tracer: update public API for smoltcp::phy::Tracer to allow custom inspection and printing of packet

Currently `smoltcp::phy::tracer::Packet` is not exported as public type, which leaves very limited
options for using `Tracer` device: traced packets only can be directly printed "as-is" and only in
limited context (inside the closure).

I'm updating API of `Tracer` device to provide access to internal fields of traced packet; this will
enable implementation of custom printing or packet inspection functions

I renamed `smoltcp::phy::tracer::Packet` to `smoltcp::phy::tracer::TracerPacket`, following prefixing
convention used by types declared in `smoltcp::phy::pcap_writer`

Author: Artem Kryvokrysenko

src/phy/mod.rs

@@ -125,7 +125,7 @@ pub use self::loopback::Loopback;
 pub use self::pcap_writer::{PcapLinkType, PcapMode, PcapSink, PcapWriter};
 #[cfg(all(feature = "phy-raw_socket", unix))]
 pub use self::raw_socket::RawSocket;
-pub use self::tracer::Tracer;
+pub use self::tracer::{Tracer, TracerDirection, TracerPacket};
 #[cfg(all(
     feature = "phy-tuntap_interface",
     any(target_os = "linux", target_os = "android")

src/phy/tracer.rs

@@ -11,12 +11,11 @@ use crate::wire::pretty_print::{PrettyIndent, PrettyPrint};
 /// device.
 pub struct Tracer<D: Device> {
     inner: D,
-    writer: fn(Instant, Packet),
+    writer: fn(Instant, TracerPacket),
 }
 
 impl<D: Device> Tracer<D> {
-    /// Create a tracer device.
-    pub fn new(inner: D, writer: fn(timestamp: Instant, packet: Packet)) -> Tracer<D> {
+    pub fn new(inner: D, writer: fn(timestamp: Instant, packet: TracerPacket)) -> Tracer<D> {
         Tracer { inner, writer }
     }
 
@@ -88,7 +87,7 @@ impl<D: Device> Device for Tracer<D> {
 #[doc(hidden)]
 pub struct RxToken<Rx: phy::RxToken> {
     token: Rx,
-    writer: fn(Instant, Packet),
+    writer: fn(Instant, TracerPacket),
     medium: Medium,
     timestamp: Instant,
 }
@@ -101,10 +100,10 @@ impl<Rx: phy::RxToken> phy::RxToken for RxToken<Rx> {
         self.token.consume(|buffer| {
             (self.writer)(
                 self.timestamp,
-                Packet {
+                TracerPacket {
                     buffer,
                     medium: self.medium,
-                    prefix: "<- ",
+                    direction: TracerDirection::RX,
                 },
             );
             f(buffer)
@@ -119,7 +118,7 @@ impl<Rx: phy::RxToken> phy::RxToken for RxToken<Rx> {
 #[doc(hidden)]
 pub struct TxToken<Tx: phy::TxToken> {
     token: Tx,
-    writer: fn(Instant, Packet),
+    writer: fn(Instant, TracerPacket),
     medium: Medium,
     timestamp: Instant,
 }
@@ -133,10 +132,10 @@ impl<Tx: phy::TxToken> phy::TxToken for TxToken<Tx> {
             let result = f(buffer);
             (self.writer)(
                 self.timestamp,
-                Packet {
+                TracerPacket {
                     buffer,
                     medium: self.medium,
-                    prefix: "-> ",
+                    direction: TracerDirection::TX,
                 },
             );
             result
@@ -148,15 +147,56 @@ impl<Tx: phy::TxToken> phy::TxToken for TxToken<Tx> {
     }
 }
 
-pub struct Packet<'a> {
+/// Packet which is being traced by [Tracer](struct.Tracer.html) device.
+pub struct TracerPacket<'a> {
     buffer: &'a [u8],
     medium: Medium,
-    prefix: &'static str,
+    direction: TracerDirection,
 }
 
-impl<'a> fmt::Display for Packet<'a> {
+impl<'a> TracerPacket<'a> {
+    /// Creates new [TracerPacket](struct.TracerPacket.html)
+    pub fn new(buffer: &'a [u8], medium: Medium, direction: TracerDirection) -> Self {
+        Self {
+            buffer,
+            medium,
+            direction,
+        }
+    }
+
+    /// Returns packet buffer
+    pub fn buffer(&'a self) -> &'a [u8] {
+        self.buffer
+    }
+
+    /// Returns packet medium
+    pub fn medium(&self) -> Medium {
+        self.medium
+    }
+
+    /// Returns direction in which packet being traced
+    pub fn direction(&self) -> TracerDirection {
+        self.direction
+    }
+}
+
+/// Direction on which packet is being traced
+#[derive(Debug, Clone, Copy)]
+pub enum TracerDirection {
+    /// Packet is received by Smoltcp interface
+    RX,
+    /// Packet is transmitted by Smoltcp interface
+    TX,
+}
+
+impl<'a> fmt::Display for TracerPacket<'a> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        let mut indent = PrettyIndent::new(self.prefix);
+        let prefix = match self.direction {
+            TracerDirection::RX => "<- ",
+            TracerDirection::TX => "-> ",
+        };
+
+        let mut indent = PrettyIndent::new(prefix);
         match self.medium {
             #[cfg(feature = "medium-ethernet")]
             Medium::Ethernet => crate::wire::EthernetFrame::<&'static [u8]>::pretty_print(