How can I store a Chars iterator in the same struct as the String it is iterating on?

As thoroughly described in Why can’t I store a value and a reference to that value in the same struct?, in general you can’t do this because it truly is unsafe. When you move memory, you invalidate references. This is why a lot of people use Rust – to not have invalid references which lead to program crashes!

Let’s look at your code:

io::stdin().read_line(&mut self.input_buf)?;
self.input_pos = self.input_buf.chars();

Between these two lines, you’ve left self.input_pos in a bad state. If a panic occurs, then the destructor of the object has the opportunity to access invalid memory! Rust is protecting you from an issue that most people never think about.

---

As also described in that answer:

There is a special case where the lifetime tracking is overzealous:
when you have something placed on the heap. This occurs when you use a
Box<T>, for example. In this case, the structure that is moved
contains a pointer into the heap. The pointed-at value will remain
stable, but the address of the pointer itself will move. In practice,
this doesn’t matter, as you always follow the pointer.

Some crates provide ways of representing this case, but they require
that the base address never move. This rules out mutating vectors,
which may cause a reallocation and a move of the heap-allocated
values.

Remember that a String is just a vector of bytes with extra preconditions added.

Instead of using one of those crates, we can also roll our own solution, which means we (read you) get to accept all the responsibility for ensuring that we aren’t doing anything wrong.

The trick here is to ensure that the data inside the String never moves and no accidental references are taken.

use std::{mem, str::Chars};

/// I believe this struct to be safe because the String is
/// heap-allocated (stable address) and will never be modified
/// (stable address). `chars` will not outlive the struct, so
/// lying about the lifetime should be fine.
///
/// TODO: What about during destruction?
///       `Chars` shouldn't have a destructor...
struct OwningChars {
    _s: String,
    chars: Chars<'static>,
}

impl OwningChars {
    fn new(s: String) -> Self {
        let chars = unsafe { mem::transmute(s.chars()) };
        OwningChars { _s: s, chars }
    }
}

impl Iterator for OwningChars {
    type Item = char;
    fn next(&mut self) -> Option<Self::Item> {
        self.chars.next()
    }
}

You might even think about putting just this code into a module so that you can’t accidentally muck about with the innards.

---

Here’s the same code using the ouroboros crate to create a self-referential struct containing the String and a Chars iterator:

use ouroboros::self_referencing; // 0.4.1
use std::str::Chars;

#[self_referencing]
pub struct IntoChars {
    string: String,
    #[borrows(string)]
    chars: Chars<'this>,
}

// All these implementations are based on what `Chars` implements itself

impl Iterator for IntoChars {
    type Item = char;

    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        self.with_mut(|me| me.chars.next())
    }

    #[inline]
    fn count(mut self) -> usize {
        self.with_mut(|me| me.chars.count())
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.with(|me| me.chars.size_hint())
    }

    #[inline]
    fn last(mut self) -> Option<Self::Item> {
        self.with_mut(|me| me.chars.last())
    }
}

impl DoubleEndedIterator for IntoChars {
    #[inline]
    fn next_back(&mut self) -> Option<Self::Item> {
        self.with_mut(|me| me.chars.next_back())
    }
}

impl std::iter::FusedIterator for IntoChars {}

// And an extension trait for convenience

trait IntoCharsExt {
    fn into_chars(self) -> IntoChars;
}

impl IntoCharsExt for String {
    fn into_chars(self) -> IntoChars {
        IntoCharsBuilder {
            string: self,
            chars_builder: |s| s.chars(),
        }
        .build()
    }
}

---

Here’s the same code using the rental crate to create a self-referential struct containing the String and a Chars iterator:

#[macro_use]
extern crate rental; // 0.5.5

rental! {
    mod into_chars {
        pub use std::str::Chars;

        #[rental]
        pub struct IntoChars {
            string: String,
            chars: Chars<'string>,
        }
    }
}

use into_chars::IntoChars;

// All these implementations are based on what `Chars` implements itself

impl Iterator for IntoChars {
    type Item = char;

    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        self.rent_mut(|chars| chars.next())
    }

    #[inline]
    fn count(mut self) -> usize {
        self.rent_mut(|chars| chars.count())
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.rent(|chars| chars.size_hint())
    }

    #[inline]
    fn last(mut self) -> Option<Self::Item> {
        self.rent_mut(|chars| chars.last())
    }
}

impl DoubleEndedIterator for IntoChars {
    #[inline]
    fn next_back(&mut self) -> Option<Self::Item> {
        self.rent_mut(|chars| chars.next_back())
    }
}

impl std::iter::FusedIterator for IntoChars {}

// And an extension trait for convenience

trait IntoCharsExt {
    fn into_chars(self) -> IntoChars;
}

impl IntoCharsExt for String {
    fn into_chars(self) -> IntoChars {
        IntoChars::new(self, |s| s.chars())
    }
}