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Uninhabited <code>repr(uX)</code> enum shouldn't have size zero

A-enumArea: Enums (discriminated unions, or more generally ADTs (algebraic data types))Area: Enums (discriminated unions, or more generally ADTs (algebraic data types))

A-layoutArea: Memory layout of typesArea: Memory layout of types

C-bugCategory: This is a bug.Category: This is a bug.

T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.

T-langRelevant to the language teamRelevant to the language team

Status: Open.

#146984 In rust-lang/rust;

· theemathas opened on Sep 24, 2025

Transitioning to superset enum variant causes unnecessary copy

A-enumArea: Enums (discriminated unions, or more generally ADTs (algebraic data types))Area: Enums (discriminated unions, or more generally ADTs (algebraic data types))

C-optimizationCategory: An issue highlighting optimization opportunities or PRs implementing suchCategory: An issue highlighting optimization opportunities or PRs implementing such

I-slowIssue: Problems and improvements with respect to performance of generated code.Issue: Problems and improvements with respect to performance of generated code.

T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.

Status: Open.

#146081 In rust-lang/rust;

· erin-desu opened on Sep 1, 2025

Diagnostic improvement request: Enums that contain non-Send aliased types produce confusing error messages

A-diagnosticsArea: Messages for errors, warnings, and lintsArea: Messages for errors, warnings, and lints

A-enumArea: Enums (discriminated unions, or more generally ADTs (algebraic data types))Area: Enums (discriminated unions, or more generally ADTs (algebraic data types))

A-threadArea: `std::thread`Area: `std::thread`

T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.

Status: Open.

#146016 In rust-lang/rust;

· mutexlox-signal opened on Aug 29, 2025

less-than ideal representation for enums that contain a large element with multiple niches

A-enumArea: Enums (discriminated unions, or more generally ADTs (algebraic data types))Area: Enums (discriminated unions, or more generally ADTs (algebraic data types))

A-layoutArea: Memory layout of typesArea: Memory layout of types

C-optimizationCategory: An issue highlighting optimization opportunities or PRs implementing suchCategory: An issue highlighting optimization opportunities or PRs implementing such

T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.

Status: Open.

#145128 In rust-lang/rust;

· lolbinarycat opened on Aug 8, 2025

ICE: layout.ty.discriminant_for_variant(tcx, idx).unwrap().val != u128::from(idx.as_u32()

A-enumArea: Enums (discriminated unions, or more generally ADTs (algebraic data types))Area: Enums (discriminated unions, or more generally ADTs (algebraic data types))

A-layoutArea: Memory layout of typesArea: Memory layout of types

C-bugCategory: This is a bug.Category: This is a bug.

I-ICEIssue: The compiler panicked, giving an Internal Compilation Error (ICE) ❄️Issue: The compiler panicked, giving an Internal Compilation Error (ICE) ❄️

needs-triageThis issue may need triage. Remove it if it has been sufficiently triaged.This issue may need triage. Remove it if it has been sufficiently triaged.

requires-debug-assertionsThis issue requires a build of rustc or tooling with debug-assertions in some wayThis issue requires a build of rustc or tooling with debug-assertions in some way

T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.

Status: Open.

#144501 In rust-lang/rust;

· matthiaskrgr opened on Jul 26, 2025

<code>#[repr]</code> on zero-variant enums leaves the reader with questions

A-diagnosticsArea: Messages for errors, warnings, and lintsArea: Messages for errors, warnings, and lints

A-enumArea: Enums (discriminated unions, or more generally ADTs (algebraic data types))Area: Enums (discriminated unions, or more generally ADTs (algebraic data types))

A-reprArea: the `#[repr(stuff)]` attributeArea: the `#[repr(stuff)]` attribute

T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.

Status: Open.

#139703 In rust-lang/rust;

· Veykril opened on Apr 12, 2025

Inefficient implementation of <code>PartialEq</code> for nested (fieldless) enums

A-codegenArea: Code generationArea: Code generation

A-enumArea: Enums (discriminated unions, or more generally ADTs (algebraic data types))Area: Enums (discriminated unions, or more generally ADTs (algebraic data types))

A-LLVMArea: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues.Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues.

C-optimizationCategory: An issue highlighting optimization opportunities or PRs implementing suchCategory: An issue highlighting optimization opportunities or PRs implementing such

E-needs-testCall for participation: An issue has been fixed and does not reproduce, but no test has been added.Call for participation: An issue has been fixed and does not reproduce, but no test has been added.

I-heavyIssue: Problems and improvements with respect to binary size of generated code.Issue: Problems and improvements with respect to binary size of generated code.

I-slowIssue: Problems and improvements with respect to performance of generated code.Issue: Problems and improvements with respect to performance of generated code.

T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.

Status: Open.

#132628 In rust-lang/rust;

· zaneduffield opened on Nov 5, 2024

repr(C) on an enum accepts discriminants that do not fit into the default C enum size (repr_c_enums_larger_than_int lint)

A-enumArea: Enums (discriminated unions, or more generally ADTs (algebraic data types))Area: Enums (discriminated unions, or more generally ADTs (algebraic data types))

A-FFIArea: Foreign function interface (FFI)Area: Foreign function interface (FFI)

A-reprArea: the `#[repr(stuff)]` attributeArea: the `#[repr(stuff)]` attribute

C-tracking-issueCategory: An issue tracking the progress of sth. like the implementation of an RFCCategory: An issue tracking the progress of sth. like the implementation of an RFC

O-windowsOperating system: WindowsOperating system: Windows

O-windows-msvcToolchain: MSVC, Operating system: WindowsToolchain: MSVC, Operating system: Windows

Status: Open.

#124403 In rust-lang/rust;

· RalfJung opened on Apr 26, 2024

repr(C, align) is unsound on enums

A-alignArea: alignment control (`repr(align(N))` and so on)Area: alignment control (`repr(align(N))` and so on)

A-enumArea: Enums (discriminated unions, or more generally ADTs (algebraic data types))Area: Enums (discriminated unions, or more generally ADTs (algebraic data types))

A-reprArea: the `#[repr(stuff)]` attributeArea: the `#[repr(stuff)]` attribute

C-bugCategory: This is a bug.Category: This is a bug.

T-langRelevant to the language teamRelevant to the language team

Status: Open.

#82101 In rust-lang/rust;

· mahkoh opened on Feb 14, 2021

attribute to reserve None value on Option&lt;enum&gt;

A-enumArea: Enums (discriminated unions, or more generally ADTs (algebraic data types))Area: Enums (discriminated unions, or more generally ADTs (algebraic data types))

A-FFIArea: Foreign function interface (FFI)Area: Foreign function interface (FFI)

C-feature-requestCategory: A feature request, i.e: not implemented / a PR.Category: A feature request, i.e: not implemented / a PR.

needs-rfcThis change is large or controversial enough that it should have an RFC accepted before doing it.This change is large or controversial enough that it should have an RFC accepted before doing it.

T-langRelevant to the language teamRelevant to the language team

Status: Open.

#59788 In rust-lang/rust;

· npmccallum opened on Apr 8, 2019

#[repr(u16)] enum doesn't pack fields as expected

A-codegenArea: Code generationArea: Code generation

A-enumArea: Enums (discriminated unions, or more generally ADTs (algebraic data types))Area: Enums (discriminated unions, or more generally ADTs (algebraic data types))

A-reprArea: the `#[repr(stuff)]` attributeArea: the `#[repr(stuff)]` attribute

A-type-systemArea: Type systemArea: Type system

C-bugCategory: This is a bug.Category: This is a bug.

T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.

T-typesRelevant to the types team, which will review and decide on the PR/issue.Relevant to the types team, which will review and decide on the PR/issue.

Status: Open.

#53657 In rust-lang/rust;

· upsuper opened on Aug 24, 2018

Unions interacting with Enum layout optimization

A-codegenArea: Code generationArea: Code generation

A-enumArea: Enums (discriminated unions, or more generally ADTs (algebraic data types))Area: Enums (discriminated unions, or more generally ADTs (algebraic data types))

F-untagged_unions`#![feature(untagged_unions)]``#![feature(untagged_unions)]`

T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.

T-langRelevant to the language teamRelevant to the language team

Status: Open.

#36394 In rust-lang/rust;

· Mark-Simulacrum opened on Sep 11, 2016