Fix null synthetic_implementors error

[GuillaumeGomez]

Fixes#68584.

r? @ollie27

cc @kinnison

[rust-highfive]

The job x86_64-gnu-llvm-7 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.

2020-01-28T12:49:21.7695359Z ========================== Starting Command Output ===========================
2020-01-28T12:49:21.7714417Z [command]/bin/bash --noprofile --norc /home/vsts/work/_temp/118662eb-0c48-471d-99ca-f1d95257267f.sh
2020-01-28T12:49:21.8012328Z 2020-01-28T12:49:21.8052957Z ##[section]Finishing: Disable git automatic line ending conversion
2020-01-28T12:49:21.8058767Z ##[section]Starting: Checkout rust-lang/rust@refs/pull/68598/merge to s
2020-01-28T12:49:21.8060387Z Task : Get sources
2020-01-28T12:49:21.8060425Z Description : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.
2020-01-28T12:49:21.8060462Z Version : 1.0.0
2020-01-28T12:49:21.8060542Z Author : Microsoft
---
2020-01-28T12:49:22.6943018Z ##[command]git remote add origin https://github.com/rust-lang/rust
2020-01-28T12:49:22.7056128Z ##[command]git config gc.auto 0
2020-01-28T12:49:22.7130788Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2020-01-28T12:49:22.7182727Z ##[command]git config --get-all http.proxy
2020-01-28T12:49:22.7328353Z ##[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/68598/merge:refs/remotes/pull/68598/merge
---
2020-01-28T13:15:28.4965256Z Compiling serde_json v1.0.40
2020-01-28T13:15:30.0268271Z Compiling tidy v0.1.0 (/checkout/src/tools/tidy)
2020-01-28T13:15:39.1227660Z Finished release [optimized] target(s) in 1m 16s
2020-01-28T13:15:39.1335538Z tidy check
2020-01-28T13:15:40.2722931Z tidy error: /checkout/src/librustdoc/html/static/main.js:1902: line longer than 100 chars
2020-01-28T13:15:41.9424896Z some tidy checks failed
2020-01-28T13:15:41.9425054Z Found 487 error codes
2020-01-28T13:15:41.9425112Z Found 0 error codes with no tests
2020-01-28T13:15:41.9425165Z Done!
2020-01-28T13:15:41.9425165Z Done!
2020-01-28T13:15:41.9425200Z 2020-01-28T13:15:41.9425252Z 2020-01-28T13:15:41.9426375Z command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/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"
2020-01-28T13:15:41.9426564Z 2020-01-28T13:15:41.9426596Z 2020-01-28T13:15:41.9435190Z failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
2020-01-28T13:15:41.9435277Z Build completed unsuccessfully in 0:01:27
2020-01-28T13:15:41.9435277Z Build completed unsuccessfully in 0:01:27
2020-01-28T13:15:41.9489069Z == clock drift check ==
2020-01-28T13:15:41.9500442Z local time: Tue Jan 28 13:15:41 UTC 2020
2020-01-28T13:15:42.4933755Z network time: Tue, 28 Jan 2020 13:15:42 GMT
2020-01-28T13:15:42.4934942Z == end clock drift check ==
2020-01-28T13:15:43.2105706Z 2020-01-28T13:15:43.2226734Z ##[error]Bash exited with code '1'.
2020-01-28T13:15:43.2239901Z ##[section]Finishing: Run build
2020-01-28T13:15:43.2255256Z ##[section]Starting: Checkout rust-lang/rust@refs/pull/68598/merge to s
2020-01-28T13:15:43.2257120Z Task : Get sources
2020-01-28T13:15:43.2257173Z Description : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.
2020-01-28T13:15:43.2257240Z Version : 1.0.0
2020-01-28T13:15:43.2257287Z Author : Microsoft
2020-01-28T13:15:43.2257287Z Author : Microsoft
2020-01-28T13:15:43.2257338Z Help : [More Information](https://go.microsoft.com/fwlink/?LinkId=798199)
2020-01-28T13:15:43.2257406Z ==============================================================================
2020-01-28T13:15:43.6539830Z Cleaning any cached credential from repository: rust-lang/rust (GitHub)
2020-01-28T13:15:43.6581909Z ##[section]Finishing: Checkout rust-lang/rust@refs/pull/68598/merge to s
2020-01-28T13:15:43.6695896Z Cleaning up task key
2020-01-28T13:15:43.6696682Z Start cleaning up orphan processes.
2020-01-28T13:15:43.6804375Z Terminate orphan process: pid (3238) (python)
2020-01-28T13:15:43.7019407Z ##[section]Finishing: Finalize Job

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)

[ollie27]

Nice, r=me once the tidy error has been fixed.

Nominating for beta backport because it looks like it's just been branched.

[Centril]

@ollie27 btw, don't forget to add a team label when adding beta-nominated. :)

[GuillaumeGomez]

Updated!

[GuillaumeGomez]

@bors: r=ollie27

[bors]

📌 Commit 0754461 has been approved by ollie27

[Mark-Simulacrum]

@rust-lang/rustdoc If this is to get into beta in time (i.e., before beta becomes stable) then it needs to be approved roughly in the next 3 days.

[GuillaumeGomez]

@Mark-Simulacrum I add the tag for it.