diff: compare DIR/FILE with FILE when stdin is a directory

src/diff.rs

@@ -19,10 +19,16 @@ use std::process::{exit, ExitCode};
 //     1 means some differences were found,
 //     and 2 means trouble.
 pub fn main(opts: Peekable<ArgsOs>) -> ExitCode {
-    let params = parse_params(opts).unwrap_or_else(|error| {
+    let mut params = parse_params(opts).unwrap_or_else(|error| {
         eprintln!("{error}");
         exit(2);
     });
+    // diff - FILE with a directory on stdin => diff DIRECTORY/FILE FILE
+    if params.from == "-" && params.to != "-" {
+        if let Some(from) = stdin_directory_entry(&params.to) {
+            params.from = from;
+        }
+    }
     // if from and to are the same file, no need to perform any comparison
     let maybe_report_identical_files = || {
         if params.report_identical_files {
@@ -140,3 +146,34 @@ fn print_files_differ(kind: &str, params: &Params) {
         params.to.to_string_lossy()
     );
 }
+
+/// When standard input is a directory, returns the path of the entry in it
+/// that has the same name as `other`, provided `other` is not a directory.
+/// Entries are reached through `/dev/fd/0`, which only names the directory
+/// itself where it links to the open file, as with Linux procfs.
+#[cfg(any(target_os = "linux", target_os = "android"))]
+fn stdin_directory_entry(other: &OsString) -> Option<OsString> {
+    use std::os::fd::AsFd;
+    use std::path::Path;
+
+    let stdin_is_dir = io::stdin()
+        .as_fd()
+        .try_clone_to_owned()
+        .ok()
+        .and_then(|fd| fs::File::from(fd).metadata().ok())
+        .is_some_and(|metadata| metadata.is_dir());
+    let other = Path::new(other);
+    if !stdin_is_dir || other.is_dir() {
+        return None;
+    }
+    Some(
+        Path::new("/dev/fd/0")
+            .join(other.file_name()?)
+            .into_os_string(),
+    )
+}
+
+#[cfg(not(any(target_os = "linux", target_os = "android")))]
+fn stdin_directory_entry(_other: &OsString) -> Option<OsString> {
+    None
+}

tests/by-util/test_diff.rs

@@ -5,6 +5,8 @@
 
 use diffutilslib::assert_diff_eq;
 
+#[cfg(any(target_os = "linux", target_os = "android"))]
+use assert_cmd::assert::OutputAssertExt;
 use assert_cmd::cargo::cargo_bin_cmd;
 use predicates::prelude::*;
 use std::fs::File;
@@ -295,6 +297,31 @@ fn binary_files() -> Result<(), Box<dyn std::error::Error>> {
     Ok(())
 }
 
+#[cfg(any(target_os = "linux", target_os = "android"))]
+#[test]
+fn stdin_is_directory() -> Result<(), Box<dyn std::error::Error>> {
+    let tmp_dir = tempdir()?;
+    let dir = tmp_dir.path().join("dir");
+    std::fs::create_dir(&dir)?;
+    std::fs::write(dir.join("notes"), "alpha\n")?;
+    let notes = tmp_dir.path().join("notes");
+    std::fs::write(&notes, "beta\n")?;
+
+    // The directory itself must be standard input, not its contents.
+    let output = std::process::Command::new(assert_cmd::cargo::cargo_bin!("diffutils"))
+        .arg("diff")
+        .arg("-")
+        .arg(&notes)
+        .stdin(File::open(&dir)?)
+        .output()?;
+    output
+        .assert()
+        .code(predicate::eq(1))
+        .failure()
+        .stdout("1c1\n< alpha\n---\n> beta\n");
+    Ok(())
+}
+
 #[test]
 fn compare_file_to_directory() -> Result<(), Box<dyn std::error::Error>> {
     let tmp_dir = tempdir()?;