Fix zero-size uninitialized boxes

[SimonSapin]

Requesting a zero-size allocation is not allowed, return a dangling pointer instead.

CC #63291 (comment)

[rust-highfive]

r? @alexcrichton

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[alexcrichton]

Looks good to me, thanks! Could this perhaps though check the alloc.size() calculation instead of recalculating the size of T and factoring in the length? Other than that though r=me with some tests

[JohnCSimon]

Ping from triage
@SimonSapin can you address @alexcrichton 's requests?
Thanks

[SimonSapin]

Done. I’ll take the liberty to carry Alex’s “r=me” from previous comment:

@bors r=alexcrichton

[bors]

📌 Commit 352b026 has been approved by alexcrichton

[rust-highfive]

The job x86_64-gnu-llvm-6.0 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.

2019-10-16T18:34:54.3912255Z ##[command]git remote add origin https://github.com/rust-lang/rust
2019-10-16T18:34:54.4098898Z ##[command]git config gc.auto 0
2019-10-16T18:34:54.4185459Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2019-10-16T18:34:54.4250759Z ##[command]git config --get-all http.proxy
2019-10-16T18:34:55.0155008Z ##[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/65174/merge:refs/remotes/pull/65174/merge
---
2019-10-16T18:42:14.4937529Z    Compiling serde_json v1.0.40
2019-10-16T18:42:16.4223505Z    Compiling tidy v0.1.0 (/checkout/src/tools/tidy)
2019-10-16T18:42:28.6972252Z     Finished release [optimized] target(s) in 1m 36s
2019-10-16T18:42:28.7052623Z tidy check
2019-10-16T18:42:28.8372721Z tidy error: /checkout/src/liballoc/tests/boxed.rs: too many trailing newlines (2)
2019-10-16T18:42:30.8079203Z some tidy checks failed
2019-10-16T18:42:30.8079301Z Found 482 error codes
2019-10-16T18:42:30.8079355Z Found 0 error codes with no tests
2019-10-16T18:42:30.8079607Z Done!
2019-10-16T18:42:30.8079607Z Done!
2019-10-16T18:42:30.8079638Z 
2019-10-16T18:42:30.8079667Z 
2019-10-16T18:42:30.8080535Z command did not execute successfully: "/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"
2019-10-16T18:42:30.8080673Z 
2019-10-16T18:42:30.8080700Z 
2019-10-16T18:42:30.8086476Z failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
2019-10-16T18:42:30.8087326Z Build completed unsuccessfully in 0:01:39
2019-10-16T18:42:30.8087326Z Build completed unsuccessfully in 0:01:39
2019-10-16T18:42:30.8137988Z == clock drift check ==
2019-10-16T18:42:30.8160231Z   local time: Wed Oct 16 18:42:30 UTC 2019
2019-10-16T18:42:30.9049248Z   network time: Wed, 16 Oct 2019 18:42:30 GMT
2019-10-16T18:42:30.9052471Z == end clock drift check ==
2019-10-16T18:42:32.2598861Z ##[error]Bash exited with code '1'.
2019-10-16T18:42:32.2633494Z ##[section]Starting: Checkout
2019-10-16T18:42:32.2635221Z ==============================================================================
2019-10-16T18:42:32.2635296Z Task         : Get sources
2019-10-16T18:42:32.2635347Z 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)

[RalfJung]

Dang, I missed that when reviewing the original patch. :/

Do the Rc/Arc uninit constructors by any chance have similar issues?

[SimonSapin]

Heap allocations for Rc or Arc are never zero-size since they contain at least the refcounts

[Centril]

Failed in #65482, @bors r-

[SimonSapin]

Removed the extra trailing newline, and ran a full ./x.py test

@bors r=alexcrichton

[bors]

📌 Commit 227db40 has been approved by alexcrichton