Say that dereferencing a pointer to a ZST is no longer undefined #467
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
The new rules were tracked in rust-lang/rust#117945 The corresponding update to the Reference was rust-lang/reference#1541
RalfJung
reviewed
Oct 19, 2024
src/exotic-sizes.md
Outdated
| type. | ||
| references, must be non-null and suitably aligned. However, dereferencing a | ||
| null pointer to a ZST is not [undefined behavior][ub], unlike pointers to | ||
| other types. |
There was a problem hiding this comment.
In fact dereferencing (as in, using the * operator on) a null pointer is never UB. addr_of!(*ptr) is always safe, even if ptr is null.
Only non-zero-sized loads, non-zero-sized stores, and field projections at non-zero field offsets have any requirements.
There was a problem hiding this comment.
I've changed it to say "loading or storing through a null pointer to a ZST"
From discussion at https://rust-lang.zulipchat.com/#narrow/channel/136281-t-opsem/topic/What.20operations.20are.20allowed.20on.20ZST.20null.20pointers.3F - applying the dereference operator to a null pointer stopped being UB a few months ago - it's better to be explicit about whether we're talking about loads and stores or about applying the dereference operator I think we don't need to mention field projections here.
RalfJung
reviewed
Oct 28, 2024
| type. | ||
| references, must be non-null and suitably aligned. However, loading or storing | ||
| through a null pointer to a ZST is not [undefined behavior][ub], unlike | ||
| pointers to other types. |
There was a problem hiding this comment.
I think it would be less confusing to say something like "loading or storing through a null pointer is UB, unless the load/store has size 0". Let's talk about the common case first.
There was a problem hiding this comment.
The subject of this section is ZSTs, so "loading or storing through a null pointer to a ZST is not undefined behavior" is the information it's supplying, and "unlike pointers to other types" is an aside.
traviscross
approved these changes
Apr 15, 2025
|
The corresponding lang FCP was: |
ChrisDenton
added a commit
to ChrisDenton/rust
that referenced
this pull request
Apr 22, 2025
Update books ## rust-lang/nomicon 1 commits in 0c10c30cc54736c5c194ce98c50e2de84eeb6e79..c76a20f0d987145dcedf05c5c073ce8d91f2e82a 2025-04-15 20:54:57 UTC to 2025-04-15 20:54:57 UTC - Say that dereferencing a pointer to a ZST is no longer undefined (rust-lang/nomicon#467) ## rust-lang/reference 7 commits in 3340922df189bddcbaad17dc3927d51a76bcd5ed..3bf3402aea982b876eb56c87da17b0685c6461d5 2025-04-18 13:44:45 UTC to 2025-04-17 17:27:01 UTC - Use `cfg(false)` instead of `cfg(FALSE)` (rust-lang/reference#1763) - Add `cfg(true)` and `cfg(false)` to conditional compilation (RFC 3695) (rust-lang/reference#1762) - Refactor rendering with `RenderCtx` (rust-lang/reference#1796) - attributes/codegen: update aarch64 features (rust-lang/reference#1791) - Simplify GenericParams grammar (rust-lang/reference#1795) - Add rule identifiers for the ABI chapter (rust-lang/reference#1793) - Remove broken footnote links from grammar summary (rust-lang/reference#1794)
ChrisDenton
added a commit
to ChrisDenton/rust
that referenced
this pull request
Apr 22, 2025
Update books ## rust-lang/nomicon 1 commits in 0c10c30cc54736c5c194ce98c50e2de84eeb6e79..c76a20f0d987145dcedf05c5c073ce8d91f2e82a 2025-04-15 20:54:57 UTC to 2025-04-15 20:54:57 UTC - Say that dereferencing a pointer to a ZST is no longer undefined (rust-lang/nomicon#467) ## rust-lang/reference 7 commits in 3340922df189bddcbaad17dc3927d51a76bcd5ed..3bf3402aea982b876eb56c87da17b0685c6461d5 2025-04-18 13:44:45 UTC to 2025-04-17 17:27:01 UTC - Use `cfg(false)` instead of `cfg(FALSE)` (rust-lang/reference#1763) - Add `cfg(true)` and `cfg(false)` to conditional compilation (RFC 3695) (rust-lang/reference#1762) - Refactor rendering with `RenderCtx` (rust-lang/reference#1796) - attributes/codegen: update aarch64 features (rust-lang/reference#1791) - Simplify GenericParams grammar (rust-lang/reference#1795) - Add rule identifiers for the ABI chapter (rust-lang/reference#1793) - Remove broken footnote links from grammar summary (rust-lang/reference#1794)
rust-timer
added a commit
to rust-lang-ci/rust
that referenced
this pull request
Apr 23, 2025
Rollup merge of rust-lang#140124 - rustbot:docs-update, r=ehuss Update books ## rust-lang/nomicon 1 commits in 0c10c30cc54736c5c194ce98c50e2de84eeb6e79..c76a20f0d987145dcedf05c5c073ce8d91f2e82a 2025-04-15 20:54:57 UTC to 2025-04-15 20:54:57 UTC - Say that dereferencing a pointer to a ZST is no longer undefined (rust-lang/nomicon#467) ## rust-lang/reference 7 commits in 3340922df189bddcbaad17dc3927d51a76bcd5ed..3bf3402aea982b876eb56c87da17b0685c6461d5 2025-04-18 13:44:45 UTC to 2025-04-17 17:27:01 UTC - Use `cfg(false)` instead of `cfg(FALSE)` (rust-lang/reference#1763) - Add `cfg(true)` and `cfg(false)` to conditional compilation (RFC 3695) (rust-lang/reference#1762) - Refactor rendering with `RenderCtx` (rust-lang/reference#1796) - attributes/codegen: update aarch64 features (rust-lang/reference#1791) - Simplify GenericParams grammar (rust-lang/reference#1795) - Add rule identifiers for the ABI chapter (rust-lang/reference#1793) - Remove broken footnote links from grammar summary (rust-lang/reference#1794)
github-actions bot
pushed a commit
to rust-lang/miri
that referenced
this pull request
Apr 23, 2025
Update books ## rust-lang/nomicon 1 commits in 0c10c30cc54736c5c194ce98c50e2de84eeb6e79..c76a20f0d987145dcedf05c5c073ce8d91f2e82a 2025-04-15 20:54:57 UTC to 2025-04-15 20:54:57 UTC - Say that dereferencing a pointer to a ZST is no longer undefined (rust-lang/nomicon#467) ## rust-lang/reference 7 commits in 3340922df189bddcbaad17dc3927d51a76bcd5ed..3bf3402aea982b876eb56c87da17b0685c6461d5 2025-04-18 13:44:45 UTC to 2025-04-17 17:27:01 UTC - Use `cfg(false)` instead of `cfg(FALSE)` (rust-lang/reference#1763) - Add `cfg(true)` and `cfg(false)` to conditional compilation (RFC 3695) (rust-lang/reference#1762) - Refactor rendering with `RenderCtx` (rust-lang/reference#1796) - attributes/codegen: update aarch64 features (rust-lang/reference#1791) - Simplify GenericParams grammar (rust-lang/reference#1795) - Add rule identifiers for the ABI chapter (rust-lang/reference#1793) - Remove broken footnote links from grammar summary (rust-lang/reference#1794)
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
The new rules were tracked in
rust-lang/rust#117945
The corresponding update to the Reference was
rust-lang/reference#1541
See also #198