Say that dereferencing a pointer to a ZST is no longer undefined #467
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The new rules were tracked in rust-lang/rust#117945 The corresponding update to the Reference was rust-lang/reference#1541
RalfJung
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Oct 19, 2024
src/exotic-sizes.md
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| 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. |
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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.
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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. |
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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.
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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
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Apr 15, 2025
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The corresponding lang FCP was: |
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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)
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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)
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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)
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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)
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The new rules were tracked in
rust-lang/rust#117945
The corresponding update to the Reference was
rust-lang/reference#1541
See also #198