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Add eprint! and eprintln! #39228

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zackw opened this issue Jan 21, 2017 · 3 comments
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Add eprint! and eprintln! #39228

zackw opened this issue Jan 21, 2017 · 3 comments

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@zackw
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zackw commented Jan 21, 2017 •
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From https://internals.rust-lang.org/t/extremely-pre-rfc-eprintln/4635 …

if you are writing command-line programs: the easiest way to emit non-fatal error messages is println!, but that sends them to stdout, which is not where error messages should go. panic! does the right thing (and thus also unwrap, expect, assert!, etc) but those are all (a) fatal and (b) properly used only for conditions that indicate a bug. The most straightforward way to write diagnostics to stderr is

writeln!(io::stderr(), "out of cheese error").unwrap();

which is significantly more typing in itself, requires you to use std::io::Write, and doesn't have println!'s defensive measures against being called at an awkward moment.

The simplest thing we could possibly add to fix this problem would be an eprintln! macro, which would be exactly the same as println! except that the output goes to stderr instead of stdout.

All of the documentation should also be updated to make it clear that it is inappropriate to use println! for error messages.

… which met with general approval and two people saying they already were adding eprintln! to all their own crates, so I'm just gonna go ahead and submit a PR. eprint! is included for completeness and symmetry; I'm not sure it's useful, but I think people would be surprised to find it missing.
zackw added a commit to zackw/rust that referenced this issue Jan 21, 2017
@zackw
Add eprint! and eprintln! macros to the prelude.
8b6bbb1 
These are exactly the same as `print!` and `println!` except that
they write to stderr instead of stdout.  Issue rust-lang#39228.
@zackw zackw mentioned this issue Jan 21, 2017
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@petrochenkov
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I'd suggest to just implement this and send a PR (in the worst case you'll need to write an RFC as well).
This was suggested many times in various places, but nobody pushed hard enough and took responsibility to actually get this implemented and merged. I encourage you to do it.
The number of likes this suggestion gathered on internals in one day shows that the feature is clearly wanted.

@steveklabnik
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This does require an RFC, as I mentioned in #39229, so I'm going to give this a close; this would be tracked by the RFC process. Thanks!

@zackw zackw mentioned this issue Apr 10, 2017
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zackw added a commit to zackw/rust that referenced this issue May 10, 2017
@zackw
Add eprint! and eprintln! macros to the prelude.
7612727 
These are exactly the same as `print!` and `println!` except that
they write to stderr instead of stdout.  Issue rust-lang#39228.
frewsxcv added a commit to frewsxcv/rust that referenced this issue May 11, 2017
@frewsxcv
Rollup merge of rust-lang#41192 - zackw:eprintln, r=alexcrichton
3187c48 
Add `eprint!` and `eprintln!` macros to the prelude.

These are exactly the same as `print!` and `println!` except that they write to stderr instead of stdout.  Issues rust-lang#39228 and rust-lang#40528; previous PR rust-lang#39229; accepted RFC rust-lang/rfcs#1869; proposed revision to The Book rust-lang/book#615.

I have _not_ revised this any since the original submission; I will do that later this week.  I wanted to get this PR in place since it's been quite a while since the RFC was merged.

Known outstanding review comments:

* [x] @steveklabnik requested a new chapter for the unstable version of The Book -- please see if the proposed revisions to the second edition cover it.
* [x] @nodakai asked if it were possible to merge the internal methods `_print` and `_eprint` - not completely, since they both refer to different internal globals which we don't want to expose, but I will see if some duplication can be factored out.

Please let me know if I missed anything.
frewsxcv added a commit to frewsxcv/rust that referenced this issue May 11, 2017
@frewsxcv
Rollup merge of rust-lang#41192 - zackw:eprintln, r=alexcrichton
a00e182 
Add `eprint!` and `eprintln!` macros to the prelude.

These are exactly the same as `print!` and `println!` except that they write to stderr instead of stdout.  Issues rust-lang#39228 and rust-lang#40528; previous PR rust-lang#39229; accepted RFC rust-lang/rfcs#1869; proposed revision to The Book rust-lang/book#615.

I have _not_ revised this any since the original submission; I will do that later this week.  I wanted to get this PR in place since it's been quite a while since the RFC was merged.

Known outstanding review comments:

* [x] @steveklabnik requested a new chapter for the unstable version of The Book -- please see if the proposed revisions to the second edition cover it.
* [x] @nodakai asked if it were possible to merge the internal methods `_print` and `_eprint` - not completely, since they both refer to different internal globals which we don't want to expose, but I will see if some duplication can be factored out.

Please let me know if I missed anything.
@musically-ut
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If you are still running into this error, update Rust to version >= 1.21.0.

I spent a while googling this error and wondering which package providing eprintln! was old before realizing that this PR had eventually been merged in.

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@steveklabnik @zackw @musically-ut @petrochenkov