Re-enable README tests in a cleverer way

This change borrows the core idea from the [doc-comment crate](https://github.com/GuillaumeGomez/doc-comment/) to run the README examples as doctests by wholesale including README.md via `include_str!`. The resulting docs are on an empty module marked as `#[doc(hidden)]` and I've also made them only run when the `derive` feature is enabled because AFAIK there isn't a good way to disable individual tests in a single item depending on features. This should still be useful since scroll's CI tests with that feature enabled so it'll ensure that the examples in the README continue to work!

Since the examples are being run as standard rusttests now I was able to do a little bit of cleanup on them as well.

README.md

@@ -37,7 +37,7 @@ A simple example demonstrates its flexibility:
 ```rust
 use scroll::{ctx, Pread, LE};
 
-fn parse() -> Result<(), scroll::Error> {
+fn main() -> Result<(), scroll::Error> {
     let bytes: [u8; 4] = [0xde, 0xad, 0xbe, 0xef];
 
     // reads a u32 out of `b` with the endianness of the host machine, at offset 0, turbofish-style
@@ -71,20 +71,13 @@ fn parse() -> Result<(), scroll::Error> {
     assert_eq!("world", world);
     Ok(())
 }
-
-fn main() {
-  parse().unwrap();
-}
 ```
 
 ### Deriving `Pread` and `Pwrite`
 
 Scroll implements a custom derive that can provide `Pread` and `Pwrite` implementations for your structs.
 
-```no_test
-#[macro_use]
-extern crate scroll_derive;
-
+```rust
 use scroll::{Pread, Pwrite, BE};
 
 #[derive(Pread, Pwrite)]
@@ -94,7 +87,7 @@ struct Data {
     three: u8,
 }
 
-fn parse() -> Result<(), scroll::Error> {
+fn main() -> Result<(), scroll::Error> {
     let bytes: [u8; 7] = [0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce, 0xff];
     // Read a single `Data` at offset zero in big-endian byte order.
     let data: Data = bytes.pread_with(0, BE)?;
@@ -108,10 +101,6 @@ fn parse() -> Result<(), scroll::Error> {
     assert_eq!(bytes, out);
     Ok(())
 }
-
-fn main() {
-  parse().unwrap();
-}
 ```
 
 This feature is **not** enabled by default, you must enable the `derive` feature in Cargo.toml to use it:
@@ -129,7 +118,7 @@ Scroll can also read/write simple types from a `std::io::Read` or `std::io::Writ
 use std::io::Cursor;
 use scroll::IOread;
 
-fn parse_io() -> Result<(), scroll::Error> {
+fn main() -> Result<(), scroll::Error> {
     let bytes_ = [0x01,0x00,0x00,0x00,0x00,0x00,0x00,0x00, 0xef,0xbe,0x00,0x00,];
     let mut bytes = Cursor::new(bytes_);
 
@@ -139,10 +128,6 @@ fn parse_io() -> Result<(), scroll::Error> {
     let bar = bytes.ioread::<u32>()?;
     Ok(())
 }
-
-fn main() {
-  parse_io().unwrap();
-}
 ```
 
 Similarly, we can write to anything that implements `std::io::Write` quite naturally:
@@ -151,18 +136,14 @@ Similarly, we can write to anything that implements `std::io::Write` quite natur
 use scroll::{IOwrite, LE, BE};
 use std::io::{Write, Cursor};
 
-fn write_io() -> Result<(), scroll::Error> {
+fn main() -> Result<(), scroll::Error> {
     let mut bytes = [0x0u8; 10];
     let mut cursor = Cursor::new(&mut bytes[..]);
     cursor.write_all(b"hello")?;
     cursor.iowrite_with(0xdeadbeef as u32, BE)?;
     assert_eq!(cursor.into_inner(), [0x68, 0x65, 0x6c, 0x6c, 0x6f, 0xde, 0xad, 0xbe, 0xef, 0x0]);
     Ok(())
 }
-
-fn main() {
-  write_io().unwrap();
-}
 ```
 
 # Advanced Uses
@@ -196,17 +177,13 @@ impl<'a> ctx::TryFromCtx<'a, Endian> for Data<'a> {
   }
 }
 
-fn parse_data() -> Result<(), scroll::Error> {
+fn main() -> Result<(), scroll::Error> {
     let bytes = b"UserName\x00\x01\x02\x03\x04";
     let data = bytes.pread_with::<Data>(0, BE)?;
     assert_eq!(data.id, 0x01020304);
     assert_eq!(data.name.to_string(), "UserName".to_string());
     Ok(())
 }
-
-fn main() {
-  parse_data().unwrap();
-}
 ```
 
 Please see the official documentation, or a simple [example](examples/data_ctx.rs) for more.

src/lib.rs

@@ -169,6 +169,19 @@ pub mod export {
     pub use ::core::mem;
 }
 
+
+#[allow(unused)]
+macro_rules! doc_comment {
+    ($x:expr) => {
+        #[doc = $x]
+        #[doc(hidden)]
+        mod readme_tests {}
+    };
+}
+
+#[cfg(feature = "derive")]
+doc_comment!(include_str!("../README.md"));
+
 #[cfg(test)]
 mod tests {
     #[allow(overflowing_literals)]