rust - PathBuf::deref() 如何返回对临时对象的引用？

Question

我偶然发现this standard library code :

impl ops::Deref for PathBuf {
    type Target = Path;
    #[inline]
    fn deref(&self) -> &Path {
        Path::new(&self.inner)
    }
}

这怎么行？这似乎是在堆栈上创建一个临时对象，然后返回对其的引用。这不是明显的终身违规吗？

Answer 1

路径::new uses unsafe code to convert an &OsStr to a &Path :

pub fn new<S: AsRef<OsStr> + ?Sized>(s: &S) -> &Path {
    unsafe { &*(s.as_ref() as *const OsStr as *const Path) }
}

如果您阅读the internal documentation for Path :

// `Path::new` current implementation relies
// on `Path` being layout-compatible with `OsStr`.
// When attribute privacy is implemented, `Path` should be annotated as `#[repr(transparent)]`.
// Anyway, `Path` representation and layout are considered implementation detail, are
// not documented and must not be relied upon.
pub struct Path {
    inner: OsStr,
}

另请参阅:

• How to wrap a borrowed value in a newtype that is also a borrowed value?