io: Remove blanket trait impls

Remove the blanket impls of `Read`, `BufRead`, and `Write`. Replace them
with a set of sane impls.

Note, we add code to the `impl_write` macro to implement both
`crate::Write` and `std::io::Write` when "std" feature is
enabled.

Fix: #2432

io/src/lib.rs

@@ -151,24 +151,6 @@ impl<'a, R: BufRead + ?Sized> BufRead for Take<'a, R> {
     }
 }
 
-#[cfg(feature = "std")]
-impl<R: std::io::Read> Read for R {
-    #[inline]
-    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
-        Ok(<R as std::io::Read>::read(self, buf)?)
-    }
-}
-
-#[cfg(feature = "std")]
-impl<R: std::io::BufRead + Read + ?Sized> BufRead for R {
-    #[inline]
-    fn fill_buf(&mut self) -> Result<&[u8]> { Ok(std::io::BufRead::fill_buf(self)?) }
-
-    #[inline]
-    fn consume(&mut self, amount: usize) { std::io::BufRead::consume(self, amount) }
-}
-
-#[cfg(not(feature = "std"))] // Conflicts with blanket impl when "std" is enabled.
 impl Read for &[u8] {
     #[inline]
     fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
@@ -179,7 +161,12 @@ impl Read for &[u8] {
     }
 }
 
-#[cfg(not(feature = "std"))] // Conflicts with blanket impl when "std" is enabled.
+#[cfg(feature = "std")]
+impl<R: std::io::Read> Read for std::io::BufReader<R> {
+    #[inline]
+    fn read(&mut self, buf: &mut [u8]) -> Result<usize> { Ok(std::io::Read::read(self, buf)?) }
+}
+
 impl BufRead for &[u8] {
     #[inline]
     fn fill_buf(&mut self) -> Result<&[u8]> { Ok(self) }
@@ -189,6 +176,15 @@ impl BufRead for &[u8] {
     fn consume(&mut self, amount: usize) { *self = &self[amount..] }
 }
 
+#[cfg(feature = "std")]
+impl<R: std::io::Read> BufRead for std::io::BufReader<R> {
+    #[inline]
+    fn fill_buf(&mut self) -> Result<&[u8]> { Ok(std::io::BufRead::fill_buf(self)?) }
+
+    #[inline]
+    fn consume(&mut self, amount: usize) { std::io::BufRead::consume(self, amount) }
+}
+
 /// Wraps an in memory reader providing the `position` function.
 pub struct Cursor<T> {
     inner: T,
@@ -262,18 +258,7 @@ pub trait Write {
     }
 }
 
-#[cfg(feature = "std")]
+#[cfg(feature = "alloc")]
-impl<W: std::io::Write> Write for W {
-    #[inline]
-    fn write(&mut self, buf: &[u8]) -> Result<usize> {
-        Ok(<W as std::io::Write>::write(self, buf)?)
-    }
-
-    #[inline]
-    fn flush(&mut self) -> Result<()> { Ok(<W as std::io::Write>::flush(self)?) }
-}
-
-#[cfg(all(feature = "alloc", not(feature = "std")))]
 impl Write for alloc::vec::Vec<u8> {
     #[inline]
     fn write(&mut self, buf: &[u8]) -> Result<usize> {
@@ -285,7 +270,6 @@ impl Write for alloc::vec::Vec<u8> {
     fn flush(&mut self) -> Result<()> { Ok(()) }
 }
 
-#[cfg(not(feature = "std"))]
 impl<'a> Write for &'a mut [u8] {
     #[inline]
     fn write(&mut self, buf: &[u8]) -> Result<usize> {
@@ -299,12 +283,20 @@ impl<'a> Write for &'a mut [u8] {
     fn flush(&mut self) -> Result<()> { Ok(()) }
 }
 
+#[cfg(feature = "std")]
+impl<W: std::io::Write> Write for std::io::BufWriter<W> {
+    #[inline]
+    fn write(&mut self, buf: &[u8]) -> Result<usize> { Ok(std::io::Write::write(self, buf)?) }
+
+    #[inline]
+    fn flush(&mut self) -> Result<()> { Ok(std::io::Write::flush(self)?) }
+}
+
 /// A sink to which all writes succeed. See [`std::io::Sink`] for more info.
 ///
 /// Created using `io::sink()`.
 pub struct Sink;
 
-#[cfg(not(feature = "std"))]
 impl Write for Sink {
     #[inline]
     fn write(&mut self, buf: &[u8]) -> Result<usize> { Ok(buf.len()) }

io/src/macros.rs

@@ -45,5 +45,16 @@ macro_rules! impl_write {
                 $flush_fn(self)
             }
         }
+
+        impl<$($bounded_ty: $bounds),*> $crate::Write for $ty {
+            #[inline]
+            fn write(&mut self, buf: &[u8]) -> $crate::Result<usize> {
+                $write_fn(self, buf)
+            }
+            #[inline]
+            fn flush(&mut self) -> $crate::Result<()> {
+                $flush_fn(self)
+            }
+        }
     }
 }