refactor(lib): fix some unused warnings (#3657)

src/body/incoming.rs

@@ -317,6 +317,13 @@ impl Body for Incoming {
 
 impl fmt::Debug for Incoming {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        #[cfg(any(
+            all(
+                any(feature = "http1", feature = "http2"),
+                any(feature = "client", feature = "server")
+            ),
+            feature = "ffi"
+        ))]
         #[derive(Debug)]
         struct Streaming;
         #[derive(Debug)]

src/body/mod.rs

@@ -35,6 +35,10 @@ pub(crate) use self::incoming::Sender;
 pub(crate) use self::length::DecodedLength;
 
 mod incoming;
+#[cfg(all(
+    any(feature = "http1", feature = "http2"),
+    any(feature = "client", feature = "server")
+))]
 mod length;
 
 fn _assert_send_sync() {

src/error.rs

@@ -98,6 +98,7 @@ pub(super) enum Parse {
     #[cfg(feature = "http1")]
     Header(Header),
     #[cfg(any(feature = "http1", feature = "http2"))]
+    #[cfg_attr(feature = "http2", allow(unused))]
     TooLarge,
     Status,
     #[cfg(all(any(feature = "client", feature = "server"), feature = "http1"))]

src/proto/h1/role.rs

@@ -1,7 +1,7 @@
-use std::{fmt, mem::MaybeUninit};
+use std::mem::MaybeUninit;
 
 #[cfg(feature = "client")]
-use std::fmt::Write;
+use std::fmt::{self, Write as _};
 #[cfg(feature = "server")]
 use std::time::Instant;
 
@@ -1605,8 +1605,10 @@ fn write_headers_original_case(
     }
 }
 
+#[cfg(feature = "client")]
 struct FastWrite<'a>(&'a mut Vec<u8>);
 
+#[cfg(feature = "client")]
 impl<'a> fmt::Write for FastWrite<'a> {
     #[inline]
     fn write_str(&mut self, s: &str) -> fmt::Result {