_STUBS").unwrap_or("");
+        no_cuda == "1" || no_cuda.to_lowercase() == "true" || no_cuda.to_lowercase() == "yes"
+    }
+}
+
 #[macro_export]
 macro_rules! no_cuda_message {
     () => {