You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: error_impl_small.rs
    path of file: ./repos/failure/src/error
    
    the code of the file until where you have to start completion: use std::heap::{Heap, Alloc, Layout};

use core::mem;
use core::ptr;

use Fail;
use backtrace::Backtrace;

pub(crate) struct ErrorImpl {
    inner: &'static mut Inner,
}

// Dynamically sized inner value
struct Inner {
    backtrace: Backtrace,
    vtable: *const VTable,
    failure: FailData,
}

unsafe impl Send for Inner { }
unsafe impl Sync for Inner { }

extern {
    type VTable;
    type FailData;
}

#[allow(dead_code)]
struct 
    
    pub(crate) fn downcast<T: Fail>(self) -> Result<T, ErrorImpl> {
        let ret: Option<T> = self.failure().downcast_ref().map(|fail| {
            unsafe {
                // drop the backtrace
                let _ = ptr::read(&self.inner.backtrace as *const Backtrace);
                // read out the fail type
                ptr::read(fail as *const T)
            }
        });
        match ret {
            Some(ret) => {
                // forget self (backtrace is dropped, failure is moved
                mem::forget(self);
                Ok(ret)
            }
            _ => Err(self)
        }
    }