Add build-vm subcommand to nh os.

This commit adds the `nh os build-vm` subcommand to build a virtual
machine activation script instead of a full system.
This includes a new option --with-bootloader/-B that applies to just build-vm,
to build a VM with a bootloader.

src/darwin.rs

@@ -95,7 +95,7 @@ impl DarwinRebuildArgs {
             }
         }
 
-        let toplevel = toplevel_for(hostname, installable);
+        let toplevel = toplevel_for(hostname, installable, String::from("toplevel"));
 
         commands::Build::new(toplevel)
             .extra_arg("--out-link")

src/interface.rs

@@ -94,6 +94,19 @@ pub enum OsSubcommand {
 
     /// List available generations from profile path
     Info(OsGenerationsArgs),
+
+    /// Build a NixOS VM image
+    BuildVm(OsBuildVmArgs),
+}
+
+#[derive(Debug, Args)]
+pub struct OsBuildVmArgs {
+    #[command(flatten)]
+    pub common: OsRebuildArgs,
+
+    /// Build with bootloader. Bootloader is bypassed by default.
+    #[arg(long, short = 'B')]
+    pub with_bootloader: bool,
 }
 
 #[derive(Debug, Args)]

src/nixos.rs

@@ -10,7 +10,7 @@ use crate::commands::Command;
 use crate::generations;
 use crate::installable::Installable;
 use crate::interface::OsSubcommand::{self};
-use crate::interface::{self, OsGenerationsArgs, OsRebuildArgs, OsReplArgs};
+use crate::interface::{self, OsGenerationsArgs, OsBuildVmArgs, OsRebuildArgs, OsReplArgs};
 use crate::update::update;
 
 const SYSTEM_PROFILE: &str = "/nix/var/nix/profiles/system";
@@ -22,10 +22,11 @@ impl interface::OsArgs {
     pub fn run(self) -> Result<()> {
         use OsRebuildVariant::*;
         match self.subcommand {
-            OsSubcommand::Boot(args) => args.rebuild(Boot),
-            OsSubcommand::Test(args) => args.rebuild(Test),
-            OsSubcommand::Switch(args) => args.rebuild(Switch),
-            OsSubcommand::Build(args) => args.rebuild(Build),
+            OsSubcommand::Boot(args) => args.rebuild(Boot, None),
+            OsSubcommand::Test(args) => args.rebuild(Test, None),
+            OsSubcommand::Switch(args) => args.rebuild(Switch, None),
+            OsSubcommand::Build(args) => args.rebuild(Build, None),
+            OsSubcommand::BuildVm(args) => args.build(BuildVm),
             OsSubcommand::Repl(args) => args.run(),
             OsSubcommand::Info(args) => args.info(),
         }
@@ -38,10 +39,22 @@ enum OsRebuildVariant {
     Switch,
     Boot,
     Test,
+    BuildVm,
+}
+
+impl OsBuildVmArgs {
+    fn build(self, variant: OsRebuildVariant) -> Result<()> {
+        use OsRebuildVariant::*;
+
+        let final_attr = Some(get_final_attr(true, self.with_bootloader));
+
+        self.common.rebuild(variant, final_attr)
+    }
 }
 
 impl OsRebuildArgs {
-    fn rebuild(self, variant: OsRebuildVariant) -> Result<()> {
+    // final_attr is the attribute of config.system.build.X to evaluate.
+    fn rebuild(self, variant: OsRebuildVariant, final_attr: Option<String>) -> Result<()> {
         use OsRebuildVariant::*;
 
         let elevate = if self.bypass_root_check {
@@ -77,13 +90,22 @@ impl OsRebuildArgs {
 
         debug!(?out_path);
 
-        let toplevel = toplevel_for(hostname, self.common.installable.clone());
+        let toplevel = toplevel_for(
+            hostname,
+            self.common.installable.clone(),
+            final_attr.unwrap_or(String::from("toplevel"))
+        );
+
+        let message = match variant {
+            BuildVm => "Building NixOS VM image",
+            _ => "Building NixOS configuration",
+        };
 
         commands::Build::new(toplevel)
             .extra_arg("--out-link")
             .extra_arg(out_path.get_path())
             .extra_args(&self.extra_args)
-            .message("Building NixOS configuration")
+            .message(message)
             .nom(!self.common.no_nom)
             .run()?;
 
@@ -165,11 +187,26 @@ impl OsRebuildArgs {
     }
 }
 
-pub fn toplevel_for<S: AsRef<str>>(hostname: S, installable: Installable) -> Installable {
+pub fn get_final_attr(build_vm: bool, with_bootloader: bool) -> String {
+    let attr = if build_vm && with_bootloader {
+        "vmWithBootLoader"
+    } else if build_vm {
+        "vm"
+    } else {
+        "toplevel"
+    };
+    String::from(attr)
+}
+
+pub fn toplevel_for<S: AsRef<str>>(
+    hostname: S,
+    installable: Installable,
+    final_attr: String,
+) -> Installable {
     let mut res = installable.clone();
     let hostname = hostname.as_ref().to_owned();
 
-    let toplevel = ["config", "system", "build", "toplevel"]
+    let toplevel = ["config", "system", "build", &final_attr]
         .into_iter()
         .map(String::from);