date: support -f - to read from stdin

So far `date -f -` was not reading from stdin. This commit fixes
this.

Closes: #6058

src/uu/date/src/date.rs

@@ -91,6 +91,7 @@ enum DateSource {
     Now,
     Custom(String),
     File(PathBuf),
+    Stdin,
     Human(TimeDelta),
 }
 
@@ -173,7 +174,10 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
             DateSource::Custom(date.into())
         }
     } else if let Some(file) = matches.get_one::<String>(OPT_FILE) {
-        DateSource::File(file.into())
+        match file.as_ref() {
+            "-" => DateSource::Stdin,
+            _ => DateSource::File(file.into()),
+        }
     } else {
         DateSource::Now
     };
@@ -240,6 +244,11 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
                     }
                 }
             }
+            DateSource::Stdin => {
+                let lines = BufReader::new(std::io::stdin()).lines();
+                let iter = lines.map_while(Result::ok).map(parse_date);
+                Box::new(iter)
+            }
             DateSource::File(ref path) => {
                 if path.is_dir() {
                     return Err(USimpleError::new(

tests/by-util/test_date.rs

@@ -426,3 +426,21 @@ fn test_date_parse_from_format() {
         .arg("+%Y-%m-%d %H:%M:%S")
         .succeeds();
 }
+
+#[test]
+fn test_date_from_stdin() {
+    new_ucmd!()
+        .arg("-f")
+        .arg("-")
+        .pipe_in(
+            "2023-03-27 08:30:00\n\
+             2023-04-01 12:00:00\n\
+             2023-04-15 18:30:00\n",
+        )
+        .succeeds()
+        .stdout_is(
+            "Mon Mar 27 08:30:00 2023\n\
+             Sat Apr  1 12:00:00 2023\n\
+             Sat Apr 15 18:30:00 2023\n",
+        );
+}