Do something with Nix configuration

src/cmds/hook.rs

@@ -179,6 +179,16 @@ pub fn run(args: &clap::ArgMatches) -> Result {
         }
     };
 
+    // Write out a nix.conf in some situations.
+    //
+    // NIX_CONF_DIR   nix_conf_dir   What to do?
+    // <unset>        <unset>        Nothing
+    // <unset>        /some/thing    Write new nix.conf, include /etc/nix/nix.conf, then /some/thing.
+    // /some/thing    <unset>        Nothing
+    // /some/thing1   /some/thing2   Write new nix.conf, include /some/thing1, then /some/thing2.
+
+
+
     writeln!(&mut handle, "}} # End.")?;
 
     Ok(0)

src/config.rs

@@ -52,6 +52,7 @@ pub struct Config {
     pub build_exe: PathBuf,
     pub watch_exe: PathBuf,
     pub direnv_exe: PathBuf,
+    pub nix_conf_dir: PathBuf,
     pub parent_dir: PathBuf,
     pub self_exe: PathBuf,
     pub messages: Messages,
@@ -63,11 +64,32 @@ struct ConfigData {
     build_exe: PathBuf,
     watch_exe: PathBuf,
     #[serde(default)]
+    nix_conf_dir: NixConfDir,
+    #[serde(default)]
     parent_dir: ParentDir,
     #[serde(default)]
     messages: Messages,
 }
 
+#[derive(Debug, Deserialize)]
+pub struct NixConfDir(pub PathBuf);
+
+impl Default for NixConfDir {
+    fn default() -> Self {
+        // From nix.conf(5): this is the "system-wide configuration file
+        // sysconfdir/nix/nix.conf". However,it doesn't seem possible or at
+        // least obvious how to find out what Nix's notion of `sysconfdir` is,
+        // so we assume it's /etc for now.
+        Self("/etc/nix".into())
+    }
+}
+
+impl AsRef<Path> for NixConfDir {
+    fn as_ref(&self) -> &Path {
+        &self.0
+    }
+}
+
 #[derive(Debug, Deserialize)]
 pub struct ParentDir(pub PathBuf);
 
@@ -124,6 +146,7 @@ impl Config {
             build_exe: datum_dir.join(config_data.build_exe).absolutize()?,
             watch_exe: datum_dir.join(config_data.watch_exe).absolutize()?,
             direnv_exe: search_path("direnv").ok_or(Error::DirenvNotFound)?,
+            nix_conf_dir: datum_dir.join(config_data.nix_conf_dir).absolutize()?,
             parent_dir: datum_dir.join(config_data.parent_dir).absolutize()?,
             self_exe: env::current_exe()?,
             messages: config_data.messages,

src/main.rs

@@ -9,6 +9,7 @@ mod cmds;
 mod config;
 mod env;
 mod error;
+mod nix;
 mod status;
 mod sums;
 

src/nix.rs

@@ -0,0 +1,59 @@
+use std::env::var_os;
+use std::path::PathBuf;
+
+// Find the system-wide nix.conf according to the same rules that Nix uses.
+//
+// nix.conf(5) has this to say:
+//
+//   Nix reads settings from two configuration files:
+//
+//   - The system-wide configuration file sysconfdir/nix/nix.conf (i.e.
+//     /etc/nix/nix.conf on most systems), or $NIX_CONF_DIR/nix.conf if
+//     NIX_CONF_DIR is set.
+//
+//   - The user configuration file $XDG_CONFIG_HOME/nix/nix.conf, or
+//     ~/.config/nix/nix.conf if XDG_CONFIG_HOME is not set.
+//
+fn find_system_nix_conf() -> PathBuf {
+    match var_os("NIX_CONF_DIF") {
+        Some(nix_conf_dir) => {
+            // Nix doesn't care if this file exists or not, so we don't either.
+            PathBuf::from(nix_conf_dir).join("nix.conf")
+        }
+        None => {
+            // It doesn't seem possible or at least obvious how to find out what
+            // Nix's notion of `sysconfdir` is, so we assume it's /etc for now.
+            PathBuf::from("/etc/nix/nix.conf")
+        }
+    }
+}
+
+// nix.conf(5) explains its format:
+//
+//   The configuration files consist of `name = value` pairs, one per line.
+//   Other files can be included with a line like `include` path, where `path`
+//   is interpreted relative to the current conf file and a missing file is an
+//   error unless `!include` is used instead. Comments start with a # character.
+//   Here is an example configuration file:
+//
+//     keep-outputs = true       # Nice for developers
+//     keep-derivations = true   # Idem
+//
+// Reading the code for Nix's applyConfigFile clarifies some things:
+//
+// - When it says "one per line" it means it: there is no way to break
+//   a setting over multiple lines.
+//
+// - Characters cannot be escaped, strings cannot be quoted.
+//
+// - When using include or !include, the filename is used as-is. It cannot
+//   contain whitespace; that would result in an error.
+//
+// - Options can contain whitespace (" \r\t") but each run of whitespace is
+//   normalized into a single space.
+//
+// - Everything is bytes; there's no encoding or decoding using character sets.
+//
+// - Included files can be relative paths. These are resolved relative to the
+//   directory of the file being read.
+//