automatically update the lockfile and support --locked

There is a problem because cargo2nix is not using the cargo binary here, which
could result in the lockfile being different than the project's lock file.

We want the behavior that cargo implements privately, to see that the lockfile
is out of date and to ask if the user wants to update it.

Cargo2nix could offload the work via metadata and json parsing, which may make
more sense as a cargo plugin.

Cargo2nix will be limited in its capability to work seemlessly with newer or
older projects if they generate lockfiles that clash with the output of
cargo2nix.  The use of the cargo library further cements the potential for
disparity.

src/main.rs

@@ -12,13 +12,14 @@ use cargo::{
     core::{
         compiler::{CompileKind, RustcTargetData},
         dependency::DepKind,
+        registry::PackageRegistry,
         resolver::{
             features::{ForceAllTargets, HasDevUnits},
             CliFeatures, Resolve,
         },
         Package, PackageId, PackageIdSpec, Workspace,
     },
-    ops::{resolve_ws_with_opts, Packages},
+    ops::{resolve_with_previous, resolve_ws_with_opts, Packages},
     util::important_paths::find_root_manifest_for_wd,
 };
 use cargo_platform::Platform;
@@ -59,6 +60,9 @@ struct Opt {
     /// Overwrite existing output filepath without prompting
     #[arg(action, long, short)]
     overwrite: bool,
+    /// Don't attempt to update the lockfile
+    #[arg(action, long, short, value_name = "LOCKED")]
+    locked: bool,
     /// Output to stdout
     #[arg(conflicts_with = "file", action, long, short)]
     stdout: bool,
@@ -77,7 +81,8 @@ fn main() -> std::io::Result<()> {
     let workspace_directory = opt.workspace_directory.unwrap_or(std::env::current_dir()?).canonicalize()?;
     let file = opt.file.unwrap_or(PathBuf::from("./Cargo.nix"));
 
-    let rendered = generate_cargo_nix(&workspace_directory).expect("Error generating nix expressions");
+    let rendered = generate_cargo_nix(&workspace_directory, opt.locked)
+        .expect("Error generating nix expressions");
 
     if opt.stdout { write_to_stdout(&rendered).expect("Error writing to stdout"); }
     else { write_to_file(&file, &rendered, &opt.overwrite).expect("Error writing to file"); }
@@ -187,10 +192,10 @@ fn write_to_stdout(rendered: &str) -> Result<()> {
     Ok(())
 }
 
-fn generate_cargo_nix(workspace_directory: &PathBuf) -> Result<String> {
+fn generate_cargo_nix(workspace_directory: &PathBuf, locked: bool) -> Result<String> {
     let config = {
         let mut config = cargo::Config::default()?;
-        config.configure(0, true, None, false, true, false, &None, &[], &[])?;
+        config.configure(0, false, None, false, locked, false, &None, &[], &[])?;
         config
     };
 
@@ -204,6 +209,21 @@ fn generate_cargo_nix(workspace_directory: &PathBuf) -> Result<String> {
     let cargo_lock_hash: String = format!("{:x}", hasher.finalize());
     let ws = Workspace::new(&root_manifest_path, &config)?;
 
+    if !locked {
+        let mut registry = PackageRegistry::new(ws.config())?;
+        let mut resolve = resolve_with_previous(
+            &mut registry,
+            &ws,
+            &CliFeatures::new_all(true),
+            HasDevUnits::Yes,
+            None,
+            None,
+            &[],
+            true,
+        )?;
+        cargo::ops::write_pkg_lockfile(&ws, &mut resolve)?;
+    }
+
     // To get a list of all packages with all features and dependencies that
     // might be enabled present, we first resolve with all features turned on
     let requested_kinds = CompileKind::from_requested_targets(ws.config(), &[])?;