Use cbindgen via Rust API (#14013)

* move cbindgen to Rust build

* add include guard

* allow cmdline build and compile-time build

* write `qiskit.h` to `crates/cext`

and use Makefile to correctly move or regenerate the file lazily

* try cleaner build setup

write header into target/qiskit.h and copy into dist/c/include with Makefile. The cargo build script tracks changes in target/qiskit.h and rebuilds if changed/deleted

Not sure if this is the best way but it seems to work :)

* try fix build deps, and fix clippy

* fix merge artifact

this is probably why CI doesn't work, we need to ensure the cdylib is built

* include dir creation as dependency

Author: Cryoris

Makefile

@@ -106,7 +106,6 @@ C_LIB_CARGO_PATH=$(C_CARGO_TARGET_DIR)/$(C_LIB_CARGO_FILENAME)
 C_QISKIT_H=$(C_DIR_INCLUDE)/qiskit.h
 C_LIBQISKIT=$(C_DIR_LIB)/$(subst _cext,,$(C_LIB_CARGO_FILENAME))
 
-
 # Run clang-format (does not apply any changes)
 cformat:
 	bash tools/run_clang_format.sh
@@ -120,22 +119,24 @@ fix_cformat:
 $(C_LIB_CARGO_PATH):
 	cargo rustc --release --crate-type cdylib -p qiskit-cext
 
-$(C_QISKIT_H): $(C_LIB_CARGO_PATH)
-	cbindgen --crate qiskit-cext --output $(C_DIR_INCLUDE)/qiskit.h --lang C
-
 $(C_DIR_LIB):
 	mkdir -p $(C_DIR_LIB)
 
-$(C_LIBQISKIT): $(C_DIR_LIB) $(C_LIB_CARGO_PATH)
+$(C_DIR_INCLUDE):
+	mkdir -p $(C_DIR_INCLUDE)
+
+$(C_LIBQISKIT): $(C_DIR_LIB)  $(C_LIB_CARGO_PATH)
 	cp $(C_LIB_CARGO_PATH) $(C_DIR_LIB)/$(subst _cext,,$(C_LIB_CARGO_FILENAME))
 
-.PHONY: cheader clib c
+$(C_QISKIT_H): $(C_DIR_INCLUDE) $(C_LIB_CARGO_PATH) 
+	cp target/qiskit.h $(C_DIR_INCLUDE)/qiskit.h
+
+.PHONY: c cheader 
 cheader: $(C_QISKIT_H)
-clib: $(C_LIBQISKIT)
-c: clib cheader
+c: $(C_LIBQISKIT) $(C_QISKIT_H)
 
 # Use ctest to run C API tests
-ctest: $(C_QISKIT_H)
+ctest: $(C_LIB_CARGO_PATH) $(C_QISKIT_H) 
 	# -S specifically specifies the source path to be the current folder
 	# -B specifically specifies the build path to be inside test/c/build
 	cmake -S. -B$(C_DIR_TEST_BUILD)
@@ -147,3 +148,5 @@ ctest: $(C_QISKIT_H)
 
 cclean:
 	rm -rf $(C_DIR_OUT) $(C_DIR_TEST_BUILD)
+	rm -f target/qiskit.h
+	cargo clean

crates/cext/build.rs

@@ -10,9 +10,36 @@
 // copyright notice, and modified files need to carry a notice indicating
 // that they have been altered from the originals.
 
-fn main() {
-    // Get the Python library directory and library name that PyO3 is using, and store it into a
-    // configuration file.
+use std::str::FromStr;
+
+extern crate cbindgen;
+
+/// This function generates the C header for Qiskit from the qiskit-cext crate.
+fn generate_qiskit_header() {
+    // Trigger this script if the header was changed/removed.
+    #![allow(clippy::print_stdout)]
+    println!("cargo:rerun-if-changed=../../target/qiskit.h");
+
+    // Pull the config from the cbindgen.toml file.
+    let config = cbindgen::Config::from_file("cbindgen.toml").unwrap();
+
+    // Ensure target path exists and then set the full filename of qiskit.h.
+    let mut path = ::std::path::PathBuf::from_str("../../target/").unwrap();
+    ::std::fs::create_dir_all(&path).expect("Could not create target directory.");
+    path.push("qiskit.h");
+
+    // Build the header.
+    cbindgen::Builder::new()
+        .with_crate(".")
+        .with_config(config)
+        .generate()
+        .expect("Unable to generate C bindings.")
+        .write_to_file(path);
+}
+
+// Get the Python library directory and library name that PyO3 is using, and store it into a
+// configuration file.
+fn write_python_config() {
     let interpreter_config = pyo3_build_config::get();
     let pyo3_lib_config = format!(
         "PYO3_PYTHON_LIB_DIR={}\nPYO3_PYTHON_LIB_NAME={}\n",
@@ -28,5 +55,10 @@ fn main() {
     match ::std::fs::write(pyo3_config_file, pyo3_lib_config) {
         Ok(_) => (),
         Err(_) => println!("cargo:warning=Failed to write Python config."),
-    }
+    };
+}
+
+fn main() {
+    generate_qiskit_header();
+    write_python_config();
 }

crates/cext/cbindgen.toml

@@ -1,6 +1,8 @@
+language = "C"
 include_version = true
-include_guard = "QISKIT_H_INCLUDED"
+include_guard = "QISKIT_H"
 style = "type"
+
 sys_includes = ["complex.h"]
 after_includes = """
 #ifdef QISKIT_C_PYTHON_INTERFACE