2y ago

std::any::Any for slices?

I recently ran into an issue where I wanted to use Any for slices. However, it only allows 'static types (based on what I read, this is because you get the same TypeId regardless of lifetimes).

I came up with this workaround which I think is safe:

 rust

    
use std::{
    any::{Any, TypeId},
    marker::PhantomData,
};

#[derive(Clone, Debug)]
pub struct AnySlice&lt;'a> {
    tid: TypeId,
    len: usize,
    ptr: *const (),
    marker: PhantomData&lt;&amp;'a ()>,
}

impl&lt;'a> AnySlice&lt;'a> {
    pub fn from_slice(s: &amp;'a [T]) -> Self {
        Self {
            len: s.len(),
            ptr: s.as_ptr() as *const (),
            tid: TypeId::of::(),
            marker: PhantomData,
        }
    }

    pub fn as_slice(&amp;self) -> Option&lt;&amp;'a [T]> {
        if TypeId::of::() != self.tid {
            return None;
        }
        Some(unsafe { std::slice::from_raw_parts(self.ptr as *const T, self.len) })
    }

    pub fn is(&amp;self) -> bool {
        TypeId::of::() == self.tid
    }
}

edit: Unfortunately it seems like Lemmy insists on mangling the code block. See the playground link below.

T: Any ensures T is also 'static. The lifetime is preserved with PhantomData. Here's a playground link with some simple tests and a mut version: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=3116a404c28317c46dbba6ed6824c8a9

It seems to pass Miri, including the mut version (which requires a bit more care to ensure there can only be one mutable reference). Any problems with doing this?