maint: Move to /rust/* to /

This makes it easier for tooling to find the Rust stuff.
Rust/non-rust is not a useful distinction in this repo anymore anyway.

nix-bindings-util/src/string_return.rs

@@ -0,0 +1,88 @@
+use anyhow::Result;
+
+/// Callback for nix_store_get_uri and other functions that return a string.
+///
+/// This function is used by the other nix_* crates, and you should never need to call it yourself.
+///
+/// Some functions in the nix library "return" strings without giving you ownership over them, by letting you pass a callback function that gets to look at that string. This callback simply turns that string pointer into an owned rust String.
+///
+/// # Safety
+///
+/// _Manual memory management_
+///
+/// Only for passing to the nix C API. Do not call this function directly.
+pub unsafe extern "C" fn callback_get_result_string(
+    start: *const ::std::os::raw::c_char,
+    n: std::os::raw::c_uint,
+    user_data: *mut std::os::raw::c_void,
+) {
+    let ret = user_data as *mut Result<String>;
+
+    if start.is_null() {
+        if n != 0 {
+            panic!("callback_get_result_string: start is null but n is not zero");
+        }
+        *ret = Ok(String::new());
+        return;
+    }
+
+    let slice = std::slice::from_raw_parts(start as *const u8, n as usize);
+
+    if (*ret).is_ok() {
+        panic!(
+            "callback_get_result_string: Result must be initialized to Err. Did Nix call us twice?"
+        );
+    }
+
+    *ret = String::from_utf8(slice.to_vec())
+        .map_err(|e| anyhow::format_err!("Nix string is not valid UTF-8: {}", e));
+}
+
+pub fn callback_get_result_string_data(vec: &mut Result<String>) -> *mut std::os::raw::c_void {
+    vec as *mut Result<String> as *mut std::os::raw::c_void
+}
+
+#[macro_export]
+macro_rules! result_string_init {
+    () => {
+        Err(anyhow::anyhow!("String was not set by Nix C API"))
+    };
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use nix_bindings_bindgen_raw as raw;
+
+    /// Typecheck the function signature against the generated bindings in nix_bindings_bindgen_raw.
+    static _CALLBACK_GET_RESULT_STRING: raw::get_string_callback = Some(callback_get_result_string);
+
+    #[test]
+    fn test_callback_get_result_string_empty() {
+        let mut ret: Result<String> = result_string_init!();
+        let start: *const std::os::raw::c_char = std::ptr::null();
+        let n: std::os::raw::c_uint = 0;
+        let user_data: *mut std::os::raw::c_void = callback_get_result_string_data(&mut ret);
+
+        unsafe {
+            callback_get_result_string(start, n, user_data);
+        }
+
+        let s = ret.unwrap();
+        assert_eq!(s, "");
+    }
+
+    #[test]
+    fn test_callback_result_string() {
+        let mut ret: Result<String> = result_string_init!();
+        let start: *const std::os::raw::c_char = b"helloGARBAGE".as_ptr() as *const i8;
+        let n: std::os::raw::c_uint = 5;
+        let user_data: *mut std::os::raw::c_void = callback_get_result_string_data(&mut ret);
+        unsafe {
+            callback_get_result_string(start, n, user_data);
+        }
+
+        let s = ret.unwrap();
+        assert_eq!(s, "hello");
+    }
+}