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Stabilize std::hint::unreachable_unchecked. #49906

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merged 1 commit into from
Apr 24, 2018
Merged

Stabilize std::hint::unreachable_unchecked. #49906

merged 1 commit into from
Apr 24, 2018
+71 −16

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kennytm
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@kennytm kennytm commented Apr 12, 2018

Closes #43751.

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r? @Mark-Simulacrum

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@rust-highfive rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Apr 12, 2018
@kennytm kennytm added the relnotes Marks issues that should be documented in the release notes of the next release. label Apr 12, 2018
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r? @sfackler

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Maybe the docs should point out that undefined behaviour can occur if runtime could reach the point that was marked unreachable_unchecked, not just if. That is, the program might behave weirdly even if it does not reach the unreachable part, because the compiler made assumptions somewhere else.

It's a nuance in wording, yet people have been banging their heads with "catching" code reaching unreachable parts while debugging random segfaults: It's spooky action at a distance.

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It's certainly worth reminding people of that since it's often not untuitive, but I want to be clear that this is not specific to unreachable: all Undefined Behavior is "time travelling" in the sense that a program that eventually executes UB has never had any meaning to begin with and thus implementations don't have to try and faithfully implement the part of the program trace before the UB operation. This is worth stressing, but if it's stressed in the documentation of one particular function, the docs should be careful to not imply that this is specific to said function.

@kennytm kennytm force-pushed the stable-unreachable branch from ab012e9 to 5fe8c59 Compare April 16, 2018 10:30
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Ping from triage @sfackler! This PR needs your review!

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@bors r+

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bors commented Apr 23, 2018

📌 Commit 5fe8c59 has been approved by sfackler

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 23, 2018
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aturon commented Apr 23, 2018

@bors: rollup

kennytm added a commit to kennytm/rust that referenced this pull request Apr 24, 2018
@kennytm
Rollup merge of rust-lang#49906 - kennytm:stable-unreachable, r=sfackler
f28f5aa 
Stabilize `std::hint::unreachable_unchecked`.

Closes rust-lang#43751.
@kennytm kennytm mentioned this pull request Apr 24, 2018
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bors added a commit that referenced this pull request Apr 24, 2018
@bors
Auto merge of #50191 - kennytm:rollup, r=kennytm
52ed3d8 
Rollup of 11 pull requests

Successful merges:

 - #49461 (std: Child::kill() returns error if process has already exited)
 - #49727 (Add Cell::update)
 - #49812 (Fix revision support for UI tests.)
 - #49829 (Add doc links to `std::os` extension traits)
 - #49906 (Stabilize `std::hint::unreachable_unchecked`.)
 - #49970 (Deprecate Read::chars and char::decode_utf8)
 - #49985 (don't see issue #0)
 - #50118 (fix search bar bug)
 - #50139 (encourage descriptive issue titles)
 - #50174 (Use FxHashMap in syntax_pos::symbol::Interner::intern.)
 - #50185 (core: Fix overflow in `int::mod_euc` when `self < 0 && rhs == MIN`)

Failed merges:
@bors bors merged commit 5fe8c59 into rust-lang:master Apr 24, 2018
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@kennytm @rust-highfive @Mark-Simulacrum @lukaslueg @hanna-kruppe @pietroalbini @sfackler @bors @aturon