feat: nix-fetchers crate

(cherry picked from commit 27d572403ac98d83d652481da6c22ad50bb00168)

rust/nix-fetchers/src/lib.rs

@@ -0,0 +1,38 @@
+use anyhow::{Context as _, Result};
+use nix_c_raw as raw;
+use nix_util::context::{self, Context};
+use std::ptr::NonNull;
+
+pub struct FetchersSettings {
+    pub(crate) ptr: NonNull<raw::fetchers_settings>,
+}
+impl Drop for FetchersSettings {
+    fn drop(&mut self) {
+        unsafe {
+            raw::fetchers_settings_free(self.ptr.as_ptr());
+        }
+    }
+}
+impl FetchersSettings {
+    pub fn new() -> Result<Self> {
+        let mut ctx = Context::new();
+        let ptr = unsafe { context::check_call!(raw::fetchers_settings_new(&mut ctx))? };
+        Ok(FetchersSettings {
+            ptr: NonNull::new(ptr).context("fetchers_settings_new unexpectedly returned null")?,
+        })
+    }
+
+    pub fn raw_ptr(&self) -> *mut raw::fetchers_settings {
+        self.ptr.as_ptr()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn fetchers_settings_new() {
+        let _ = FetchersSettings::new().unwrap();
+    }
+}