Understanding the Lifetime Mismatch

In Rust, lifetime elision rules assign an implicit lifetime to parameters like &mut self. When you write a method signature like fn implicit_second_lifetime(&mut self) -> Ref<'a> inside an impl<'a> MutRef<'a> block, Rust expands &mut self to use a separate, anonymous lifetime. The signature effectively becomes:

fn implicit_second_lifetime<'b>(&'b mut self) -> Ref<'a>

Here, lifetime 'b represents the duration of the mutable borrow of self during this method call, while 'a is the lifetime of the slice stored inside MutRef.

Why Re-Borrowing Binds to the Outer Lifetime

When you access self.mut_slice inside implicit_second_lifetime, you are re-borrowing data through the reference &'b mut self. Rust's borrow checker enforces a fundamental safety rule: you cannot dereference a pointer with lifetime 'b to extract a reference with a longer lifetime 'a (since 'a outlives 'b).

Because you only hold permission to access self for the duration of 'b, any slice or reference derived by dereferencing self is constrained by 'b. Trying to cast or coerce this derived slice into Ref<'a> violates safety because self might be moved, dropped, or aliased after lifetime 'b ends.

The Trait Trap: Why Standard Iterator Doesn't Work

The standard Rust Iterator trait definition looks like this:

pub trait Iterator {
    type Item;
    fn next(&mut self) -> Option<Self::Item>;
}

Notice that next takes &mut self with an implicit lifetime tied only to that specific method call. If Self::Item contains a reference with lifetime 'a, that reference cannot be tied to the temporary borrow of &mut self. This is why attempting to write an iterator that yields slices borrowed from a buffer held inside self fails with standard iterators.

Solutions and Modern Approaches

1. Use Generic Associated Types (GATs) / Streaming Iterators

If you need an iterator where returned items borrow from self on each step, you need a Lending Iterator (also known as a Streaming Iterator). With Rust's support for Generic Associated Types (GATs), you can define a trait where the item's lifetime is tied to &mut self:

pub trait LendingIterator {
    type Item<'a> where Self: 'a;
    fn next<'a>(&'a mut self) -> Option<Self::Item<'a>>;
}

2. Splitting Borrows or Returning Owned Data

If GATs add too much complexity, alternative strategies include:

  • Separate Buffer from Iterator: Pass the mutable buffer into a method directly rather than storing it inside the struct that acts as the iterator.
  • Return Indices or Ranges: Instead of returning slices like Ref<'a>, return std::ops::Range<usize> indices. The caller can then slice the original buffer independently.
  • Use &'a mut self Explicitly: As shown in your working example, declaring fn explicit_single_lifetime(&'a mut self) works, but keep in mind this mutably borrows self for its entire remaining lifetime, rendering self unusable afterward.