Add doc for impl From for Error

[phungleson]

As part of issue #51430 (cc @skade).

The impl is very simple, let me know if we need to go into any details.

[rust-highfive]

r? @alexcrichton

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[dtolnay]

@bors delegate=skade

[bors]

✌️ @skade can now approve this pull request

[pietroalbini]

Ping from triage @skade! This PR needs your review.

[TimNN]

Ping from triage @skade / @rust-lang/docs: This PR requires your review.

[GuillaumeGomez]

repr? (please write the full word).

[GuillaumeGomez]

Please add an example.

[phungleson]

Hey, I am currently traveling, will look at this again in around 3 weeks.

[rust-highfive]

The job x86_64-gnu-llvm-5.0 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

[00:04:40] travis_fold:start:tidy
travis_time:start:tidy
tidy check
[00:04:41] tidy error: /checkout/src/libstd/io/error.rs:223: trailing whitespace
[00:04:42] some tidy checks failed
[00:04:42] 
[00:04:42] 
[00:04:42] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "--no-vendor" "--quiet"
[00:04:42] 
[00:04:42] 
[00:04:42] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:04:42] Build completed unsuccessfully in 0:00:55
[00:04:42] Build completed unsuccessfully in 0:00:55
[00:04:42] make: *** [tidy] Error 1
[00:04:42] Makefile:79: recipe for target 'tidy' failed

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:00ac71e1
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
---
travis_time:end:05677c85:start=1537940447065740989,finish=1537940447070032233,duration=4291244
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:05006e2c
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\1|;y|!|/|'); if [ -f "$EXE" ]; then printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" "$CORE"; gdb --batch -q -c "$CORE" "$EXE" -iex 'set auto-load off' -iex 'dir src/' -iex 'set sysroot .' -ex bt -ex q; echo travis_fold":"end:crashlog; fi; done || true
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:0144f2d0
travis_time:start:0144f2d0
$ cat ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
cat: ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers: No such file or directory
travis_fold:end:after_failure.5
travis_fold:start:after_failure.6
travis_time:start:13ae7763
$ dmesg | grep -i kill

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[phungleson]

@GuillaumeGomez an example was added

[GuillaumeGomez]

Please remove fn main() here and just keep its code.

[frewsxcv]

To expand on @GuillaumeGomez 's comment, you don't need to add the main function in documentation examples. See also:

https://doc.rust-lang.org/book/first-edition/documentation.html#documentation-as-tests

[phungleson]

Cool thanks @frewsxcv @GuillaumeGomez main has been removed

[GuillaumeGomez]

Just thought: shouldn't it be "an Error"?

[frewsxcv]

@phungleson Hope you don't mind I pushed a small grammar change. Thanks for the docs contribution!

@bors r+ rollup

[bors]

📌 Commit 4f3896d has been approved by frewsxcv

[phungleson]

@frewsxcv not at all thanks!

[bors]

⌛ Testing commit 4f3896d with merge b9adc33...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: frewsxcv
Pushing b9adc33 to master...