Tracking Issue for nested field access in offset_of #120140
Description
Feature gate: #![feature(offset_of_nested)]
This is a tracking issue for the use of nested field access in offset_of. This is currently implemented with dot-separated fields, like in C. The original RFC for offset_of was rust-lang/rfcs#3308.
Public API
pub macro offset_of($Container:ty, $($fields:expr)+ $(,)?) { ... }
struct Struct {
field: A,
}
struct A {
a: ((u8, u8), bool),
}
const OFFSET: usize = offset_of!(Struct, field.a.0.1);Steps / History
- Implementation: Included in initial offset_of implementation
- Final comment period (FCP)1
- Stabilization PR - Stabilize offset_of_nested #128284
Unresolved Questions
- None yet.
Activity
est31 commentedon Jan 20, 2024
This has been split off from #106655 to get the non-nested
offset_offmacro stabilized sooner.How the syntax for nested access should look like is still subject to discussion, especially also how it relates to enums.
Some links:
offset_of. Contains some of the syntax discussion.offset_ofalso contains some syntax discussion.tgross35 commentedon May 22, 2024
Could the top example be updated to show what
Structlooks like?GKFX commentedon Jun 2, 2024
How close is this to stabilization? It looks like this is in use by the Linux kernel (https://lore.kernel.org/rust-for-linux/CAH5fLgjP98pS1wsP=YXP5Yr79Y62VF7XPKjbj+G75B3SOFt80g@mail.gmail.com/), as well as the standard library (for an enum):
rust/library/core/src/option.rs
Line 787 in 13e2d72
Stabilizing this may restrict future changes to the offset_of parsing logic (compiler/rustc_parse/src/parser/expr.rs:1191, parse_floating_field_access). Currently this has enum variants treated the same as fields and (with parsing support only) array indexing using
[expr]syntax.dingxiangfei2009 commentedon Jun 11, 2024
Cross-posting stabilization report and request because the two feature gates need to stabilize together for best utility.
Report
cc @Darksonn @ojeda
Amanieu commentedon Jun 17, 2024
@rfcbot fcp merge
rfcbot commentedon Jun 17, 2024
Team member @Amanieu has proposed to merge this. The next step is review by the rest of the tagged team members:
No concerns currently listed.
Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!
cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.
tmandry commentedon Jun 20, 2024
I'm unclear on whether this needs an RFC. I would defer to libs-api on that question though, since this is more a question of the syntax the macro supports than the capability it provides, and I think the syntax is more in the libs-api realm than the lang one (though it may be a bit of both).
Regarding the syntax, there is still an open question about supporting enum variants in the future. I'm personally okay with moving forward with this subset for now, especially since I can imagine a syntax I don't dislike:
offset_of!(Struct, (field.a as MyEnum::Variant).0.1). But I'm just documenting my thinking here for future reference; we should leave the bikeshed out of this issue if at all possible.@rfcbot reviewed
nikomatsakis commentedon Jul 12, 2024
@rfcbot reviewed
I don't feel this requires an RFC. I think it is a natural extension to the existing functionality.
rfcbot commentedon Jul 18, 2024
🔔 This is now entering its final comment period, as per the review above. 🔔
pnkfelix commentedon Jul 19, 2024
@rfcbot reviewed
rfcbot commentedon Jul 28, 2024
The final comment period, with a disposition to merge, as per the review above, is now complete.
As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.
This will be merged soon.
Rollup merge of rust-lang#128284 - GKFX:stabilize-offset-of-nested, r…
Unrolled build for rust-lang#128284