Fix Ubuntu CI: link system OpenBLAS directly via build.rs

The openblas-src -> openblas-build -> ureq -> native-tls dependency chain
fails to compile on Ubuntu CI because native-tls 0.2.17 is incompatible
with the latest Rust stable (missing Protocol::Tlsv13 match arm).

Replace blas-src/openblas with a build.rs that emits
`cargo:rustc-link-lib=openblas` directly, linking the system library
(libopenblas-dev) without any download infrastructure. The accelerate
feature (macOS) continues using blas-src as before.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: igerber

rust/Cargo.toml

@@ -17,7 +17,7 @@ extension-module = ["pyo3/extension-module"]
 # Platform BLAS backends (optional, activated for pre-built wheels)
 # When enabled, ndarray's .dot() and general_mat_vec_mul dispatch to BLAS dgemv/dgemm
 accelerate = ["ndarray/blas", "dep:blas-src", "blas-src/accelerate"]
-openblas = ["ndarray/blas", "dep:blas-src", "blas-src/openblas"]
+openblas = ["ndarray/blas"]
 
 [dependencies]
 # PyO3 0.22 supports Python 3.8-3.13

rust/build.rs

@@ -0,0 +1,12 @@
+/// Build script for diff_diff_rust.
+///
+/// When the `openblas` feature is enabled, links against the system OpenBLAS
+/// library directly. This avoids the `openblas-src` -> `openblas-build` ->
+/// `ureq` -> `native-tls` dependency chain, which has Rust compiler
+/// compatibility issues. Requires `libopenblas-dev` (Ubuntu) or
+/// `openblas-devel` (CentOS/manylinux) to be installed.
+fn main() {
+    if std::env::var("CARGO_FEATURE_OPENBLAS").is_ok() {
+        println!("cargo:rustc-link-lib=openblas");
+    }
+}

rust/src/lib.rs

@@ -3,10 +3,12 @@
 //! This module provides optimized implementations of computationally
 //! intensive operations used in difference-in-differences analysis.
 
-// Pull in BLAS linker flags when platform features are enabled.
+// Pull in BLAS linker flags for macOS Accelerate.
 // blas-src is a linker-only crate — extern crate is required to ensure
-// the Accelerate framework (macOS) or OpenBLAS (Linux) is actually linked.
-#[cfg(any(feature = "accelerate", feature = "openblas"))]
+// the Accelerate framework is actually linked.
+// For OpenBLAS (Linux), linking is handled by build.rs instead of blas-src
+// to avoid the openblas-src -> ureq -> native-tls dependency chain.
+#[cfg(feature = "accelerate")]
 extern crate blas_src;
 
 use pyo3::prelude::*;