Merge pull request #1427 from google/fix-extern-warning #2183

Job	Run time
Address Sanitizer	13m 58s
Examples Rust nightly	3m 36s
Examples Rust beta	3m 28s
Test Rust nightly	8m 35s
Examples Rust stable	3m 40s
Test Rust beta	8m 43s
Test Rust stable	8m 59s
Examples macOS	4m 27s
Test macOS	9m 15s
Examples Windows (gnu)	6m 6s
Examples Windows (msvc)	4m 41s
Test Windows (msvc)	13m 10s
Clippy	32s
Format	13s
Test forcing wrapper generation	6m 46s
Outdated	8s
Docs	40s
	1h 36m 57s

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