Fix warnings when compiling stdlib with --test

[fhahn]

This PR siliences some warnings when compiling stdlib with --test. Mostly remove some unused imports and added a few #[allow(..)].

I also marked some signal handling functions with #[cfg(not(test))], because they are only called through rt::lang_start, which is also marked as #[cfg(not(test))]

[rust-highfive]

r? @aturon

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[steveklabnik]

Looks good to me, but this isn't my specialty in the codebase, so I'll let someone else r+

[alexcrichton]

I think these cfg annotations should be combined, right?

[fhahn]

@alexcrichton Thanks for your feedback! I've updated the commit. Now almost all warnings on Linux should be gone. The only remaining warnings are about the use of deprecated items in src/libstd/dynamic_lib.rs (https://github.com/rust-lang/rust/pull/30458/files#diff-59794c31034a5097419f3f376a30b4a4R135 and https://github.com/rust-lang/rust/pull/30458/files#diff-59794c31034a5097419f3f376a30b4a4R168), even though I've added #[allow(deprecated)].

[alexcrichton]

I think this may have slightly changed the semantics here as Answer lives over the call to panic!, but changing the declaration to let _answer = ... and removing the drop should have the same semantics.

[alexcrichton]

Thanks @fhahn! Looks good to me!

[fhahn]

@alexcrichton thanks for your feedback! I've updated the PR again.

[alexcrichton]

@bors: r+ abe9b4088989912f0cfa2348e6a103131791f3df

Thanks!

[bors]

⌛ Testing commit abe9b40 with merge efa0caf...

[bors]

💔 Test failed - auto-linux-64-x-android-t

[fhahn]

I've update the PR to fix the fail on android

[sanxiyn]

@bors r+

[bors]

📌 Commit 217bebb has been approved by sanxiyn

[bors]

⌛ Testing commit 217bebb with merge 1d71545...

[bors]

💔 Test failed - auto-linux-64-x-android-t

[fhahn]

The build on Android fails, because I marked os::raw::tests::unix as test. I could disable it again for Android (or in general), but shouldn"t the types of nlink_t in libc and os::raw be equal?

[sanxiyn]

I agree they should match on Android too, but I think it could be done on a separate PR.

[fhahn]

I've excluded the test from Android for now (at least I hope) and opened a PR for libc. After this lands, we should enable the test for android again.

[sanxiyn]

@bors r+

[bors]

📌 Commit e27cbef has been approved by sanxiyn

[bors]

⌛ Testing commit e27cbef with merge a06bb97...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt