bytes inside implicitly const-promoted expressions are immutable #1554

RalfJung · 2024-08-02T12:42:46Z

This needs to be stated explicitly due to rust-lang/unsafe-code-guidelines#493: with code like

let x: &'static Option<Cell<i32>> = &None;

it is currently UB to write to this memory, even though there is an UnsafeCell. This is because const-promotion notices that the value has no UnsafeCell and therefore places it in read-only global memory. However, our aliasing rules for interior mutability are likely not going to do such value-based reasoning, so we need to explicitly say that const-promotion is allowed to make things read-only here -- that's what this PR does.

Given that we have not yet said what exactly our aliasing model is, this could be considered a NOP for now. But it doesn't hurt to be explicit, and it seems likely that will eventually turn into a meaningful distinction.

Cc @rust-lang/opsem

traviscross · 2024-08-20T23:45:53Z

src/behavior-considered-undefined.md

+  Moreover, the bytes [pointed to] by a shared reference, including transitively through other references (both shared and mutable) and `Box`es, are immutable;
+  transitivity includes those references stored in fields of compound types.


Let's remove this from the diff if there's not meant to be any change here (it appears that just a line break was added to this otherwise unchanged language).

src/behavior-considered-undefined.md

Update books ## rust-lang/book 4 commits in 04bc1396bb857f35b5dda1d773c9571e1f253304..e7d217be2a75ef1753f0988d6ccaba4d7e376259 2024-08-14 01:19:47 UTC to 2024-08-13 16:51:00 UTC - Backport/forward port ch12 (rust-lang/book#4008) - Found some more things to fix in ch7; I forgot to update the snapshot (rust-lang/book#4007) - Remove redundant sentence. Send to nostarch (rust-lang/book#4006) - Fix: typo (rust-lang/book#4003) ## rust-lang/edition-guide 5 commits in aeeb287d41a0332c210da122bea8e0e91844ab3e..eeba2cb9c37ab74118a4fb5e5233f7397e4a91f8 2024-08-19 23:28:06 UTC to 2024-08-15 15:12:33 UTC - 2024: Add rustdoc combined doctests (rust-lang/edition-guide#320) - Update for unsafe attributes stabilization (rust-lang/edition-guide#319) - 2024: Add macro-fragment-specifiers. (rust-lang/edition-guide#312) - Fix deprecated_safe_2024 link (rust-lang/edition-guide#317) - Add 2024 unsafe functions (rust-lang/edition-guide#304) ## rust-embedded/book 1 commits in 019f3928d8b939ec71b63722dcc2e46330156441..ff5d61d56f11e1986bfa9652c6aff7731576c37d 2024-08-20 07:26:19 UTC to 2024-08-20 07:26:19 UTC - Use aligned address to demonstrate HardFault (rust-embedded/book#374) ## rust-lang/nomicon 1 commits in 6ecf95c5f2bfa0e6314dfe282bf775fd1405f7e9..14649f15d232d509478206ee9ed5105641aa60d0 2024-08-14 14:49:09 UTC to 2024-08-14 14:49:09 UTC - CI: Switch to merge queue (rust-lang/nomicon#459) ## rust-lang/reference 14 commits in 62cd0df95061ba0ac886333f5cd7f3012f149da1..0668397076da350c404dadcf07b6cbc433ad3743 2024-08-11 21:06:12 +0000 to 2024-08-27 21:47:20 +0000 - Update enum.md (rust-lang/reference#1354) - Be consistent about how "Edition differences" is capitalized (rust-lang/reference#1586) - Sync denied lints with upstream (rust-lang/reference#1589) - const_eval: update for const-fn float stabilization (rust-lang/reference#1566) - Add spec identifier syntax to destructors.md (rust-lang/reference#1571) - Say that `pub(in path)` can't depend on `use` statements (rust-lang/reference#1559) - bytes inside implicitly const-promoted expressions are immutable (rust-lang/reference#1554) - Tweak `repr(transparent)` to mention requiring *at most* one non-1-ZST (rust-lang/reference#1568) - operator expressions: add &raw (rust-lang/reference#1567) - Rewrite the automatic std link translation, and switch to automatic links (rust-lang/reference#1578) - Add some basic docs for unsafe attrs (rust-lang/reference#1539) - don't capitalize Undefined Behavior (rust-lang/reference#1575) - add the `const` operand to docs for inline assembly (rust-lang/reference#1556) - Typo: 'a' to 'an' in type-coercions.md (rust-lang/reference#1572) ## rust-lang/rust-by-example 1 commits in 8f94061936e492159f4f6c09c0f917a7521893ff..859786c5bc99301bbc22fc631a5c2b341860da08 2024-08-26 10:30:48 UTC to 2024-08-26 10:30:48 UTC - Update primitives.md with examples (rust-lang/rust-by-example#1878) ## rust-lang/rustc-dev-guide 7 commits in 43d83780db545a1ed6d45773312fc578987e3968..fa928a6d19e1666d8d811dfe3fd35cdad3b4e459 2024-08-26 14:46:50 UTC to 2024-08-12 21:07:49 UTC - Fix x.py reference (rust-lang/rustc-dev-guide#2049) - Update `stabilization_guide.md` (rust-lang/rustc-dev-guide#2034) - Explain the internal `#[rustc_*]` TEST attributes used for debugging and inside tests (rust-lang/rustc-dev-guide#2046) - missing char (rust-lang/rustc-dev-guide#2047) - Replace direct http links to rustc-dev-guide.rust-lang.org (rust-lang/rustc-dev-guide#2044) - Update index.html, 39. The MIR: fix typo (rust-lang/rustc-dev-guide#2043) - Update LLVM docs (rust-lang/rustc-dev-guide#2039)

Rollup merge of rust-lang#129617 - rustbot:docs-update, r=ehuss Update books ## rust-lang/book 4 commits in 04bc1396bb857f35b5dda1d773c9571e1f253304..e7d217be2a75ef1753f0988d6ccaba4d7e376259 2024-08-14 01:19:47 UTC to 2024-08-13 16:51:00 UTC - Backport/forward port ch12 (rust-lang/book#4008) - Found some more things to fix in ch7; I forgot to update the snapshot (rust-lang/book#4007) - Remove redundant sentence. Send to nostarch (rust-lang/book#4006) - Fix: typo (rust-lang/book#4003) ## rust-lang/edition-guide 5 commits in aeeb287d41a0332c210da122bea8e0e91844ab3e..eeba2cb9c37ab74118a4fb5e5233f7397e4a91f8 2024-08-19 23:28:06 UTC to 2024-08-15 15:12:33 UTC - 2024: Add rustdoc combined doctests (rust-lang/edition-guide#320) - Update for unsafe attributes stabilization (rust-lang/edition-guide#319) - 2024: Add macro-fragment-specifiers. (rust-lang/edition-guide#312) - Fix deprecated_safe_2024 link (rust-lang/edition-guide#317) - Add 2024 unsafe functions (rust-lang/edition-guide#304) ## rust-embedded/book 1 commits in 019f3928d8b939ec71b63722dcc2e46330156441..ff5d61d56f11e1986bfa9652c6aff7731576c37d 2024-08-20 07:26:19 UTC to 2024-08-20 07:26:19 UTC - Use aligned address to demonstrate HardFault (rust-embedded/book#374) ## rust-lang/nomicon 1 commits in 6ecf95c5f2bfa0e6314dfe282bf775fd1405f7e9..14649f15d232d509478206ee9ed5105641aa60d0 2024-08-14 14:49:09 UTC to 2024-08-14 14:49:09 UTC - CI: Switch to merge queue (rust-lang/nomicon#459) ## rust-lang/reference 14 commits in 62cd0df95061ba0ac886333f5cd7f3012f149da1..0668397076da350c404dadcf07b6cbc433ad3743 2024-08-11 21:06:12 +0000 to 2024-08-27 21:47:20 +0000 - Update enum.md (rust-lang/reference#1354) - Be consistent about how "Edition differences" is capitalized (rust-lang/reference#1586) - Sync denied lints with upstream (rust-lang/reference#1589) - const_eval: update for const-fn float stabilization (rust-lang/reference#1566) - Add spec identifier syntax to destructors.md (rust-lang/reference#1571) - Say that `pub(in path)` can't depend on `use` statements (rust-lang/reference#1559) - bytes inside implicitly const-promoted expressions are immutable (rust-lang/reference#1554) - Tweak `repr(transparent)` to mention requiring *at most* one non-1-ZST (rust-lang/reference#1568) - operator expressions: add &raw (rust-lang/reference#1567) - Rewrite the automatic std link translation, and switch to automatic links (rust-lang/reference#1578) - Add some basic docs for unsafe attrs (rust-lang/reference#1539) - don't capitalize Undefined Behavior (rust-lang/reference#1575) - add the `const` operand to docs for inline assembly (rust-lang/reference#1556) - Typo: 'a' to 'an' in type-coercions.md (rust-lang/reference#1572) ## rust-lang/rust-by-example 1 commits in 8f94061936e492159f4f6c09c0f917a7521893ff..859786c5bc99301bbc22fc631a5c2b341860da08 2024-08-26 10:30:48 UTC to 2024-08-26 10:30:48 UTC - Update primitives.md with examples (rust-lang/rust-by-example#1878) ## rust-lang/rustc-dev-guide 7 commits in 43d83780db545a1ed6d45773312fc578987e3968..fa928a6d19e1666d8d811dfe3fd35cdad3b4e459 2024-08-26 14:46:50 UTC to 2024-08-12 21:07:49 UTC - Fix x.py reference (rust-lang/rustc-dev-guide#2049) - Update `stabilization_guide.md` (rust-lang/rustc-dev-guide#2034) - Explain the internal `#[rustc_*]` TEST attributes used for debugging and inside tests (rust-lang/rustc-dev-guide#2046) - missing char (rust-lang/rustc-dev-guide#2047) - Replace direct http links to rustc-dev-guide.rust-lang.org (rust-lang/rustc-dev-guide#2044) - Update index.html, 39. The MIR: fix typo (rust-lang/rustc-dev-guide#2043) - Update LLVM docs (rust-lang/rustc-dev-guide#2039)

Update books ## rust-lang/book 4 commits in 04bc1396bb857f35b5dda1d773c9571e1f253304..e7d217be2a75ef1753f0988d6ccaba4d7e376259 2024-08-14 01:19:47 UTC to 2024-08-13 16:51:00 UTC - Backport/forward port ch12 (rust-lang/book#4008) - Found some more things to fix in ch7; I forgot to update the snapshot (rust-lang/book#4007) - Remove redundant sentence. Send to nostarch (rust-lang/book#4006) - Fix: typo (rust-lang/book#4003) ## rust-lang/edition-guide 5 commits in aeeb287d41a0332c210da122bea8e0e91844ab3e..eeba2cb9c37ab74118a4fb5e5233f7397e4a91f8 2024-08-19 23:28:06 UTC to 2024-08-15 15:12:33 UTC - 2024: Add rustdoc combined doctests (rust-lang/edition-guide#320) - Update for unsafe attributes stabilization (rust-lang/edition-guide#319) - 2024: Add macro-fragment-specifiers. (rust-lang/edition-guide#312) - Fix deprecated_safe_2024 link (rust-lang/edition-guide#317) - Add 2024 unsafe functions (rust-lang/edition-guide#304) ## rust-embedded/book 1 commits in 019f3928d8b939ec71b63722dcc2e46330156441..ff5d61d56f11e1986bfa9652c6aff7731576c37d 2024-08-20 07:26:19 UTC to 2024-08-20 07:26:19 UTC - Use aligned address to demonstrate HardFault (rust-embedded/book#374) ## rust-lang/nomicon 1 commits in 6ecf95c5f2bfa0e6314dfe282bf775fd1405f7e9..14649f15d232d509478206ee9ed5105641aa60d0 2024-08-14 14:49:09 UTC to 2024-08-14 14:49:09 UTC - CI: Switch to merge queue (rust-lang/nomicon#459) ## rust-lang/reference 14 commits in 62cd0df95061ba0ac886333f5cd7f3012f149da1..0668397076da350c404dadcf07b6cbc433ad3743 2024-08-11 21:06:12 +0000 to 2024-08-27 21:47:20 +0000 - Update enum.md (rust-lang/reference#1354) - Be consistent about how "Edition differences" is capitalized (rust-lang/reference#1586) - Sync denied lints with upstream (rust-lang/reference#1589) - const_eval: update for const-fn float stabilization (rust-lang/reference#1566) - Add spec identifier syntax to destructors.md (rust-lang/reference#1571) - Say that `pub(in path)` can't depend on `use` statements (rust-lang/reference#1559) - bytes inside implicitly const-promoted expressions are immutable (rust-lang/reference#1554) - Tweak `repr(transparent)` to mention requiring *at most* one non-1-ZST (rust-lang/reference#1568) - operator expressions: add &raw (rust-lang/reference#1567) - Rewrite the automatic std link translation, and switch to automatic links (rust-lang/reference#1578) - Add some basic docs for unsafe attrs (rust-lang/reference#1539) - don't capitalize Undefined Behavior (rust-lang/reference#1575) - add the `const` operand to docs for inline assembly (rust-lang/reference#1556) - Typo: 'a' to 'an' in type-coercions.md (rust-lang/reference#1572) ## rust-lang/rust-by-example 1 commits in 8f94061936e492159f4f6c09c0f917a7521893ff..859786c5bc99301bbc22fc631a5c2b341860da08 2024-08-26 10:30:48 UTC to 2024-08-26 10:30:48 UTC - Update primitives.md with examples (rust-lang/rust-by-example#1878) ## rust-lang/rustc-dev-guide 7 commits in 43d83780db545a1ed6d45773312fc578987e3968..fa928a6d19e1666d8d811dfe3fd35cdad3b4e459 2024-08-26 14:46:50 UTC to 2024-08-12 21:07:49 UTC - Fix x.py reference (rust-lang/rustc-dev-guide#2049) - Update `stabilization_guide.md` (rust-lang/rustc-dev-guide#2034) - Explain the internal `#[rustc_*]` TEST attributes used for debugging and inside tests (rust-lang/rustc-dev-guide#2046) - missing char (rust-lang/rustc-dev-guide#2047) - Replace direct http links to rustc-dev-guide.rust-lang.org (rust-lang/rustc-dev-guide#2044) - Update index.html, 39. The MIR: fix typo (rust-lang/rustc-dev-guide#2043) - Update LLVM docs (rust-lang/rustc-dev-guide#2039)

RalfJung mentioned this pull request Aug 2, 2024

const-eval interning: accept interior mutable pointers in final value rust-lang/rust#128543

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bytes inside implicitly const-promoted expressions are immutable

ff494bf

RalfJung force-pushed the promoted-immutable branch from 4cb1985 to ff494bf Compare August 21, 2024 06:31

traviscross approved these changes Aug 22, 2024

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Merged via the queue into rust-lang:master with commit 3d7ee4f Aug 22, 2024
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Update books rust-lang/rust#129617

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bytes inside implicitly const-promoted expressions are immutable #1554

bytes inside implicitly const-promoted expressions are immutable #1554

RalfJung commented Aug 2, 2024 •

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traviscross Aug 20, 2024 •

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		Moreover, the bytes [pointed to] by a shared reference, including transitively through other references (both shared and mutable) and `Box`es, are immutable;
		transitivity includes those references stored in fields of compound types.

bytes inside implicitly const-promoted expressions are immutable #1554

bytes inside implicitly const-promoted expressions are immutable #1554

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