chore(nix): move rustSimple from users.Profpatsch.writers

I think it’s solid enough to use in a wider context.

Change-Id: If53e8bbb6b90fa88d73fb42730db470e822ea182
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3055
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: lukegb <lukegb@tvl.fyi>

nix/writers/tests/rust.nix

@@ -0,0 +1,68 @@
+{ depot, pkgs, ... }:
+
+let
+  inherit (depot.nix.writers)
+    rustSimple
+    rustSimpleLib
+    rustSimpleBin
+    ;
+
+  inherit (pkgs)
+    coreutils
+    ;
+
+  run = drv: depot.nix.runExecline.local "run-${drv.name}" {} [
+    "if" [ drv ]
+    "importas" "out" "out"
+    "${coreutils}/bin/touch" "$out"
+  ];
+
+  rustTransitiveLib = rustSimpleLib {
+    name = "transitive";
+  } ''
+    pub fn transitive(s: &str) -> String {
+      let mut new = s.to_string();
+      new.push_str(" 1 2 3");
+      new
+    }
+
+    #[cfg(test)]
+    mod tests {
+      use super::*;
+
+      #[test]
+      fn test_transitive() {
+        assert_eq!(transitive("foo").as_str(), "foo 1 2 3")
+      }
+    }
+  '';
+
+  rustTestLib = rustSimpleLib {
+    name = "test_lib";
+    dependencies = [ rustTransitiveLib ];
+  } ''
+    extern crate transitive;
+    use transitive::{transitive};
+    pub fn test() -> String {
+      transitive("test")
+    }
+  '';
+
+  rustWithLib = run (rustSimple {
+    name = "rust-with-lib";
+    dependencies = [ rustTestLib ];
+  } ''
+    extern crate test_lib;
+
+    fn main() {
+      assert_eq!(test_lib::test(), String::from("test 1 2 3"));
+    }
+  '');
+
+
+in depot.nix.utils.drvTargets {
+  inherit
+    rustTransitiveLib
+    rustWithLib
+    ;
+}