style(lib): Address cast-possible-truncation clippy findings.

Cargo.toml

@@ -115,7 +115,6 @@ restriction = { level = "warn", priority = -2 }
 
 arithmetic_side_effects = "allow" # TODO: consider
 as_conversions = "allow" # TODO: tricky
-cast_possible_truncation = "allow" # TODO: consider
 cast_precision_loss = "allow" # TODO: consider
 checked_conversions = "allow"
 else_if_without_else = "allow"

src/proto/h1/encode.rs

@@ -142,13 +142,18 @@ impl Encoder {
             }
             Kind::Length(remaining) => {
                 trace!("sized write, len = {}", len);
-                if len as u64 > *remaining {
-                    let limit = *remaining as usize;
-                    *remaining = 0;
-                    BufKind::Limited(msg.take(limit))
-                } else {
-                    *remaining -= len as u64;
-                    BufKind::Exact(msg)
+                match usize::try_from(*remaining) {
+                    // Holding more than is owed, so write only what is left.
+                    Ok(limit) if limit < len => {
+                        *remaining = 0;
+                        BufKind::Limited(msg.take(limit))
+                    }
+                    // Ok(_) => Owed at least what we hold, write all of it.
+                    // Err(_) => Owed more than `usize` can represent, write all of it.
+                    Ok(_) | Err(_) => {
+                        *remaining -= len as u64;
+                        BufKind::Exact(msg)
+                    }
                 }
             }
             #[cfg(feature = "server")]
@@ -241,6 +246,7 @@ impl Encoder {
                         dst.buffer(msg);
                         !self.is_last
                     }
+                    #[allow(clippy::cast_possible_truncation, reason="usize::MAX > len > remaining, cast truncation is impossible")]
                     Ordering::Greater => {
                         dst.buffer(msg.take(remaining as usize));
                         !self.is_last
@@ -328,11 +334,7 @@ where
     }
 }
 
-#[cfg(target_pointer_width = "32")]
-const USIZE_BYTES: usize = 4;
-
-#[cfg(target_pointer_width = "64")]
-const USIZE_BYTES: usize = 8;
+const USIZE_BYTES: usize = std::mem::size_of::<usize>();
 
 // each byte will become 2 hex
 const CHUNK_SIZE_MAX_BYTES: usize = USIZE_BYTES * 2;
@@ -369,6 +371,7 @@ impl Buf for ChunkSize {
     }
 
     #[inline]
+    #[allow(clippy::cast_possible_truncation)]
     fn advance(&mut self, cnt: usize) {
         assert!(cnt <= self.remaining());
         self.pos += cnt as u8; // just asserted cnt fits in u8
@@ -385,12 +388,14 @@ impl fmt::Debug for ChunkSize {
 }
 
 impl fmt::Write for ChunkSize {
+    #[allow(clippy::cast_possible_truncation, reason="bytes is structurally always less than u8::MAX")]
     fn write_str(&mut self, num: &str) -> fmt::Result {
         use std::io::Write;
         (&mut self.bytes[self.len.into()..])
             .write_all(num.as_bytes())
             .expect("&mut [u8].write() cannot error");
-        self.len += num.len() as u8; // safe because bytes is never bigger than 256
+        debug_assert!(u8::try_from(num.len()).is_ok());
+        self.len += num.len() as u8; // safe because bytes is never bigger than 255
         Ok(())
     }
 }

src/proto/h2/ping.rs

@@ -361,12 +361,12 @@ impl Shared {
 // ===== impl Bdp =====
 
 /// Any higher than this likely will be hitting the TCP flow control.
-const BDP_LIMIT: usize = 1024 * 1024 * 16;
+const BDP_LIMIT: WindowSize = 1024 * 1024 * 16;
 
 impl Bdp {
     fn calculate(&mut self, bytes: usize, rtt: Duration) -> Option<WindowSize> {
         // No need to do any math if we're at the limit.
-        if self.bdp as usize == BDP_LIMIT {
+        if self.bdp == BDP_LIMIT {
             self.stabilize_delay();
             return None;
         }
@@ -396,7 +396,8 @@ impl Bdp {
         // if the current `bytes` sample is at least 2/3 the previous
         // bdp, increase to double the current sample.
         if bytes >= self.bdp as usize * 2 / 3 {
-            self.bdp = (bytes * 2).min(BDP_LIMIT) as WindowSize;
+            self.bdp = WindowSize::try_from((bytes * 2).min(BDP_LIMIT as usize))
+                .unwrap_or(BDP_LIMIT);
             trace!("BDP increased to {}", self.bdp);
 
             self.stable_count = 0;