Removed the dead links from cell.rs

[DevQps]

Description

Removed some dead links. The code used to refer to the method borrow that (I think?) used to exist on Box. However, now it is implemented by the Borrow trait. I find nothing on the Borrow docs that the borrow of Borrow can panic as was stated in the documentation. So I removed these lines.

closes #60442

[rust-highfive]

r? @alexcrichton

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[rust-highfive]

The job x86_64-gnu-llvm-6.0 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.

travis_time:end:074e9790:start=1556965254992540817,finish=1556965255780384680,duration=787843863
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
$ export GCP_CACHE_BUCKET=rust-lang-ci-cache
$ export AWS_ACCESS_KEY_ID=AKIA46X5W6CZEJZ6XT55
---
[01:07:40]  Documenting core v0.0.0 (/checkout/src/libcore)
[01:07:56] error: `[borrow_mut]` cannot be resolved, ignoring it...
[01:07:56]    --> src/libcore/cell.rs:930:10
[01:07:56]     |
[01:07:56] 930 |     /// [`borrow_mut`] method instead if `self` isn't mutable.
[01:07:56]     |
[01:07:56] note: lint level defined here
[01:07:56]    --> src/libcore/lib.rs:64:9
[01:07:56]     |
[01:07:56]     |
[01:07:56] 64  | #![deny(intra_doc_link_resolution_failure)] // rustdoc is run without -D warnings
[01:07:56]     |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[01:07:56]     = help: to escape `[` and `]` characters, just add '\' before them like `\[` or `\]`
[01:07:56] 
[01:07:56] error: `[borrow_mut]` cannot be resolved, ignoring it...
[01:07:56]    --> src/libcore/cell.rs:933:51
[01:07:56]     |
[01:07:56] 933 |     /// not what you want. In case of doubt, use [`borrow_mut`] instead.
[01:07:56]     |
[01:07:56]     = help: to escape `[` and `]` characters, just add '\' before them like `\[` or `\]`
[01:07:56] 
[01:07:57] error: aborting due to 2 previous errors
[01:07:57] error: aborting due to 2 previous errors
[01:07:57] 
[01:07:57] error: Could not document `core`.
[01:07:57] 
[01:07:57] Caused by:
[01:07:57]   process didn't exit successfully: `/checkout/obj/build/bootstrap/debug/rustdoc --edition=2018 --crate-name core src/libcore/lib.rs --color always --target x86_64-unknown-linux-gnu -o /checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/doc --markdown-css rust.css --markdown-no-toc --generate-redirect-pages --resource-suffix 1.36.0 --index-page /checkout/src/doc/index.md -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/release/deps -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/release/deps` (exit code: 1)
[01:07:57] 
[01:07:57] 
[01:07:57] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "rustdoc" "--target" "x86_64-unknown-linux-gnu" "-j" "4" "--release" "--locked" "--color" "always" "--features" "panic-unwind backtrace" "--manifest-path" "/checkout/src/libstd/Cargo.toml" "-Z" "unstable-options" "-p" "core" "--" "--markdown-css" "rust.css" "--markdown-no-toc" "--generate-redirect-pages" "--resource-suffix" "1.36.0" "--index-page" "/checkout/src/doc/index.md"
[01:07:57] 
[01:07:57] 
[01:07:57] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap doc
[01:07:57] Build completed unsuccessfully in 0:07:49
---
travis_time:end:0b6bc508:start=1556969345678496687,finish=1556969345683963528,duration=5466841
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:112d37fe
$ 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:0a754442
travis_time:start:0a754442
$ 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:07592aa5
$ dmesg | grep -i kill

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[DevQps]

@alexcrichton I am not sure what to do with this error message actually. It seems a bit unrelated to the things I did. What do you suggest on doing?

[ehuss]

I'm not sure I understand this PR. For me, the links seem to be working.

https://doc.rust-lang.org/nightly/core/cell/struct.RefCell.html#method.try_borrow clicking on the borrow link jumps to the RefCell::borrow method.

https://doc.rust-lang.org/nightly/core/cell/struct.RefCell.html#method.try_borrow_mut clicking on the borrow_mut link jumps to the RefCell::borrow_mut method.

https://doc.rust-lang.org/nightly/core/cell/struct.RefCell.html#method.get_mut clicking on the borrow_mut link jumps to the RefCell::borrow_mut method.

RefCell implements its own borrow methods.

As for the original issue, the link https://doc.rust-lang.org/std/boxed/struct.Box.html#method.borrow isn't invalid per se, it's just that the methods are hidden. If you expand the methods for impl<T> Borrow<T> for Box<T>, you can see the method is highlighted. That is part of #60200, where those aren't expanding when they should.

[DevQps]

Hey @ehuss

I think I am finally putting all the pieces together! I see that the issue has already been closed because of the reasons that you provided (thanks for that). I will close this PR now, since the issue is mainly due to the trait implementations not unfolding.