This version only adds support for target_os = "hermit" via hermit_abi crate.
The rest of the code in the crate has not been changed.

Simple codebase, looks good. Did not look at the cfg(windows) code.

One of the more foundational crates found in the dependency tree of a lot of
rust programs, because both clap and env_logger pull this in.

In my review I've fully read the source code and can confirm that I fully
understand what's happening in here. The unix and hermit targets are
extremely straight-forward. As for the windows target, that's a bit more
complicated, but still manageable in the end. Windows doesn't have a clear
API for determining whether something is a (pseudo) TTY, so the heuristics
provided by this crate are as good as it's going to get.

This crate has quite a few unsafe code sections, but that's sometimes
required for providing a safe interface. In this case, we need it because the
underlying functions for unix (libc) are unsafe, and the same applies to a
bunch of winapi functions used in the heuristics for windows.

The bits of unsafe code that's not just wrapping an unsafe function provided
by another library are all in the windows heuristics, and involved
provisioning buffers that winapi calls can write info back into and some
pointer magic. While I couldn't spot an issue with this, a look into the issue
tracker revealed that other's have. The buffer creation on the heap is not
necessarily aligned properly, meaning that there's a possible soundness issue
on windows targets here.

Last but not least, we have to talk about maintenance and and alternatives:
The soundness issue mentioned above has been known for over a year, with a fix
first pushed to a PR shortly after. Even though there's been reviews from
well-known rustaceans, the author hasn't merged this PR yet, and in general
there hasn't been any relevant activity for a while.

An alternative implementation that has been derived from atty is
is-terminal, which has taken this into consideration and has taken over the
rest of the implementation from here, with a slightly different API. They're
also switching the underlying implementations around, reducing the amount of
unsafe.

As for why I'm still rating this as positive: With the exception of the
potential unsoundness bug on windows, this crate is still okay-ish to be used.
Also: Getting rid of it soon is not realistic, because it's in the dependency
tree of quite a few widely-used crates.

To summarize: Widely used, foundational crate. Except for on windows, this is
perfectly fine, on windows there's an unsoundness bug, and there has not been
an update or other activity from upstream. is-terminal is a good
alternative, which was derived from this crate, but includes the fix for the
unsoundess bug.

atty is a micro-crate with little code and no dependency.
The code looks fine to me. Complicated just because of multi-platform support.
I trust it.

malicious potential: 0%
dependencies:libc, winapi, hermit-abi
unsafe:6 (because working with low level system calls)
NONE: FFI, asm!, file-read, file-write, no_mangle, network-access, build.rs, proc_macro, process::command, macro_rules
published to crates.io by: softprops (unknown to me)

atty is a micro-crate with little code and no dependency.
The code looks fine to me. Complicated just because of multi-platform support.
I trust it.

malicious potential: 0%
dependencies:libc, winapi, hermit-abi
unsafe:6 (because working with low level system calls)
NONE: FFI, asm!, file-read, file-write, no_mangle, network-access, build.rs, proc_macro, process::command, macro_rules
published to crates.io by: softprops (unknown to me)

Small platform-functionality wrapper, nothing exciting.

atty is a micro-crate with little code and no dependency.
The code looks fine to me. Complicated just because of multi-platform support.
I trust it.

malicious potential: 0%
dependencies:libc, winapi, hermit-abi
unsafe:6 (because working with low level system calls)
NONE: FFI, asm!, file-read, file-write, no_mangle, network-access, build.rs, proc_macro, process::command, macro_rules
published to crates.io by: softprops (unknown to me)