Add leading_ones and trailing_ones methods to the primitive integer types

[thomcc]

I was surprised these were missing (given that leading_zeros and trailing_zeros exist), and they seem trivial and hopefully not controversial.

Note that there's some precedent in that count_ones and count_zeros are both supported even though only one of these has an intrinsic.

I'm not sure if these need a rustc_const_unstable flag (the tests don't seem to mind that it's missing). I just made them const, since there's not really any reason for these to be non-const when the _zeros variants are const.

Note: My understanding is trivial stuff like (hopefully) this can land without an RFC, but I'm not fully sure about the process though. Questions like "when does the tracking issue get filed?", are a total mystery to me. So, any guidance is appreciated, and sorry in advance if I should have gone through some more involved process for this.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @sfackler (or someone else) soon.

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[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.

2020-01-12T22:59:22.8626142Z ##[command]git remote add origin https://github.com/rust-lang/rust
2020-01-12T22:59:22.8714535Z ##[command]git config gc.auto 0
2020-01-12T22:59:22.8763998Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2020-01-12T22:59:22.8823552Z ##[command]git config --get-all http.proxy
2020-01-12T22:59:22.8960278Z ##[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/68165/merge:refs/remotes/pull/68165/merge
---
2020-01-12T23:04:46.5662187Z * 588 error codes
2020-01-12T23:04:46.5663186Z * highest error code: E0745
2020-01-12T23:04:46.6255245Z thread 'main' panicked at 'assertion failed: `(left != right)`
2020-01-12T23:04:46.6262014Z   left: `0`,
2020-01-12T23:04:46.6271921Z  right: `0`: "none" should be used when there is no issue, not "0"', src/tools/tidy/src/features.rs:417:21
2020-01-12T23:04:46.6272172Z 
2020-01-12T23:04:46.6272198Z 
2020-01-12T23:04:46.6272198Z 
2020-01-12T23:04:46.6327210Z 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"
2020-01-12T23:04:46.6327382Z 
2020-01-12T23:04:46.6327437Z 
2020-01-12T23:04:46.6327500Z failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
2020-01-12T23:04:46.6327550Z Build completed unsuccessfully in 0:01:29
2020-01-12T23:04:46.6327550Z Build completed unsuccessfully in 0:01:29
2020-01-12T23:04:46.6357025Z == clock drift check ==
2020-01-12T23:04:46.6357105Z   local time: Sun Jan 12 23:04:46 UTC 2020
2020-01-12T23:04:46.9198906Z   network time: Sun, 12 Jan 2020 23:04:46 GMT
2020-01-12T23:04:46.9199012Z == end clock drift check ==
2020-01-12T23:04:47.6672070Z 
2020-01-12T23:04:47.6796371Z ##[error]Bash exited with code '1'.
2020-01-12T23:04:47.6834069Z ##[section]Starting: Checkout
2020-01-12T23:04:47.6835561Z ==============================================================================
2020-01-12T23:04:47.6835610Z Task         : Get sources
2020-01-12T23:04:47.6835651Z Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.

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)

[thomcc]

Uh, it's mad about the #[unstable(..., issue = "0")], which seems fair. Should I fix that by filing a tracking issue and putting it in? Or should I put none for now, and wait to get some form of goahead for filing the tracking issue

[Mark-Simulacrum]

It's fine for CI to be failing for now, once we get libs team signoff (i.e., @sfackler reviews this) then we can move ahead with filing a tracking issue and so forth.

[crlf0710]

Tracking issue should be #57969, just reopen it.

[sfackler]

Seem reasonble to me. r=me with the tracking issue updated.

[JohnCSimon]

Ping from triage:
@sfackler @thomcc What is the status of this PR? It's sat idle for over a week.

[thomcc]

I think I've done what was asked? I don't think I can reopen the tracking issue though.

[tesuji]

You could always creat a new one.

[LukasKalbertodt]

I reopened the tracking issue.

@bors r=sfackler

[bors]

📌 Commit 783a7dc has been approved by sfackler

[bors]

⌛ Testing commit 783a7dc with merge 1d5f6d4...

[bors]

☀️ Test successful - checks-azure
Approved by: sfackler
Pushing 1d5f6d4 to master...

[rust-highfive]

📣 Toolstate changed by #68165!

Tested on commit 1d5f6d4.
Direct link to PR: #68165

🎉 rustc-guide on linux: test-fail → test-pass (cc @JohnTitor @amanjeev @spastorino @mark-i-m, @rust-lang/infra).