feat: write diff status header

src/diff.rs

@@ -1,7 +1,4 @@
-use std::{
+use std::fmt;
-  fmt,
-  io,
-};
 
 use rustc_hash::{
   FxBuildHasher,
@@ -14,6 +11,8 @@ use crate::{
   Version,
 };
 
+const HEADER_STYLE: yansi::Style = yansi::Style::new().bold().underline();
+
 #[derive(Default)]
 struct Diff<T> {
   old: T,
@@ -42,10 +41,10 @@ impl fmt::Display for DiffStatus {
 }
 
 pub fn diff<'a>(
-  writer: &mut dyn io::Write,
+  writer: &mut dyn fmt::Write,
   paths_old: impl Iterator<Item = &'a StorePath>,
   paths_new: impl Iterator<Item = &'a StorePath>,
-) -> io::Result<()> {
+) -> fmt::Result {
   let mut paths =
     FxHashMap::<&str, Diff<Vec<Option<&Version>>>>::with_hasher(FxBuildHasher);
 
@@ -92,7 +91,16 @@ pub fn diff<'a>(
     a_status.cmp(&b_status).then_with(|| a_name.cmp(b_name))
   });
 
+  let mut last_status = None::<DiffStatus>;
+
   for (name, _versions, status) in diffs {
+    if last_status != Some(status) {
+      last_status = Some(status);
+      HEADER_STYLE.fmt_prefix(writer)?;
+      writeln!(writer, "{status:?} packages:")?;
+      HEADER_STYLE.fmt_suffix(writer)?;
+    }
+
     write!(writer, "{status} {name}")?;
     writeln!(writer)?;
   }

src/main.rs

@@ -1,5 +1,8 @@
 use std::{
-  fmt::Write as _,
+  fmt::{
+    self,
+    Write as _,
+  },
   io::{
     self,
     Write as _,
@@ -21,6 +24,14 @@ use dix::{
 };
 use yansi::Paint as _;
 
+struct WriteFmt<W: io::Write>(W);
+
+impl<W: io::Write> fmt::Write for WriteFmt<W> {
+  fn write_str(&mut self, string: &str) -> fmt::Result {
+    self.0.write_all(string.as_bytes()).map_err(|_| fmt::Error)
+  }
+}
+
 #[derive(clap::Parser, Debug)]
 #[command(version, about)]
 struct Cli {
@@ -92,7 +103,7 @@ fn real_main() -> Result<()> {
 
   drop(connection);
 
-  let mut out = io::stdout();
+  let mut out = WriteFmt(io::stdout());
 
   #[expect(clippy::pattern_type_mismatch)]
   diff(