Maintain command-line compatibility with Dustin's petname #2

This time it's the `--dir` option.

README.md

@@ -39,9 +39,14 @@ If you have [installed Cargo][install-cargo], you can install rust-petname with
 `cargo install petname`. This puts a `petname` binary in `~/.cargo/bin`, which
 the Cargo installation process will probably have added to your `PATH`.
 
-The `petname` binary from rust-petname is drop-in compatible with the original
-`petname`. It's more strict when validating arguments, but for most uses it
-should behave the same.
+The `petname` binary from rust-petname is _mostly_ drop-in compatible with the
+original `petname`. It has more options and it's stricter when validating
+arguments, but for most uses it should behave the same[^differences].
+
+[^differences]:
+ When using the `--dir` option, Dustin Kirkland's _petname_ looks for a
+ file named `names.txt` whereas this looks for `nouns.txt` first before
+ checking for `names.txt`.
 
 ```shellsession
 $ petname -h
@@ -181,6 +186,8 @@ command-line too. Below are the most important:
 - When using custom word lists with `--dir <DIR>`, nouns are now found in a file
  named appropriately `DIR/nouns.txt`. Previously this was `names.txt` but this
  was confusing; the term "names" is overloaded enough already.
+ - For compatibility, if `nouns.txt` is not found, an attempt will be made to
+ load nouns from `names.txt`.
 - The option `--count 0` is no longer a synonym for `--stream`. Use `--stream`
  instead. It's not an error to pass `--count 0`, but it will result in zero
  names being generated.

src/main.rs

@@ -150,7 +150,15 @@ impl Words {
  Ok(Self::Custom(
  read_file_to_string(dirname.join("adjectives.txt"))?,
  read_file_to_string(dirname.join("adverbs.txt"))?,
- read_file_to_string(dirname.join("nouns.txt"))?,
+ // Load `nouns.txt`, but fall back to trying `names.txt` for
+ // compatibility with Dustin Kirkland's _petname_.
+ match read_file_to_string(dirname.join("nouns.txt")) {
+ Ok(nouns) => nouns,
+ Err(err) => match read_file_to_string(dirname.join("names.txt")) {
+ Ok(nouns) => nouns,
+ Err(_) => Err(err)?, // Error from `nouns.txt`.
+ },
+ },
  ))
  }
 }