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For many of Nix's consumers it be convenient to easily convert a Nix
error into a std::io::Error. That's currently not possible because of
the InvalidPath, InvalidUtf8, and UnsupportedOperation types that have
no equivalent in std::io::Error.
However, very few of Nix's public APIs actually return those unusual
errors. So a more useful API would be for Nix's standard error type to
implement Into<std::io::Error>.
This commit makes Error a simple NewType around Errno. For most
functions it's a drop-in replacement. There are only three exceptions:
* clearenv now returns a bespoke error type. It was the only Nix
function whose error couldn't be cleanly mapped onto an Errno.
* sys::signal::signal now returns Error(Errno::ENOTSUP) instead of
Error::UnsupportedOperation when the user passes an incompatible
argument to `handler`.
* When a NixPath exceeds PATH_MAX, it will now return
Error(Errno::ENAMETOOLONG) instead of Error::InvalidPath.
In the latter two cases there is now some abiguity about whether the
error code was generated by Nix or by the OS. But I think the ambiguity
is worth it for the sake of being able to implement Into<io::Error>.
This commit also introduces Error::Sys() as a migration aid. Previously
that as an enum variant. Now it's a function, but it will work in many
of the same contexts as the original.
Fixes #1155
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