From Failure into std::error::Error

[bltavares]

Choose one: a 🙋 feature

This PR moves from failure crate into a boxed std::error::Error using the anyhow.

Needs

• From Failure into std::error::Error random-access-disk#36
• From Failure into std::error::Error random-access-memory#25
• Built on top of Bump to random-access-storage 3.0.0 and other deps #100 updates (last commit only to review)

Checklist

• tests pass
• tests and/or benchmarks are included
• documentation is changed or added

Context

• Change stack from failure to std::error::Error #101

Semver Changes

major/minor as a breaking changing on the API.

[yoshuawuyts]

Maybe define an alias in the crate root?

[bltavares]

I've wanted to do that, but trait alias are only available on nightly:

error[E0658]: trait aliases are experimental
 --> examples\async.rs:6:1
  |
6 | trait Storage = RandomAccess<Error = Box<dyn std::error::Error + Send + Sync>> + Debug + Send;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: for more information, see https://github.com/rust-lang/rust/issues/41517

[bltavares]

I'll open an issue to track rust-lang/rust#41517 and move it on after this lands.

[yoshuawuyts]

Using ? will map the error for you.

[bltavares]

Now I remember why I've done this .map_err.

My previous attempt to use ? led me to unsized types:

error[E0277]: the size for values of type `dyn std::error::Error + std::marker::Send + std::marker::Sync` cannot be known at compilation time
  --> src\storage\mod.rs:80:59
   |
80 |         instance.bitfield.write(0, &header.to_vec()).await?;
   |                                                           ^ doesn't have a size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `dyn std::error::Error + std::marker::Send + std::marker::Sync`
   = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
   = note: required because of the requirements on the impl of `std::error::Error` for `std::boxed::Box<dyn std::error::Error + std::marker::Send + std::marker::Sync>`
   = note: required because of the requirements on the impl of `std::convert::From<std::boxed::Box<dyn std::error::Error + std::marker::Send + std::marker::Sync>>` for `anyhow::Error`
   = note: required by `std::convert::From::from`

It seems that anyhow::Error does not implement std::error::Error directly. This issue dtolnay/anyhow#63 led me to attempt calling .into(), which also failed:

error[E0284]: type annotations needed
  --> src\storage\mod.rs:80:60
   |
80 |         instance.bitfield.write(0, &header.to_vec()).await.into()?;
   |         ---------------------------------------------------^^^^--
   |         |                                                  |
   |         |                                                  cannot infer type
   |         this method call resolves to `T`
   |
   = note: cannot resolve `<_ as std::ops::Try>::Ok == _`

Which also didn't work even with a type specification:

error[E0277]: the trait bound `std::result::Result<(), anyhow::Error>: std::convert::From<std::result::Result<(), std::boxed::Box<dyn std::error::Error + std::marker::Send + std::marker::Sync>>>` is not satisfied
  --> src\storage\mod.rs:83:93
   |
83 |         let result: anyhow::Result<()> = instance.bitfield.write(0, &header.to_vec()).await.into();
   |                                                                                             ^^^^ the trait `std::convert::From<std::result::Result<(), std::boxed::Box<dyn std::error::Error + std::marker::Send + std::marker::Sync>>>` is not implemented for `std::result::Result<(), anyhow::Error>`
   |
   = note: required because of the requirements on the impl of `std::convert::Into<std::result::Result<(), anyhow::Error>>` for `std::result::Result<(), std::boxed::Box<dyn std::error::Error + std::marker::Send + std::marker::Sync>>`

There is an upstream issue documenting this conversion error dtolnay/anyhow#66, but I'm not sure if it will fixed, as there is a conflict on the impl of the conversions as documented on the previous discussion.

That led me to .map_err and it compiled without needing to implement a local structure to wrap stderr.

[yoshuawuyts]

Suggested change

	self.data.write(offset, &data).map_err(|e| anyhow!(e))
	Ok(self.data.write(offset, &data)?)