chore: checkpoint before Python removal

vendor/serde_json/build.rs

@@ -0,0 +1,30 @@
+use std::env;
+
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+
+    println!("cargo:rustc-check-cfg=cfg(fast_arithmetic, values(\"32\", \"64\"))");
+
+    // Decide ideal limb width for arithmetic in the float parser and string
+    // parser.
+    let target_arch = env::var_os("CARGO_CFG_TARGET_ARCH").unwrap();
+    let target_pointer_width = env::var_os("CARGO_CFG_TARGET_POINTER_WIDTH").unwrap();
+    if target_arch == "aarch64"
+        || target_arch == "loongarch64"
+        || target_arch == "mips64"
+        || target_arch == "powerpc64"
+        || target_arch == "riscv64"
+        || target_arch == "wasm32"
+        || target_arch == "x86_64"
+        || target_pointer_width == "64"
+    {
+        // The above list of architectures are ones that have native support for
+        // 64-bit arithmetic, but which have some targets using a smaller
+        // pointer width. Examples include aarch64-unknown-linux-gnu_ilp32 and
+        // x86_64-unknown-linux-gnux32. So our choice of limb width is not
+        // equivalent to using usize everywhere.
+        println!("cargo:rustc-cfg=fast_arithmetic=\"64\"");
+    } else {
+        println!("cargo:rustc-cfg=fast_arithmetic=\"32\"");
+    }
+}