common: Make file_offset dependency optional

This feature is off by default.
`file_offset` only supports `unix` and `windows` build targets.

The additional code implements `FileExt` `std::fs::File`.
`FileExt` is currently only used in the `datafile` module.

common/Cargo.toml

@@ -6,7 +6,7 @@ license = "MIT/Apache-2.0"
 
 [dependencies]
 arrayvec = "0.5.2"
-file_offset = "0.1.0"
+file_offset = { version = "0.1.0", optional = true }
 serde = { version = "1.0.23", optional = true }
 
 [dev-dependencies]

common/src/io.rs

@@ -1,12 +1,8 @@
-use file_offset::FileExt as FileOffsetExt;
 use std::error;
 use std::fmt;
-use std::fs::File;
 use std::io;
 use std::io::Read;
 
-use num::Cast;
-
 struct SeekOverflow(());
 
 pub fn seek_overflow() -> io::Error {
@@ -48,16 +44,20 @@ pub trait FileExt {
     fn read_offset_retry(&self, buffer: &mut [u8], offset: u64) -> io::Result<usize>;
 }
 
-impl FileExt for File {
+#[cfg(feature = "file_offset")]
+impl FileExt for std::fs::File {
     fn read_offset_retry(&self, buffer: &mut [u8], offset: u64) -> io::Result<usize> {
+        use num::Cast;
+
         // Make sure the additions in this function don't overflow
         let _end_offset = offset
             .checked_add(buffer.len().u64())
             .ok_or_else(seek_overflow)?;
 
         let mut read = 0;
         while read != buffer.len() {
-            match self.read_offset(&mut buffer[read..], offset + read.u64()) {
+            match file_offset::FileExt::read_offset(self, &mut buffer[read..], offset + read.u64())
+            {
                 Ok(0) => break,
                 Ok(r) => read += r,
                 Err(ref e) if e.kind() == io::ErrorKind::Interrupted => {}

common/src/lib.rs

@@ -3,6 +3,7 @@
 extern crate quickcheck;
 
 extern crate arrayvec;
+#[cfg(feature = "file_offset")]
 extern crate file_offset;
 #[cfg(feature = "serde")]
 extern crate serde;

datafile/Cargo.toml

@@ -5,7 +5,7 @@ authors = ["heinrich5991 <[email protected]>"]
 license = "MIT/Apache-2.0"
 
 [dependencies]
-common = { path = "../common/" }
+common = { path = "../common/", features = ["file_offset"] }
 hexdump = "0.1.1"
 itertools = ">=0.3.0,<0.5.0"
 log = "0.3.0"