Clarifying how the alignment of the struct works

[Valloric]

The docs were not specifying how to compute the alignment of the struct, so I had to spend some time trying to figure out how that works. Found the answer on this page:

The total size of this struct is 5, but the most-aligned field is b with alignment 2, so we round up to 6 and give the struct an alignment of 2 bytes.

[rust-highfive]

r? @bluss

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[Valloric]

cc @steveklabnik

[rust-highfive]

The job x86_64-gnu-llvm-3.9 of your PR failed on Travis (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.

[00:04:47] travis_fold:start:tidy
travis_time:start:tidy
tidy check
[00:04:48] tidy error: /checkout/src/libcore/mem.rs:231: trailing whitespace
[00:04:49] some tidy checks failed
[00:04:49] 
[00:04:49] 
[00:04:49] 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" "--quiet"
[00:04:49] 
[00:04:49] 
[00:04:49] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:04:49] Build completed unsuccessfully in 0:01:44
[00:04:49] Build completed unsuccessfully in 0:01:44
[00:04:49] make: *** [tidy] Error 1
[00:04:49] Makefile:79: recipe for target 'tidy' failed

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:30c63490
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
---
travis_time:end:02723ba4:start=1530750794457176390,finish=1530750794464305509,duration=7129119
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:0b6026ae
$ head -30 ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
head: cannot open ‘./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers’ for reading: No such file or directory
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:05b37dff
$ dmesg | grep -i kill

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)

[hanna-kruppe]

This isn't the whole truth, one can increase the alignment requirements of a struct beyond what its fields require with repr(align(N)). (One can also lower it with repr(packed), but that also omits padding between fields, so the entire layout algorithm is rather different for repr(C, packed) structs).

[Valloric]

@rkruppe

This isn't the whole truth, one can increase the alignment requirements of a struct beyond what its fields require with repr(align(N)).

Mentioned this as well.

[pietroalbini]

Ping from triage @bluss! This PR needs your review.

[cramertj]

@bors r+ rollup

[bors]

📌 Commit dc425e2 has been approved by cramertj