crates/core/bytestream/writer: Generalize writer trait for std writer…

…s and feature gate in that no-std writers are disabled on writing

crates/zune-core/src/bytestream/writer/no_std_writer.rs

@@ -1,3 +1,6 @@
+// We cannot use the below impls and std ones because we'll re-implement the
+// same trait fot &[u8] which is blanketed by write. Ending up with two separate implementations
+#![cfg(not(feature = "std"))]
 use crate::bytestream::{ZByteIoError, ZByteWriterTrait};
 
 impl ZByteWriterTrait for &mut [u8] {

crates/zune-core/src/bytestream/writer/std_writer.rs

@@ -1,10 +1,10 @@
 #![cfg(feature = "std")]
-use std::fs::File;
-use std::io::{BufWriter, Write};
+
+use std::io::Write;
 
 use crate::bytestream::ZByteIoError;
 
-impl crate::bytestream::ZByteWriterTrait for &mut BufWriter<File> {
+impl<T: Write> crate::bytestream::ZByteWriterTrait for T {
     fn write_bytes(&mut self, buf: &[u8]) -> Result<usize, ZByteIoError> {
         self.write(buf).map_err(ZByteIoError::StdIoError)
     }
@@ -20,6 +20,8 @@ impl crate::bytestream::ZByteWriterTrait for &mut BufWriter<File> {
         self.flush().map_err(ZByteIoError::StdIoError)
     }
     fn reserve_capacity(&mut self, _: usize) -> Result<(), ZByteIoError> {
+        // we can't reserve capacity, sorry to implementations where this
+        // matters
         Ok(())
     }
 }