How to create an in-memory object that can be used as a Reader, Writer, or Seek in Rust?

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In fact there is a way: Cursor<T>!
(please also read Shepmaster’s answer on why often it’s even easier)

In the documentation you can see that there are the following impls:

impl<T> Seek for Cursor<T> where T: AsRef<[u8]>
impl<T> Read for Cursor<T> where T: AsRef<[u8]>
impl Write for Cursor<Vec<u8>>
impl<T> AsRef<[T]> for Vec<T>

From this you can see that you can use the type Cursor<Vec<u8>> just as an ordinary file, because Read, Write and Seek are implemented for that type!

Little example (Playground):

use std::io::{Cursor, Read, Seek, SeekFrom, Write};

// Create fake "file"
let mut c = Cursor::new(Vec::new());

// Write into the "file" and seek to the beginning
c.write_all(&[1, 2, 3, 4, 5]).unwrap();
c.seek(SeekFrom::Start(0)).unwrap();

// Read the "file's" contents into a vector
let mut out = Vec::new();
c.read_to_end(&mut out).unwrap();

println!("{:?}", out);

For a more useful example, check the documentation linked above.

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