Apply clippy to doctest

taiki-e/github-actions@96af4dc

```
error: unreachable `pub` item
   --> pin-project-internal/src/lib.rs:359:1
    |
359 | pub trait PinnedDrop {
    | ---^^^^^^^^^^^^^^^^^
    | |
    | help: consider restricting its visibility: `pub(crate)`
    |
    = help: or consider exporting it for use by other crates
    = note: requested on the command line with `-D unreachable-pub`

error: unsafe impl missing a safety comment
   --> pin-project-internal/src/lib.rs:328:1
    |
328 | unsafe impl<T: Unpin, U> UnsafeUnpin for Struct<T, U> {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: consider adding a safety comment on the preceding line
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#undocumented_unsafe_blocks
    = note: requested on the command line with `-D clippy::undocumented-unsafe-blocks`

error: unreachable `pub` item
   --> pin-project-internal/src/lib.rs:503:1
    |
503 | pub trait PinnedDrop {
    | ---^^^^^^^^^^^^^^^^^
    | |
    | help: consider restricting its visibility: `pub(crate)`
    |
    = help: or consider exporting it for use by other crates
    = note: requested on the command line with `-D unreachable-pub`

error: unreachable `pub` item
   --> pin-project-internal/src/lib.rs:555:1
    |
555 | pub trait PinnedDrop {
    | ---^^^^^^^^^^^^^^^^^
    | |
    | help: consider restricting its visibility: `pub(crate)`
    |
    = help: or consider exporting it for use by other crates
    = note: requested on the command line with `-D unreachable-pub`
```

Author: taiki-e

pin-project-internal/src/lib.rs

@@ -4,10 +4,12 @@
 
 #![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::needless_doctest_main)]
@@ -325,6 +327,7 @@ use proc_macro::TokenStream;
 ///     unpinned: U,
 /// }
 ///
+/// # #[allow(clippy::undocumented_unsafe_blocks)]
 /// unsafe impl<T: Unpin, U> UnsafeUnpin for Struct<T, U> {}
 /// ```
 ///
@@ -356,6 +359,7 @@ use proc_macro::TokenStream;
 ///
 /// ```
 /// # use std::pin::Pin;
+/// # #[allow(unreachable_pub)]
 /// pub trait PinnedDrop {
 ///     fn drop(self: Pin<&mut Self>);
 /// }
@@ -500,6 +504,7 @@ pub fn pin_project(args: TokenStream, input: TokenStream) -> TokenStream {
 ///
 /// ```
 /// # use std::pin::Pin;
+/// # #[allow(unreachable_pub)]
 /// pub trait PinnedDrop {
 ///     fn drop(self: Pin<&mut Self>);
 /// }
@@ -552,6 +557,7 @@ pub fn pin_project(args: TokenStream, input: TokenStream) -> TokenStream {
 ///
 /// ```
 /// # use std::pin::Pin;
+/// # #[allow(unreachable_pub)]
 /// pub trait PinnedDrop {
 ///     unsafe fn drop(self: Pin<&mut Self>);
 /// }

src/lib.rs

@@ -93,10 +93,12 @@ see [examples] directory for more examples and generated code.
 #![no_std]
 #![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,
+    ))
 ))]
 #![warn(
     // Lints that may help when writing public library.
@@ -170,6 +172,7 @@ pub use pin_project_internal::pinned_drop;
 ///     field_2: V,
 /// }
 ///
+/// # #[allow(clippy::undocumented_unsafe_blocks)]
 /// unsafe impl<K, V> UnsafeUnpin for Struct<K, V> where K: Unpin + Clone {}
 /// ```
 ///

tests/lint/Cargo.toml

@@ -6,6 +6,7 @@ publish = false
 
 [lib]
 path = "lib.rs"
+doctest = false
 
 [dependencies]
 pin-project = { path = "../.." }