feat(users/Profpatsch/execline): add exec helpers

Most tools end by execing into their argv, so here’s a small rust
function which does the boilerplate.

Change-Id: I9748955cf53828e02f04d7e8d74fbaf10c1158b5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2453
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>

users/Profpatsch/execline/default.nix

@@ -0,0 +1,27 @@
+{ depot, pkgs, lib, ... }:
+
+let
+  exec-helpers = depot.users.Profpatsch.writers.rustSimpleLib {
+    name = "exec-helpers";
+  } ''
+    use std::os::unix::process::CommandExt;
+    use std::ffi::OsStr;
+    use std::os::unix::ffi::OsStrExt;
+    pub fn exec_into_args<'a, I>(prog_name: &str, env_additions: I) -> !
+      where
+        I: IntoIterator<Item = (&'a [u8], &'a [u8])>,
+    {
+        let mut argv = std::env::args_os();
+        let prog = argv.nth(1).expect(&format!("{}: first argument must be an executable", prog_name));
+        let args = argv;
+        let env = env_additions.into_iter().map(|(k,v)| (OsStr::from_bytes(k), OsStr::from_bytes(v)));
+        let err = std::process::Command::new(prog).args(args).envs(env).exec();
+        panic!("{}: exec failed: {:?}", prog_name, err);
+    }
+  '';
+
+in {
+  inherit
+    exec-helpers
+    ;
+}