Add open question about limiting impl Trait positions

Author: tmandry

text/0000-return-position-impl-trait-in-traits.md

@@ -649,6 +649,7 @@ There are a number of crates that do desugaring like this manually or with proce
 
 - Should we stabilize this feature together with `async fn` to mitigate hazards of writing a trait that is not forwards-compatible with its desugaring? (See [drawbacks].)
 - Resolution of [#112194: RPITIT is allowed to name any in-scope lifetime parameter, unlike inherent RPIT methods](https://github.com/rust-lang/rust/issues/112194)
+- Should we limit the legal positions for `impl Trait` to positions that are nameable using upcoming features like return-type notation (RTN)? (See [this comment](https://github.com/rust-lang/rfcs/pull/3425#pullrequestreview-1467880633) for an example.)
 
 # Future possibilities
 [future-possibilities]: #future-possibilities