to use it to find the extension in a file name.
```rust
-# fn find(_: &str, _: char) -> Option<usize> { None }
+# fn find(haystack: &str, needle: char) -> Option<usize> { haystack.find(needle) }
fn main() {
let file_name = "foobar.rs";
match find(file_name, '.') {
sense to put it into a function:
```rust
-# fn find(_: &str, _: char) -> Option<usize> { None }
+# fn find(haystack: &str, needle: char) -> Option<usize> { haystack.find(needle) }
// Returns the extension of the given file name, where the extension is defined
// as all characters following the first `.`.
// If `file_name` has no `.`, then `None` is returned.
to get rid of the case analysis:
```rust
-# fn find(_: &str, _: char) -> Option<usize> { None }
+# fn find(haystack: &str, needle: char) -> Option<usize> { haystack.find(needle) }
// Returns the extension of the given file name, where the extension is defined
// as all characters following the first `.`.
// If `file_name` has no `.`, then `None` is returned.