Using anstream with tracing_appender's `RollingFileAppender`

[Chaoses-Ib]

anstream's StripStream requires S: AsLockedWrite to implement std::io::Write. AsLockedWrite is implemented for Box<dyn std::io::Write> and tracing_appender's RollingFileAppender implements std::io::Write, so it looks like what is needed is just a Box. However, it doesn't actually work:

let file_appender = tracing_appender::rolling::daily(logs, "prefix");
let stream = Box::new(file_appender) as Box<dyn std::io::Write + Send>;
let strip_stream = anstream::StripStream::new(stream);
let (non_blocking, guard) = tracing_appender::non_blocking(strip_stream);

error[E0277]: the trait bound `Box<dyn std::io::Write + Send>: AsLockedWrite` is not satisfied
   --> src\main.rs:26:72
    |
26  |             let (non_blocking, guard) = tracing_appender::non_blocking(strip_stream);
    |                                         ------------------------------ ^^^^^^^^^^^^ the trait `AsLockedWrite` is not implemented for `Box<dyn std::io::Write + Send>`, which is required by `StripStream<Box<dyn std::io::Write + Send>>: std::io::Write`
    |                                         |
    |                                         required by a bound introduced by this call
    |
    = help: the trait `AsLockedWrite` is implemented for `Box<dyn std::io::Write>`
    = note: required for `StripStream<Box<dyn std::io::Write + Send>>` to implement `std::io::Write`
note: required by a bound in `non_blocking`
   --> C:\Users\Chaoses\.cargo\registry\src\index.crates.io-6f17d22bba15001f\tracing-appender-0.2.3\src\lib.rs:193:24
    |
193 | pub fn non_blocking<T: Write + Send + 'static>(writer: T) -> (NonBlocking, WorkerGuard) {
    |                        ^^^^^ required by this bound in `non_blocking`

For more information about this error, try `rustc --explain E0277`.

This error message doesn't make sense since stream already satisfies Box<dyn std::io::Write + Send>. And if I remove Send, there will be another error:

error[E0277]: `dyn std::io::Write` cannot be sent between threads safely
   --> src\main.rs:26:72
    |
26  |             let (non_blocking, guard) = tracing_appender::non_blocking(strip_stream);
    |                                         ------------------------------ ^^^^^^^^^^^^ `dyn std::io::Write` cannot be sent between threads safely
    |                                         |
    |                                         required by a bound introduced by this call
    |
    = help: the trait `Send` is not implemented for `dyn std::io::Write`, which is required by `StripStream<Box<dyn std::io::Write>>: Send`
    = note: required for `Unique<dyn std::io::Write>` to implement `Send`
note: required because it appears within the type `Box<dyn std::io::Write>`
   --> C:\Users\Chaoses\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib/rustlib/src/rust\library\alloc\src\boxed.rs:234:12
    |
234 | pub struct Box<
    |            ^^^
note: required because it appears within the type `StripStream<Box<dyn std::io::Write>>`
   --> C:\Users\Chaoses\.cargo\registry\src\index.crates.io-6f17d22bba15001f\anstream-0.6.15\src\strip.rs:7:12
    |
7   | pub struct StripStream<S>
    |            ^^^^^^^^^^^
note: required by a bound in `non_blocking`
   --> C:\Users\Chaoses\.cargo\registry\src\index.crates.io-6f17d22bba15001f\tracing-appender-0.2.3\src\lib.rs:193:32
    |
193 | pub fn non_blocking<T: Write + Send + 'static>(writer: T) -> (NonBlocking, WorkerGuard) {
    |                                ^^^^ required by this bound in `non_blocking`

For more information about this error, try `rustc --explain E0277`.

Any idea about what's going wrong?

[epage]

Likely the problem is that Box<dyn std::io::Write> is a distinct type from Box<dyn std::io::Write + Send>. I'd be open to more impls existing for more variants of Box.

[Chaoses-Ib]

@DaniPopes Thanks for your work! However, it seems that #224 doesn't impl AsLockedWrite for dyn std::io::Write + Send yet? The example code still raises the error:

   Compiling anstyle v1.0.8 (https://github.com/rust-cli/anstyle.git?rev=f4a1f986c06d82e44396680aa42d159283295dcf#f4a1f986)
   Compiling colorchoice v1.0.2 (https://github.com/rust-cli/anstyle.git?rev=f4a1f986c06d82e44396680aa42d159283295dcf#f4a1f986)
   Compiling anstyle-parse v0.2.5 (https://github.com/rust-cli/anstyle.git?rev=f4a1f986c06d82e44396680aa42d159283295dcf#f4a1f986)
   Compiling anstyle-query v1.1.1 (https://github.com/rust-cli/anstyle.git?rev=f4a1f986c06d82e44396680aa42d159283295dcf#f4a1f986)
   Compiling anstyle-wincon v3.0.4 (https://github.com/rust-cli/anstyle.git?rev=f4a1f986c06d82e44396680aa42d159283295dcf#f4a1f986)
   Compiling anstream v0.6.15 (https://github.com/rust-cli/anstyle.git?rev=f4a1f986c06d82e44396680aa42d159283295dcf#f4a1f986)
   Compiling test v0.1.0
error[E0277]: the trait bound `Box<dyn std::io::Write + Send>: AsLockedWrite` is not satisfied
   --> src\main.rs:26:68
    |
26  |         let (non_blocking, guard) = tracing_appender::non_blocking(strip_stream);
    |                                     ------------------------------ ^^^^^^^^^^^^ the trait `AsLockedWrite` is not implemented for `Box<dyn std::io::Write + Send>`, which is required by `AutoStream<Box<dyn std::io::Write + Send>>: std::io::Write`
    |                                     |
    |                                     required by a bound introduced by this call
    |
    = help: the following other types implement trait `AsLockedWrite`:
              (dyn std::io::Write + 'static)
              Buffer
              Stderr
              StderrLock<'static>
              Stdout
              StdoutLock<'static>
              Vec<u8>
              std::fs::File
    = note: required for `AutoStream<Box<dyn std::io::Write + Send>>` to implement `std::io::Write`
note: required by a bound in `non_blocking`
   --> C:\Users\Chaoses\.cargo\registry\src\index.crates.io-6f17d22bba15001f\tracing-appender-0.2.3\src\lib.rs:193:24
    |
193 | pub fn non_blocking<T: Write + Send + 'static>(writer: T) -> (NonBlocking, WorkerGuard) {
    |                        ^^^^^ required by this bound in `non_blocking`

For more information about this error, try `rustc --explain E0277`.

One more thing I noticed later is that AutoStream<Box<dyn std::io::Write>> doesn't impl Write anymore, i.e. the following code works before:

let stream = Box::new(Vec::new()) as Box<dyn std::io::Write>;
let strip_stream = anstream::AutoStream::new(stream, anstream::ColorChoice::Never);
fn test_write<T: std::io::Write + 'static>(writer: T) -> () {
    ()
}
test_write(strip_stream);

But raises an error on f4a1f98:

error[E0277]: the trait bound `Box<dyn std::io::Write>: AsLockedWrite` is not satisfied
  --> src\main.rs:30:20
   |
30 |         test_write(strip_stream);
   |         ---------- ^^^^^^^^^^^^ the trait `AsLockedWrite` is not implemented for `Box<dyn std::io::Write>`, which is required by `AutoStream<Box<dyn std::io::Write>>: std::io::Write`
   |         |
   |         required by a bound introduced by this call
   |
   = help: the following other types implement trait `AsLockedWrite`:
             (dyn std::io::Write + 'static)
             Buffer
             Stderr
             StderrLock<'static>
             Stdout
             StdoutLock<'static>
             Vec<u8>
             std::fs::File
   = note: required for `AutoStream<Box<dyn std::io::Write>>` to implement `std::io::Write`
note: required by a bound in `test_write`
  --> src\main.rs:27:26
   |
27 |         fn test_write<T: std::io::Write + 'static>(writer: T) -> () {
   |                          ^^^^^^^^^^^^^^ required by this bound in `test_write`

I'm not an expert on Rust's trait objects, but it looks like impl AsLockedWrite for dyn std::io::Write doesn't automatically impl for Box<dyn std::io::Write>. This post may be useful: Is impl Trait1 for dyn Trait2 ever useful?

[DaniPopes]

I missed that in #224, sorry about that. It should be fixed here: #225

[epage]

@Chaoses-Ib thanks for testing this!