Require that a <_ as Try>::Residual implement Residual

The `Residual` trait was even more experimental than `Try`, but now that RFC3721 is merged, I think it would make sense to require this.

Author: scottmcm

tests/ui/manual_try_fold.rs

@@ -2,24 +2,31 @@
 #![allow(clippy::unnecessary_fold, unused)]
 #![warn(clippy::manual_try_fold)]
 #![feature(try_trait_v2)]
+#![feature(try_trait_v2_residual)]
 //@no-rustfix
-use std::ops::{ControlFlow, FromResidual, Try};
+use std::ops::{ControlFlow, FromResidual, Residual, Try};
 
 #[macro_use]
 extern crate proc_macros;
 
 // Test custom `Try` with more than 1 argument
 struct NotOption(i32, i32);
 
+struct NotOptionResidual;
+
 impl<R> FromResidual<R> for NotOption {
     fn from_residual(_: R) -> Self {
         todo!()
     }
 }
 
+impl Residual<()> for NotOptionResidual {
+    type TryType = NotOption;
+}
+
 impl Try for NotOption {
     type Output = ();
-    type Residual = ();
+    type Residual = NotOptionResidual;
 
     fn from_output(_: Self::Output) -> Self {
         todo!()
@@ -34,15 +41,21 @@ impl Try for NotOption {
 #[derive(Default)]
 struct NotOptionButWorse(i32);
 
+struct NotOptionButWorseResidual;
+
 impl<R> FromResidual<R> for NotOptionButWorse {
     fn from_residual(_: R) -> Self {
         todo!()
     }
 }
 
+impl Residual<()> for NotOptionButWorseResidual {
+    type TryType = NotOptionButWorse;
+}
+
 impl Try for NotOptionButWorse {
     type Output = ();
-    type Residual = ();
+    type Residual = NotOptionButWorseResidual;
 
     fn from_output(_: Self::Output) -> Self {
         todo!()

tests/ui/manual_try_fold.stderr

@@ -1,5 +1,5 @@
 error: usage of `Iterator::fold` on a type that implements `Try`
-  --> tests/ui/manual_try_fold.rs:59:10
+  --> tests/ui/manual_try_fold.rs:72:10
    |
 LL |         .fold(Some(0i32), |sum, i| sum?.checked_add(*i))
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `try_fold` instead: `try_fold(0i32, |sum, i| ...)`
@@ -8,19 +8,19 @@ LL |         .fold(Some(0i32), |sum, i| sum?.checked_add(*i))
    = help: to override `-D warnings` add `#[allow(clippy::manual_try_fold)]`
 
 error: usage of `Iterator::fold` on a type that implements `Try`
-  --> tests/ui/manual_try_fold.rs:64:10
+  --> tests/ui/manual_try_fold.rs:77:10
    |
 LL |         .fold(NotOption(0i32, 0i32), |sum, i| NotOption(0i32, 0i32));
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `try_fold` instead: `try_fold(..., |sum, i| ...)`
 
 error: usage of `Iterator::fold` on a type that implements `Try`
-  --> tests/ui/manual_try_fold.rs:68:10
+  --> tests/ui/manual_try_fold.rs:81:10
    |
 LL |         .fold(NotOptionButWorse(0i32), |sum, i| NotOptionButWorse(0i32));
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `try_fold` instead: `try_fold(0i32, |sum, i| ...)`
 
 error: usage of `Iterator::fold` on a type that implements `Try`
-  --> tests/ui/manual_try_fold.rs:99:10
+  --> tests/ui/manual_try_fold.rs:112:10
    |
 LL |         .fold(Some(0i32), |sum, i| sum?.checked_add(*i))
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `try_fold` instead: `try_fold(0i32, |sum, i| ...)`