Say that dereferencing a pointer to a ZST is no longer undefined #467
Conversation
The new rules were tracked in rust-lang/rust#117945 The corresponding update to the Reference was rust-lang/reference#1541
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Outdated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.
| 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.
The new rules were tracked in
rust-lang/rust#117945
The corresponding update to the Reference was
rust-lang/reference#1541
See also #198