Reject huge alignments on macos with system allocator only

[glandium]

ef8804b addressed #30170 by rejecting
huge alignments at the allocator API level, transforming a specific
platform bug/limitation into an enforced API limitation on all
platforms.

This change essentially reverts that commit, and instead makes alloc()
itself return AllocErr::Unsupported when receiving huge alignments.

This was discussed in #32838 (comment)
and following.

[glandium]

Cc @SimonSapin

[TimNN]

Your PR failed on Travis. 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.

[00:00:46] configure: rust.quiet-tests     := True
---
[00:04:16] error: attributes are not yet allowed on `if` expressions
[00:04:16]    --> liballoc_system/lib.rs:137:17
[00:04:16]     |
[00:04:16] 137 |                 #[cfg(target_os = "macos")]
[00:04:16]     |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
[00:04:16]
[00:04:16] error: aborting due to previous error
[00:04:16]
[00:04:16] error: Could not compile `alloc_system`.
[00:04:16]
[00:04:16] Caused by:
[00:04:16]   process didn't exit successfully: `/checkout/obj/build/bootstrap/debug/rustc --crate-name alloc_system liballoc_system/lib.rs --color always --error-format json --crate-type lib --emit=dep-info,link -C opt-level=2 -C metadata=ed94ac9e95a953a9 -C extra-filename=-ed94ac9e95a953a9 --out-dir /checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/release/deps --target x86_64-unknown-linux-gnu -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/release/deps -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/release/deps --extern core=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/release/deps/libcore-811292bee4a6b310.rlib --extern libc=/checkout/obj/build/x86_64-unknown-linux-gnu/stageux-gnu/stage0/lib/rustlib/x86_64-unknown-linux-gnu
---
113580 ./obj/build/bootstrap/debug/incremental/bootstrap-1xj36c8o119z7/s-ezq968k9js-1lqr5ph-1qldka3x57wek

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.

[SimonSapin]

Looks good to me. I think we discussed doing exactly this, but just to be sure let’s get some more libs team eyes on it.

@rfcbot fcp merge

[rfcbot]

Team member @SimonSapin has proposed to merge this. The next step is review by the rest of the tagged teams:

• @Kimundi
• @SimonSapin
• @alexcrichton
• @aturon
• @dtolnay
• @sfackler
• @withoutboats

No concerns currently listed.

Once a majority of reviewers approve (and none object), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[alexcrichton]

@bors: r=SimonSapin

[bors]

📌 Commit 98175a8 has been approved by SimonSapin

[SimonSapin]

@bors: p=1

I’m working on large-ish changes that should go on top of this to avoid merge conflicts.

[bors]

⌛ Testing commit 98175a8 with merge 20338a5...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: SimonSapin
Pushing 20338a5 to master...