Apply clippy to doctest

taiki-e/github-actions@96af4dc

```
error: redundant closure
   --> src/lib.rs:150:49
    |
150 |             Box::new(self.child.iter().flat_map(|c| c.method())) as Box<dyn Iterator<Item = _>>
    |                                                 ^^^^^^^^^^^^^^ help: replace the closure with the method itself: `Type::method`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure_for_method_calls
    = note: `-D clippy::redundant-closure-for-method-calls` implied by `-D clippy::pedantic`
    = help: to override `-D clippy::pedantic` add `#[allow(clippy::redundant_closure_for_method_calls)]`

error: this loop never actually loops
   --> src/lib.rs:475:14
    |
475 |           1 => loop {
    |  ______________^
476 | |             panic!()
477 | |         },
    | |_________^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#never_loop
    = note: `-D clippy::never-loop` implied by `-D clippy::all`
    = help: to override `-D clippy::all` add `#[allow(clippy::never_loop)]`

error: this `.fold` can be written more succinctly using another method
   --> src/lib.rs:538:10
    |
538 |     iter.fold(0, |sum, x| sum + x)
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `sum()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_fold
    = note: `-D clippy::unnecessary-fold` implied by `-D clippy::all`
    = help: to override `-D clippy::all` add `#[allow(clippy::unnecessary_fold)]`
```

Author: taiki-e

src/lib.rs

@@ -142,6 +142,7 @@ struct Type {
 
 impl Type {
     #[auto_enum(Iterator)]
+#   #[allow(clippy::redundant_closure_for_method_calls)]
     fn method(&self) -> impl Iterator<Item = ()> + '_ {
         if self.child.is_empty() {
             Some(()).into_iter()
@@ -468,6 +469,7 @@ You can also skip that branch explicitly by `#[never]` attribute.
 use auto_enums::auto_enum;
 
 #[auto_enum(Iterator)]
+# #[allow(clippy::never_loop)]
 fn foo(x: i32) -> impl Iterator<Item = i32> {
     match x {
         0 => 1..10,
@@ -527,6 +529,7 @@ function is unnecessary.
 # #![feature(proc_macro_hygiene, stmt_expr_attributes)]
 use auto_enums::auto_enum;
 
+# #[allow(clippy::unnecessary_fold)]
 fn foo(x: i32) -> i32 {
     #[auto_enum(Iterator)]
     let iter = match x {
@@ -870,10 +873,12 @@ Please be careful if you return another traits with the same name.
 
 #![doc(test(
     no_crate_inject,
-    attr(
-        deny(warnings, rust_2018_idioms, single_use_lifetimes),
-        allow(dead_code, unused_variables)
-    )
+    attr(allow(
+        dead_code,
+        unused_variables,
+        clippy::undocumented_unsafe_blocks,
+        clippy::unused_trait_names,
+    ))
 ))]
 #![forbid(unsafe_code)]
 #![allow(clippy::doc_link_with_quotes)]