Documents reference equality by address (#54197)

[lucasloisp]

Clarification of the use of ptr::eq to test equality of references via address by pointer coercion, Fixes #54197 .

The same example as in ptr::eq docs is shown here to clarify that
PartialEq compares values pointed-to instead of via address (which can be desired in some cases)

[rust-highfive]

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[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.

[00:04:37] travis_fold:start:tidy
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tidy check
[00:04:37] tidy error: /checkout/src/libstd/primitive_docs.rs:912: trailing whitespace
[00:04:37] tidy error: /checkout/src/libstd/primitive_docs.rs:914: trailing whitespace
[00:04:37] tidy error: /checkout/src/libstd/primitive_docs.rs:917: trailing whitespace
[00:04:37] tidy error: /checkout/src/libstd/primitive_docs.rs:933: trailing whitespace
[00:04:37] tidy error: /checkout/src/libstd/primitive_docs.rs:940: trailing whitespace
[00:04:38] some tidy checks failed
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[00:04:38] 
[00:04:38] 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"
[00:04:38] 
[00:04:38] 
[00:04:38] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:04:38] Build completed unsuccessfully in 0:00:47
[00:04:38] Build completed unsuccessfully in 0:00:47
[00:04:38] Makefile:79: recipe for target 'tidy' failed
[00:04:38] make: *** [tidy] Error 1

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
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$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
---
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$ 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
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$ 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
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$ 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)

[TimNN]

Thanks for your PR @lucasloisp! You should hear from @aidanhs or some one else soon.

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

[QuietMisdreavus]

Usually we want to demonstrate assert_eq!() instead of assert!() when you're asserting equality like this, but i feel like this gets the point across better if you can see the == being used, since we're talking about the use of == versus ptr::eq in the first place. (Just leaving this comment for posterity.)

[QuietMisdreavus]

This looks great, thanks so much! Sorry for letting this one slip through the cracks.

@bors r+ rollup

[bors]

📌 Commit 68236e0 has been approved by QuietMisdreavus

[QuietMisdreavus]

Oh, one thing before this gets put in. Can you edit the PR description to add the line "Fixes #54197" so GitHub will automatically close it when it's merged?