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The seemingly contradictory #[repr(Rust, C)] is allowed #116825

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fmease opened this issue Oct 16, 2023 · 4 comments
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The seemingly contradictory #[repr(Rust, C)] is allowed #116825

fmease opened this issue Oct 16, 2023 · 4 comments
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@fmease
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A-attributes Area: Attributes (`#[…]`, `#![…]`) A-repr Area: the `#[repr(stuff)]` attribute C-bug Category: This is a bug. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-lang Relevant to the language team, which will review and decide on the PR/issue.
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1.74.0

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@fmease
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fmease commented Oct 16, 2023 •
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#[repr(Rust)] is already on beta and will soon hit stable.

I just found out that #[repr(C, Rust)] successfully compiles while I expected it to error due to contradictory representations.

Further, I wonder if #[repr(Rust, transparent)] should be allowed? Currently it errors with E0692, transparent struct cannot have other repr hints. (see err's response)
@rustbot rustbot added the needs-triage This issue may need triage. Remove it if it has been sufficiently triaged. label Oct 16, 2023
@fmease fmease added A-attributes Area: Attributes (`#[…]`, `#![…]`) T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. C-bug Category: This is a bug. and removed needs-triage This issue may need triage. Remove it if it has been sufficiently triaged. labels Oct 16, 2023
@fmease fmease changed the title #[repr(Rust)] and other representations Behavior of #[repr(Rust, OTHER)] Oct 16, 2023
@fmease fmease changed the title Behavior of #[repr(Rust, OTHER)] Behavior of #[repr(Rust, OTHER_REPR_HINT)] Oct 16, 2023
@Mark-Simulacrum Mark-Simulacrum added this to the 1.74.0 milestone Oct 16, 2023
@fmease fmease added the T-lang Relevant to the language team, which will review and decide on the PR/issue. label Oct 17, 2023
@fmease fmease self-assigned this Oct 17, 2023
@compiler-errors
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compiler-errors commented Oct 17, 2023 •
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Seems pretty obvious we should have exactly one repr, and only allow repr modifiers like packed afterwards. (Rust, transparent) seems like it's asking for two mutually exclusive things.

@fmease fmease changed the title Behavior of #[repr(Rust, OTHER_REPR_HINT)] The seemingly contrardirectory #[repr(Rust, C)] is allowed Oct 17, 2023
@fmease fmease changed the title The seemingly contrardirectory #[repr(Rust, C)] is allowed The seemingly contradictory #[repr(Rust, C)] is allowed Oct 17, 2023
@fmease fmease mentioned this issue Oct 17, 2023
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@marmeladema
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Should a C-struct be able to contain a Rust-struct ?
Currently the compiler is happy about this kind of construct too: https://play.rust-lang.org/?version=beta&mode=debug&edition=2021&gist=0588a57ac361ed85664914d86cb14060

If not, it might deserve it's own issue?

@workingjubilee
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That seems kosher to me? It might not be if

#[repr(C)]
pub struct ReprC(ReprRust, ReprRust);

But that's a different matter and has more to do with defining tuples in C being unclear.

fmease added a commit to fmease/rust that referenced this issue Oct 19, 2023
@fmease
Rollup merge of rust-lang#116829 - fmease:rust-aint-c, r=compiler-errors
2eb6e5f 
Make `#[repr(Rust)]` incompatible with other (non-modifier) representation hints like `C` and `simd`

Read more about this change here: rust-lang#116829 (comment).

Fixes [after backport] rust-lang#116825.
rust-timer added a commit to rust-lang-ci/rust that referenced this issue Oct 19, 2023
@rust-timer
Unrolled build for rust-lang#116829
3fa0308 
Rollup merge of rust-lang#116829 - fmease:rust-aint-c, r=compiler-errors

Make `#[repr(Rust)]` incompatible with other (non-modifier) representation hints like `C` and `simd`

Read more about this change here: rust-lang#116829 (comment).

Fixes [after backport] rust-lang#116825.
@fmease
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fmease commented Oct 21, 2023

Fixed in #116829 and backported in #117005. Closing as completed.

@fmease fmease closed this as completed Oct 21, 2023
@fmease fmease added the A-repr Area: the `#[repr(stuff)]` attribute label Nov 2, 2024
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A-attributes Area: Attributes (`#[…]`, `#![…]`) A-repr Area: the `#[repr(stuff)]` attribute C-bug Category: This is a bug. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-lang Relevant to the language team, which will review and decide on the PR/issue.
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@marmeladema @compiler-errors @Mark-Simulacrum @fmease @workingjubilee @rustbot