Don't re-write an identical lockfile (#2386)

#1910 has a workaround for
this because changing the timestamp may invalidate the repository rule.

Instead, just don't do the write if the contents is identical.

crate_universe/src/cli/generate.rs

@@ -1,9 +1,10 @@
 //! The cli entrypoint for the `generate` subcommand
 
 use std::fs;
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 
 use anyhow::{bail, Context as AnyhowContext, Result};
+use cargo_lock::Lockfile;
 use clap::Parser;
 
 use crate::config::Config;
@@ -128,8 +129,21 @@ pub fn generate(opt: GenerateOptions) -> Result<()> {
         write_lockfile(lock_content, &lockfile, opt.dry_run)?;
     }
 
-    // Write the updated Cargo.lock file
-    fs::write(&opt.cargo_lockfile, cargo_lockfile.to_string())
+    update_cargo_lockfile(&opt.cargo_lockfile, cargo_lockfile)?;
+
+    Ok(())
+}
+
+fn update_cargo_lockfile(path: &Path, cargo_lockfile: Lockfile) -> Result<()> {
+    let old_contents = fs::read_to_string(path).ok();
+    let new_contents = cargo_lockfile.to_string();
+
+    // Don't overwrite identical contents because timestamp changes may invalidate repo rules.
+    if old_contents.as_ref() == Some(&new_contents) {
+        return Ok(());
+    }
+
+    fs::write(path, new_contents)
         .context("Failed to write Cargo.lock file back to the workspace.")?;
 
     Ok(())