truncate: fix error message for file not found

Change the error message for when the reference file (the `-r` argument)
is not found to match GNU coreutils. This commit also eliminates a
redundant call to `File::open`; the file need not be opened because the
size in bytes can be read from the result of `std::fs::metadata()`.

src/uu/truncate/src/truncate.rs

@@ -11,7 +11,8 @@
 extern crate uucore;
 
 use clap::{App, Arg};
-use std::fs::{metadata, File, OpenOptions};
+use std::fs::{metadata, OpenOptions};
+use std::io::ErrorKind;
 use std::path::Path;
 
 #[derive(Eq, PartialEq)]
@@ -174,13 +175,14 @@ fn truncate(
                 TruncateMode::Reduce => (),
                 _ => crash!(1, "you must specify a relative ‘--size’ with ‘--reference’"),
             };
-            let _ = match File::open(Path::new(rfilename)) {
-                Ok(m) => m,
-                Err(f) => crash!(1, "{}", f.to_string()),
-            };
             match metadata(rfilename) {
                 Ok(meta) => meta.len(),
-                Err(f) => crash!(1, "{}", f.to_string()),
+                Err(f) => match f.kind() {
+                    ErrorKind::NotFound => {
+                        crash!(1, "cannot stat '{}': No such file or directory", rfilename)
+                    }
+                    _ => crash!(1, "{}", f.to_string()),
+                },
             }
         }
         None => 0,

tests/by-util/test_truncate.rs

@@ -245,3 +245,11 @@ fn test_invalid_numbers() {
     new_ucmd!().args(&["-s", "0XB", "file"]).fails().stderr_contains("Invalid number: ‘0XB’");
     new_ucmd!().args(&["-s", "0B", "file"]).fails().stderr_contains("Invalid number: ‘0B’");
 }
+
+#[test]
+fn test_reference_file_not_found() {
+    new_ucmd!()
+        .args(&["-r", "a", "b"])
+        .fails()
+        .stderr_contains("cannot stat 'a': No such file or directory");
+}