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avoid shared ref in UnsafeCell::get #56012

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RalfJung commented Nov 16, 2018

Avoid taking a shared reference in UnsafeCell::get. This should be taking a raw reference (see rust-lang/rfcs#2582), but that operation is not currently available, so I propose we exploit repr(transparent) instead and cast the pointer around.

This is required to make UnsafeCell::get pass the stacked borrows implementation in miri (currently, UnsafeCell::get is on a whitelist, but that is of course not very satisfying). It shouldn't affect normal execution/codegen. Would be great if we could get this landed and shrink miri's whitelist!

Cc @nikomatsakis

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rust-highfive commented Nov 16, 2018

r? @joshtriplett

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Centril commented Nov 16, 2018

Seems trivially correct to me due to #[repr(transparent)] but it don't hurt to be extra careful so...
r? @nikomatsakis

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Ha, what a glorious hack.

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nikomatsakis commented Nov 16, 2018

@bors r+

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bors commented Nov 16, 2018

📌 Commit 41434e0 has been approved by nikomatsakis

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RalfJung commented Nov 17, 2018

I added a comment referring to repr(transparent)

@bors r=nikomatsakis

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bors commented Nov 17, 2018

📌 Commit 25d46f3 has been approved by nikomatsakis

kennytm added a commit to kennytm/rust that referenced this pull request Nov 17, 2018

Rollup merge of rust-lang#56012 - RalfJung:unsafe-cell, r=nikomatsakis
avoid shared ref in UnsafeCell::get

Avoid taking a shared reference in `UnsafeCell::get`. This *should* be taking a raw reference (see rust-lang/rfcs#2582), but that operation is not currently available, so I propose we exploit `repr(transparent)` instead and cast the pointer around.

This is required to make `UnsafeCell::get` pass the [stacked borrows implementation](https://www.ralfj.de/blog/2018/11/16/stacked-borrows-implementation.html) in miri (currently, `UnsafeCell::get` is on a whitelist, but that is of course not very satisfying). It shouldn't affect normal execution/codegen. Would be great if we could get this landed and shrink miri's whitelist!

Cc @nikomatsakis

bors added a commit that referenced this pull request Nov 17, 2018

Auto merge of #56026 - kennytm:rollup, r=kennytm
Rollup of 17 pull requests

Successful merges:

 - #55564 (test/linkage-visibility: Ignore on musl targets)
 - #55672 (miri: accept extern types in structs if they are the only field)
 - #55767 (Disable some pretty-printers when gdb is rust-enabled)
 - #55827 (A few tweaks to iterations/collecting)
 - #55834 (Forward the ABI of the non-zero sized fields of an union if they have the same ABI)
 - #55857 (remove unused dependency)
 - #55867 (do not panic just because cargo failed)
 - #55916 (Make miri value visitor usfeful for mutation)
 - #55919 (core/tests/num: Simplify `test_int_from_str_overflow()` test code)
 - #55949 (ty: return impl Iterator from Predicate::walk_tys)
 - #55953 (#53488 Refactoring UpvarId)
 - #55962 (rustdoc: properly calculate spans for intra-doc link resolution errors)
 - #55963 (Stress test for MPSC)
 - #55999 (Update Cargo submodule)
 - #56011 (Replace data.clone() by Arc::clone(&data) in mutex doc.)
 - #56012 (avoid shared ref in UnsafeCell::get)
 - #56017 (std: Add debugging for a failing test on appveyor)

Failed merges:

 - #56010 (fix intra-link resolution spans in block comments)

r? @ghost

pietroalbini added a commit to pietroalbini/rust that referenced this pull request Nov 18, 2018

Rollup merge of rust-lang#56012 - RalfJung:unsafe-cell, r=nikomatsakis
avoid shared ref in UnsafeCell::get

Avoid taking a shared reference in `UnsafeCell::get`. This *should* be taking a raw reference (see rust-lang/rfcs#2582), but that operation is not currently available, so I propose we exploit `repr(transparent)` instead and cast the pointer around.

This is required to make `UnsafeCell::get` pass the [stacked borrows implementation](https://www.ralfj.de/blog/2018/11/16/stacked-borrows-implementation.html) in miri (currently, `UnsafeCell::get` is on a whitelist, but that is of course not very satisfying). It shouldn't affect normal execution/codegen. Would be great if we could get this landed and shrink miri's whitelist!

Cc @nikomatsakis

bors added a commit that referenced this pull request Nov 19, 2018

Auto merge of #56051 - pietroalbini:rollup, r=pietroalbini
Rollup of 25 pull requests

Successful merges:

 - #55562 (Add powerpc- and powerpc64-unknown-linux-musl targets)
 - #55564 (test/linkage-visibility: Ignore on musl targets)
 - #55827 (A few tweaks to iterations/collecting)
 - #55834 (Forward the ABI of the non-zero sized fields of an union if they have the same ABI)
 - #55857 (remove unused dependency)
 - #55862 (in which the E0618 "expected function" diagnostic gets a makeover)
 - #55867 (do not panic just because cargo failed)
 - #55894 (miri enum discriminant handling: Fix treatment of pointers, better error when it is undef)
 - #55916 (Make miri value visitor usfeful for mutation)
 - #55919 (core/tests/num: Simplify `test_int_from_str_overflow()` test code)
 - #55923 (reword #[test] attribute error on fn items)
 - #55935 (appveyor: Use VS2017 for all our images)
 - #55949 (ty: return impl Iterator from Predicate::walk_tys)
 - #55952 (Update to Clang 7 on CI.)
 - #55953 (#53488 Refactoring UpvarId)
 - #55962 (rustdoc: properly calculate spans for intra-doc link resolution errors)
 - #55963 (Stress test for MPSC)
 - #55968 (Clean up some non-mod-rs stuff.)
 - #55970 (Miri backtrace improvements)
 - #56007 (CTFE: dynamically make sure we do not call non-const-fn)
 - #56011 (Replace data.clone() by Arc::clone(&data) in mutex doc.)
 - #56012 (avoid shared ref in UnsafeCell::get)
 - #56016 (Add VecDeque::resize_with)
 - #56027 (docs: Add missing backtick in object_safety.rs docs)
 - #56043 (remove "approx env bounds" if we already know from trait)

Failed merges:

r? @ghost

bors added a commit that referenced this pull request Nov 19, 2018

Auto merge of #56051 - pietroalbini:rollup, r=pietroalbini
Rollup of 25 pull requests

Successful merges:

 - #55562 (Add powerpc- and powerpc64-unknown-linux-musl targets)
 - #55564 (test/linkage-visibility: Ignore on musl targets)
 - #55827 (A few tweaks to iterations/collecting)
 - #55834 (Forward the ABI of the non-zero sized fields of an union if they have the same ABI)
 - #55857 (remove unused dependency)
 - #55862 (in which the E0618 "expected function" diagnostic gets a makeover)
 - #55867 (do not panic just because cargo failed)
 - #55894 (miri enum discriminant handling: Fix treatment of pointers, better error when it is undef)
 - #55916 (Make miri value visitor usfeful for mutation)
 - #55919 (core/tests/num: Simplify `test_int_from_str_overflow()` test code)
 - #55923 (reword #[test] attribute error on fn items)
 - #55935 (appveyor: Use VS2017 for all our images)
 - #55949 (ty: return impl Iterator from Predicate::walk_tys)
 - #55952 (Update to Clang 7 on CI.)
 - #55953 (#53488 Refactoring UpvarId)
 - #55962 (rustdoc: properly calculate spans for intra-doc link resolution errors)
 - #55963 (Stress test for MPSC)
 - #55968 (Clean up some non-mod-rs stuff.)
 - #55970 (Miri backtrace improvements)
 - #56007 (CTFE: dynamically make sure we do not call non-const-fn)
 - #56011 (Replace data.clone() by Arc::clone(&data) in mutex doc.)
 - #56012 (avoid shared ref in UnsafeCell::get)
 - #56016 (Add VecDeque::resize_with)
 - #56027 (docs: Add missing backtick in object_safety.rs docs)
 - #56043 (remove "approx env bounds" if we already know from trait)

Failed merges:

r? @ghost

bors added a commit that referenced this pull request Nov 19, 2018

Auto merge of #56051 - pietroalbini:rollup, r=pietroalbini
Rollup of 26 pull requests

Successful merges:

 - #55562 (Add powerpc- and powerpc64-unknown-linux-musl targets)
 - #55564 (test/linkage-visibility: Ignore on musl targets)
 - #55827 (A few tweaks to iterations/collecting)
 - #55834 (Forward the ABI of the non-zero sized fields of an union if they have the same ABI)
 - #55857 (remove unused dependency)
 - #55862 (in which the E0618 "expected function" diagnostic gets a makeover)
 - #55867 (do not panic just because cargo failed)
 - #55894 (miri enum discriminant handling: Fix treatment of pointers, better error when it is undef)
 - #55916 (Make miri value visitor useful for mutation)
 - #55919 (core/tests/num: Simplify `test_int_from_str_overflow()` test code)
 - #55923 (reword #[test] attribute error on fn items)
 - #55935 (appveyor: Use VS2017 for all our images)
 - #55949 (ty: return impl Iterator from Predicate::walk_tys)
 - #55952 (Update to Clang 7 on CI.)
 - #55953 (#53488 Refactoring UpvarId)
 - #55962 (rustdoc: properly calculate spans for intra-doc link resolution errors)
 - #55963 (Stress test for MPSC)
 - #55968 (Clean up some non-mod-rs stuff.)
 - #55970 (Miri backtrace improvements)
 - #56007 (CTFE: dynamically make sure we do not call non-const-fn)
 - #56011 (Replace data.clone() by Arc::clone(&data) in mutex doc.)
 - #56012 (avoid shared ref in UnsafeCell::get)
 - #56016 (Add VecDeque::resize_with)
 - #56027 (docs: Add missing backtick in object_safety.rs docs)
 - #56043 (remove "approx env bounds" if we already know from trait)
 - #56059 (Increase `Duration` approximate equal threshold to 1us)

bors added a commit that referenced this pull request Nov 19, 2018

Auto merge of #56051 - pietroalbini:rollup, r=pietroalbini
Rollup of 26 pull requests

Successful merges:

 - #55562 (Add powerpc- and powerpc64-unknown-linux-musl targets)
 - #55564 (test/linkage-visibility: Ignore on musl targets)
 - #55827 (A few tweaks to iterations/collecting)
 - #55834 (Forward the ABI of the non-zero sized fields of an union if they have the same ABI)
 - #55857 (remove unused dependency)
 - #55862 (in which the E0618 "expected function" diagnostic gets a makeover)
 - #55867 (do not panic just because cargo failed)
 - #55894 (miri enum discriminant handling: Fix treatment of pointers, better error when it is undef)
 - #55916 (Make miri value visitor useful for mutation)
 - #55919 (core/tests/num: Simplify `test_int_from_str_overflow()` test code)
 - #55923 (reword #[test] attribute error on fn items)
 - #55935 (appveyor: Use VS2017 for all our images)
 - #55949 (ty: return impl Iterator from Predicate::walk_tys)
 - #55952 (Update to Clang 7 on CI.)
 - #55953 (#53488 Refactoring UpvarId)
 - #55962 (rustdoc: properly calculate spans for intra-doc link resolution errors)
 - #55963 (Stress test for MPSC)
 - #55968 (Clean up some non-mod-rs stuff.)
 - #55970 (Miri backtrace improvements)
 - #56007 (CTFE: dynamically make sure we do not call non-const-fn)
 - #56011 (Replace data.clone() by Arc::clone(&data) in mutex doc.)
 - #56012 (avoid shared ref in UnsafeCell::get)
 - #56016 (Add VecDeque::resize_with)
 - #56027 (docs: Add missing backtick in object_safety.rs docs)
 - #56043 (remove "approx env bounds" if we already know from trait)
 - #56059 (Increase `Duration` approximate equal threshold to 1us)

kennytm added a commit to pietroalbini/rust that referenced this pull request Nov 19, 2018

Rollup merge of rust-lang#56012 - RalfJung:unsafe-cell, r=nikomatsakis
avoid shared ref in UnsafeCell::get

Avoid taking a shared reference in `UnsafeCell::get`. This *should* be taking a raw reference (see rust-lang/rfcs#2582), but that operation is not currently available, so I propose we exploit `repr(transparent)` instead and cast the pointer around.

This is required to make `UnsafeCell::get` pass the [stacked borrows implementation](https://www.ralfj.de/blog/2018/11/16/stacked-borrows-implementation.html) in miri (currently, `UnsafeCell::get` is on a whitelist, but that is of course not very satisfying). It shouldn't affect normal execution/codegen. Would be great if we could get this landed and shrink miri's whitelist!

Cc @nikomatsakis

bors added a commit that referenced this pull request Nov 19, 2018

Auto merge of #56051 - pietroalbini:rollup, r=pietroalbini
Rollup of 25 pull requests

Successful merges:

 - #55562 (Add powerpc- and powerpc64-unknown-linux-musl targets)
 - #55564 (test/linkage-visibility: Ignore on musl targets)
 - #55827 (A few tweaks to iterations/collecting)
 - #55834 (Forward the ABI of the non-zero sized fields of an union if they have the same ABI)
 - #55857 (remove unused dependency)
 - #55862 (in which the E0618 "expected function" diagnostic gets a makeover)
 - #55867 (do not panic just because cargo failed)
 - #55894 (miri enum discriminant handling: Fix treatment of pointers, better error when it is undef)
 - #55916 (Make miri value visitor useful for mutation)
 - #55919 (core/tests/num: Simplify `test_int_from_str_overflow()` test code)
 - #55923 (reword #[test] attribute error on fn items)
 - #55949 (ty: return impl Iterator from Predicate::walk_tys)
 - #55952 (Update to Clang 7 on CI.)
 - #55953 (#53488 Refactoring UpvarId)
 - #55962 (rustdoc: properly calculate spans for intra-doc link resolution errors)
 - #55963 (Stress test for MPSC)
 - #55968 (Clean up some non-mod-rs stuff.)
 - #55970 (Miri backtrace improvements)
 - #56007 (CTFE: dynamically make sure we do not call non-const-fn)
 - #56011 (Replace data.clone() by Arc::clone(&data) in mutex doc.)
 - #56012 (avoid shared ref in UnsafeCell::get)
 - #56016 (Add VecDeque::resize_with)
 - #56027 (docs: Add missing backtick in object_safety.rs docs)
 - #56043 (remove "approx env bounds" if we already know from trait)
 - #56059 (Increase `Duration` approximate equal threshold to 1us)

@bors bors merged commit 25d46f3 into rust-lang:master Nov 19, 2018

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@RalfJung RalfJung deleted the RalfJung:unsafe-cell branch Nov 30, 2018

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