Prepare for being included via crates.io into std

[alexcrichton]

This commit prepares the libc crate to be included directly into the
standard library via crates.io. More details about this can be found on
rust-lang/rust#56092, but the main idea is that this crate now depends
on core/compiler-builtins explicitly (but off-by-default).

The main caveat here is that this activates no_core when building as
part of libstd, which means that it needs to explicitly have an iter
and option module for the expansion of for loops to work.

[rust-highfive]

@alexcrichton: no appropriate reviewer found, use r? to override

[alexcrichton]

r? @gnzlbg

[gnzlbg]

LGTM.

[gnzlbg]

I don't know if this should enable use_std as well to be sure - it should be enabled by default anyways as long as libstd doesn't use default-features=false.

[alexcrichton]

Nah this specifically actually doesn't want use_std because std isn't built yet! (this'll be a dependency of std)

[gnzlbg]

The style checks are failing

[alexcrichton]

@bors: r=gnzlbg

[bors]

📌 Commit aa5304f has been approved by gnzlbg

[alexcrichton]

@bors: r=gnzlbg

[bors]

📌 Commit 05ff162 has been approved by gnzlbg

[bors]

🔒 Merge conflict

This pull request and the master branch diverged in a way that cannot be automatically merged. Please rebase on top of the latest master branch, and let the reviewer approve again.

How do I rebase?

Assuming self is your fork and upstream is this repository, you can resolve the conflict following these steps:

1. git checkout rustc-test (switch to your branch)
2. git fetch upstream master (retrieve the latest master)
3. git rebase upstream/master -p (rebase on top of it)
4. Follow the on-screen instruction to resolve conflicts (check git status if you got lost).
5. git push self rustc-test --force-with-lease (update this PR)

You may also read Git Rebasing to Resolve Conflicts by Drew Blessing for a short tutorial.

Please avoid the "Resolve conflicts" button on GitHub. It uses git merge instead of git rebase which makes the PR commit history more difficult to read.

Sometimes step 4 will complete without asking for resolution. This is usually due to difference between how Cargo.lock conflict is handled during merge and rebase. This is normal, and you should still perform step 5 to update this PR.

Error message

Auto-merging src/windows.rs
Auto-merging src/unix/mod.rs
Auto-merging src/lib.rs
CONFLICT (content): Merge conflict in src/lib.rs
Auto-merging libc-test/build.rs
Automatic merge failed; fix conflicts and then commit the result.

[bors]

☔ The latest upstream changes (presumably #1129) made this pull request unmergeable. Please resolve the merge conflicts.

[alexcrichton]

@bors: r=gnzlbg

[bors]

📌 Commit 91bd079 has been approved by gnzlbg

[bors]

⌛ Testing commit 91bd079 with merge 03310f8...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: gnzlbg
Pushing 03310f8 to master...