Custom Debug impl for io::Error

[retep998]

Fixes #26408

Example output when using Result::unwrap:

thread '<main>' panicked at 'called `Result::unwrap()` on an `Err` value: Error { repr: Os { code: 2, message: "The system cannot find the file specified.\r\n" } }', src/libcore\result.rs:731

This could technically be considered a breaking change for any code that depends on the format of the Debug output for io::Error but I sincerely hope nobody wrote code like that.

[rust-highfive]

r? @aturon

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[sfackler]

👍

[reem]

+1 from me as well, definitely addresses the concerns from #26408

[sfackler]

It may be a bit nicer to look at if we output Os { code: 6, message: "foobar" } instead of Os(6, "foobar") imo.

[reem]

Agreed. I like the setup in the latest commit.

[arielb1]

Test please.

[retep998]

@arielb1 Test added

[sfackler]

@bors r+

[bors]

📌 Commit c8aec53 has been approved by sfackler

[bors]

⌛ Testing commit c8aec53 with merge 8ca4351...

[bors]

💔 Test failed - auto-linux-32-opt

[sfackler]

@bors retry

[bors]

⌛ Testing commit c8aec53 with merge b53c6cc...

[bors]

💔 Test failed - auto-win-gnu-64-opt

[retep998]

shakes fist at spurious failures

[sfackler]

@bors retry

[bors]

⌛ Testing commit c8aec53 with merge ecfcd2a...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt

[pnkfelix]

/me cant believe that we still havent solved the problem of including the requested path in the error information here (for the specific example of "no such file or directory")

[retep998]

@pnkfelix That would involve storing the requested path in the io::Error and since that won't fit into the i32 we'd have to Box it in the Custom variant. And we'd have to add code to every function that could potentially return a no such file error to check for that error code and instead return the fat custom error with the requested path.

[nobody4t]

why I still got this error when I run the code from Mac 10.13.6 with rustc version 1.29.0.