Is there a way to create a type alias of a std::slice::Iter?

user_5399309 0
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I'm trying to create a type alias of a Peekable slice::Iter, but the compiler keeps complaining that I need a lifetime parameter.

This iterator is used in several places, and always iterates over the same type. To make the code more concise, I would like to use a type alias.

I have already tried (code examples below):

  • Adding a lifetime parameter to the struct it is iterating over, and pass this as the argument to the generic, to no avail.
  • Adding a lifetime parameter inside the iterator
use std::{iter::Peekable, slice::Iter};

pub struct MyStruct {
    pub arg1: i32,
    pub arg2: i32,
    pub arg3: MyEnum,
}

pub enum MyEnum {
    Default,
}

// mutable since I want to call .peek() and .next() on my iterator
type MyIterator<'a> = &'a mut Peekable<Iter<MyStruct>>;

Notice that the above works just fine if I don't use a type alias, i.e. I use it directly in code:

pub fn my_fn(it: &mut Peekable<Iter<MyStruct>>) -> i32 { /* ... */ }

I keep getting the following error:

error[E0106]: missing lifetime specifier
  --> src/lib.rs:14:40
   |
14 | type MyIterator<'a> = &'a mut Peekable<Iter<MyStruct>>;
   |                                        ^^^^^^^^^^^^^^ expected lifetime parameter

I have also tried:

  • ... = &'a Peekable<Iter<&'a MyStruct>>;
  • ... = &'a Peekable<Iter<MyStruct+ 'a>>;

Neither work, even when adding a lifetime parameter to the MyStruct. I don't really understand how Rust wants me to define the lifetime parameter. (In general I don't fully understand this concept yet as I just started programming in Rust)

type-alias rust lifetime
1 Answers

I assume that Rust is just inferring the lifetime parameter for Iter in the function parameter context. But as E_net4 hinted at in their comment, the Iter you are probably using is defined as Iter<'a, T: 'a>. The correct type definition is:

type MyIterator<'a> = &'a mut Peekable<Iter<'a, MyStruct>>;
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