Rust Analyzer: Next few years(rust-analyzer.github.io)

    fn foo() -> Iterator {
        vec![1, 2, 3].into_iter()
    }

    warning: trait objects without an explicit `dyn` are deprecated
     --> src/main.rs:1:13
      |
    1 | fn foo() -> Iterator {
      |             ^^^^^^^^ help: use `dyn`: `dyn Iterator`
      |
      = note: `#[warn(bare_trait_objects)]` on by default
    
    error[E0191]: the value of the associated type `Item` (from trait `std::iter::Iterator`) must be specified
     --> src/main.rs:1:13
      |
    1 | fn foo() -> Iterator {
      |             ^^^^^^^^ help: specify the associated type: `Iterator<Item = Type>`

    fn foo() -> dyn Iterator<Item = i32> {
        vec![1, 2, 3].into_iter()
    }

    error[E0746]: return type cannot have an unboxed trait object
     --> src/main.rs:1:13
      |
    1 | fn foo() -> dyn Iterator<Item = usize> {
      |             ^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
      |
      = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
    help: use `impl Iterator<Item = usize>` as the return type, as all return paths are of type `std::vec::IntoIter<{integer}>`, which implements `Iterator<Item = usize>`
      |
    1 | fn foo() -> impl Iterator<Item = usize> {
      |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^

    error[E0746]: return type cannot have an unboxed trait object
     --> src/main.rs:1:13
      |
    1 | fn foo() -> dyn Iterator<Item = usize> {
      |             ^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
      |
      = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
      = note: if all the returned values were of the same type you could use `impl Iterator<Item = usize>` as the return type
      = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
      = note: you can create a new `enum` with a variant for each returned type
    help: return a boxed trait object instead
      |
    1 | fn foo() -> Box<dyn Iterator<Item = usize>> {
    2 |     if true {
    3 |         Box::new(vec![1, 2, 3].into_iter())
    4 |     } else {
    5 |         Box::new(None.into_iter())