df: show error when file argument does not exist

For example:

    $ df not-a-file
    df: not-a-file: No such file or directory

Fixes #3373.

src/uu/df/src/df.rs

@@ -12,9 +12,9 @@ mod filesystem;
 mod table;
 
 use uucore::display::Quotable;
-use uucore::error::{UError, UResult};
-use uucore::format_usage;
+use uucore::error::{UError, UResult, USimpleError};
 use uucore::fsext::{read_fs_list, MountInfo};
+use uucore::{format_usage, show};
 
 use clap::{crate_version, Arg, ArgMatches, Command};
 
@@ -311,10 +311,17 @@ where
 
     // Convert each path into a `Filesystem`, which contains
     // both the mount information and usage information.
-    paths
-        .iter()
-        .filter_map(|p| Filesystem::from_path(&mounts, p))
-        .collect()
+    let mut result = vec![];
+    for path in paths {
+        match Filesystem::from_path(&mounts, path) {
+            Some(fs) => result.push(fs),
+            None => show!(USimpleError::new(
+                1,
+                format!("{}: No such file or directory", path.as_ref().display())
+            )),
+        }
+    }
+    result
 }
 
 #[derive(Debug)]
@@ -361,6 +368,12 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
         }
     };
 
+    // This can happen if paths are given as command-line arguments
+    // but none of the paths exist.
+    if filesystems.is_empty() {
+        return Ok(());
+    }
+
     // The running total of filesystem sizes and usage.
     //
     // This accumulator is computed in case we need to display the

tests/by-util/test_df.rs

@@ -297,3 +297,16 @@ fn test_output_field_no_more_than_once() {
         .fails()
         .usage_error("option --output: field 'target' used more than once");
 }
+
+#[test]
+fn test_nonexistent_file() {
+    new_ucmd!()
+        .arg("does-not-exist")
+        .fails()
+        .stderr_only("df: does-not-exist: No such file or directory");
+    new_ucmd!()
+        .args(&["--output=file", "does-not-exist", "."])
+        .fails()
+        .stderr_is("df: does-not-exist: No such file or directory\n")
+        .stdout_is("File            \n.               \n");
+}