message: Clean up code and add comment
diff: @@ -438,17 +438,10 @@ impl PreopenDirBuilder {
  /// Make this preopened directory appear to the WASI program as `alias`
  pub fn alias(&mut self, alias: &str) -> &mut Self {
  - let alias = if let Some(first_byte) = alias.as_bytes().first() {
  - if *first_byte == b'/' {
  - &alias[1..]
  - } else {
  - alias
  - }
  - } else {
  - alias
  - }
  - .to_string();
  - self.alias = Some(alias);
  + // We mount at preopened dirs at `/` by default and multiple `/` in a row
  + // are equal to a single `/`.
  + let alias = alias.trim_start_matches('/');
  + self.alias = Some(alias.to_string());
  self
  }
  