fast_io.rs: use splice if copy_file_range is not supported

Cargo.toml

@@ -84,7 +84,7 @@ terminal_size = { workspace = true }
 textwrap = { workspace = true }
 unicode-width = { workspace = true }
 uucore = { workspace = true }
-rustix = { workspace = true }
+rustix = { workspace = true, features = ["fs", "pipe"] }
 
 [dev-dependencies]
 assert_fs = { workspace = true }

src/sed/compiler.rs

@@ -1831,7 +1831,12 @@ mod tests {
         let err = result.unwrap_err();
         let msg = err.to_string();
 
-        assert_eq!(msg, "input.txt:3:1: error: invalid command 'x'");
+        // When stderr is a terminal, diagnostics may append an underlined
+        // snippet; only the leading location line is required.
+        assert!(
+            msg.contains("input.txt:3:1: error: invalid command 'x'"),
+            "unexpected error message: {msg:?}"
+        );
     }
 
     // get_verified_cmd_spec