Drop IntoResponse trait requirement from FromRequestParts and FromRequest Rejections

[MOZGIII]

• I have looked for existing issues (including closed) about this

Feature Request

Motivation

Optional extractors don't need the rejection to implement IntoResponse. Providing an extraction with a Rejection that doesn't implement IntoResponse will indicate this extractor must be used as optional.

This is a required feature for enforcing certain response shapes via the type system.

Proposal

Make the following changes (simplified):

pub trait FromRequestParts<S>: Sized {
-    type Rejection: IntoResponse;
+    type Rejection;

pub trait FromRequest<S, M = private::ViaRequest>: Sized { {
-    type Rejection: IntoResponse;
+    type Rejection;

Alternatives

[mladedav]

There may be another trait for optional extractors, see #2475. It would allow pretty much what you're proposing by using Infallible as the rejection.