Use sublice patterns to avoid computing the len

[tesuji]

r? @Centril

[rust-highfive]

The job mingw-check of your PR failed (pretty log, 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.

2020-03-16T16:46:32.9564341Z ========================== Starting Command Output ===========================
2020-03-16T16:46:32.9566323Z [command]/bin/bash --noprofile --norc /home/vsts/work/_temp/f8b35c0d-2d2a-4f39-abaf-814c605dcde7.sh
2020-03-16T16:46:32.9566515Z 
2020-03-16T16:46:32.9569290Z ##[section]Finishing: Disable git automatic line ending conversion
2020-03-16T16:46:32.9584233Z ##[section]Starting: Checkout rust-lang/rust@refs/pull/70046/merge to s
2020-03-16T16:46:32.9586744Z Task         : Get sources
2020-03-16T16:46:32.9586960Z Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.
2020-03-16T16:46:32.9587208Z Version      : 1.0.0
2020-03-16T16:46:32.9587349Z Author       : Microsoft
---
2020-03-16T16:46:34.2449250Z ##[command]git remote add origin https://github.com/rust-lang/rust
2020-03-16T16:46:34.2455644Z ##[command]git config gc.auto 0
2020-03-16T16:46:34.2466840Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2020-03-16T16:46:34.2475089Z ##[command]git config --get-all http.proxy
2020-03-16T16:46:34.2487343Z ##[command]git -c http.extraheader="AUTHORIZATION: basic ***" fetch --force --tags --prune --progress --no-recurse-submodules --depth=2 origin +refs/heads/*:refs/remotes/origin/* +refs/pull/70046/merge:refs/remotes/pull/70046/merge
---
2020-03-16T16:50:56.4014062Z     Checking panic_unwind v0.0.0 (/checkout/src/libpanic_unwind)
2020-03-16T16:50:59.0289413Z error[E0614]: type `u8` cannot be dereferenced
2020-03-16T16:50:59.0290174Z    --> src/libstd/sys_common/wtf8.rs:603:71
2020-03-16T16:50:59.0291044Z     |
2020-03-16T16:50:59.0291813Z 603 |             [.., 0xED, b2 @ 0xA0..=0xAF, b3] => Some(decode_surrogate(*b2, *b3)),
2020-03-16T16:50:59.0293291Z 
2020-03-16T16:50:59.0293752Z error[E0614]: type `u8` cannot be dereferenced
2020-03-16T16:50:59.0294260Z    --> src/libstd/sys_common/wtf8.rs:603:76
2020-03-16T16:50:59.0294675Z     |
2020-03-16T16:50:59.0294675Z     |
2020-03-16T16:50:59.0295391Z 603 |             [.., 0xED, b2 @ 0xA0..=0xAF, b3] => Some(decode_surrogate(*b2, *b3)),
2020-03-16T16:50:59.0296631Z 
2020-03-16T16:50:59.0305447Z error[E0614]: type `u8` cannot be dereferenced
2020-03-16T16:50:59.0306062Z    --> src/libstd/sys_common/wtf8.rs:611:71
2020-03-16T16:50:59.0306469Z     |
2020-03-16T16:50:59.0306469Z     |
2020-03-16T16:50:59.0314629Z 611 |             [0xED, b2 @ 0xB0..=0xBF, b3, ..] => Some(decode_surrogate(*b2, *b3)),
2020-03-16T16:50:59.0315684Z 
2020-03-16T16:50:59.0316079Z error[E0614]: type `u8` cannot be dereferenced
2020-03-16T16:50:59.0316590Z    --> src/libstd/sys_common/wtf8.rs:611:76
2020-03-16T16:50:59.0317019Z     |
2020-03-16T16:50:59.0317019Z     |
2020-03-16T16:50:59.0317590Z 611 |             [0xED, b2 @ 0xB0..=0xBF, b3, ..] => Some(decode_surrogate(*b2, *b3)),
2020-03-16T16:50:59.0318604Z 
2020-03-16T16:50:59.8044800Z error: aborting due to 4 previous errors
2020-03-16T16:50:59.8045795Z 
2020-03-16T16:50:59.8046547Z For more information about this error, try `rustc --explain E0614`.
---
2020-03-16T16:50:59.8051893Z   local time: Mon Mar 16 16:50:59 UTC 2020
2020-03-16T16:50:59.8052165Z   network time: Mon, 16 Mar 2020 16:50:59 GMT
2020-03-16T16:50:59.8052381Z == end clock drift check ==
2020-03-16T16:51:00.7745993Z 
2020-03-16T16:51:00.7819399Z ##[error]Bash exited with code '1'.
2020-03-16T16:51:00.7851120Z ##[section]Finishing: Run build
2020-03-16T16:51:00.7890565Z ##[section]Starting: Checkout rust-lang/rust@refs/pull/70046/merge to s
2020-03-16T16:51:00.7894411Z Task         : Get sources
2020-03-16T16:51:00.7894812Z Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.
2020-03-16T16:51:00.7895068Z Version      : 1.0.0
2020-03-16T16:51:00.7895235Z Author       : Microsoft
2020-03-16T16:51:00.7895235Z Author       : Microsoft
2020-03-16T16:51:00.7895496Z Help         : [More Information](https://go.microsoft.com/fwlink/?LinkId=798199)
2020-03-16T16:51:00.7895817Z ==============================================================================
2020-03-16T16:51:01.0938404Z Cleaning any cached credential from repository: rust-lang/rust (GitHub)
2020-03-16T16:51:01.0986799Z ##[section]Finishing: Checkout rust-lang/rust@refs/pull/70046/merge to s
2020-03-16T16:51:01.1062505Z Cleaning up task key
2020-03-16T16:51:01.1063697Z Start cleaning up orphan processes.
2020-03-16T16:51:01.1242381Z Terminate orphan process: pid (10921) (python)
2020-03-16T16:51:01.1411253Z ##[section]Finishing: Finalize Job

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[Centril]

Nice. @bors r+

[bors]

📌 Commit e1bc9af has been approved by Centril