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Suboptimal layout for nested enums

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.

#147341 In rust-lang/rust;

· moulins opened on Oct 4, 2025

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

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

Enum layout should allow multiple untagged variants if they have nonoverlapping values

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.

#139148 In rust-lang/rust;

· lolbinarycat opened on Mar 30, 2025

ICE using <code>repr(align(…))</code> type as trait object receiver (arbitrary <code>self</code>-type)

A-dyn-traitArea: trait objects, vtable layoutArea: trait objects, vtable layout

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

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

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

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

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

requires-nightlyThis issue requires a nightly compiler in some way.This issue requires a nightly compiler in some way.

S-bug-has-testStatus: This bug is tracked inside the repo by a `known-bug` test.Status: This bug is tracked inside the repo by a `known-bug` test.

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.

#138564 In rust-lang/rust;

· steffahn opened on Mar 16, 2025

Should <code>Option&lt;u128&gt;</code> use a smaller (maybe usize) tag?

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

C-discussionCategory: Discussion or questions that doesn't represent real issues.Category: Discussion or questions that doesn't represent real issues.

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.

#138332 In rust-lang/rust;

· scottmcm opened on Mar 11, 2025

ICE: <code>fn_abi_of_instance(foo, [])</code> failed: Layout(Unknown(Foo))

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

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

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) ❄️

S-bug-has-testStatus: This bug is tracked inside the repo by a `known-bug` test.Status: This bug is tracked inside the repo by a `known-bug` test.

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.

#138008 In rust-lang/rust;

· matthiaskrgr opened on Mar 4, 2025

Layout optimization fails for enums with variants having values 0 and 255

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

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

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

Status: Open.

#136444 In rust-lang/rust;

· Kixunil opened on Feb 2, 2025

rustc accepts types requiring a greater-than-48-bit address space

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

C-discussionCategory: Discussion or questions that doesn't represent real issues.Category: Discussion or questions that doesn't represent real issues.

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.

#132525 In rust-lang/rust;

· shao-hua-li opened on Nov 2, 2024

Rust doesn't use niche in reference (or pointer) to slice

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

E-needs-designThis issue needs exploration and design to see how and if we can fix/implement itThis issue needs exploration and design to see how and if we can fix/implement it

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.

#132235 In rust-lang/rust;

· VorfeedCanal opened on Oct 27, 2024

ICE: Layout::compute: unexpected type <code>_</code>

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) ❄️

P-mediumMedium priorityMedium priority

regression-from-stable-to-stablePerformance or correctness regression from one stable version to another.Performance or correctness regression from one stable version to another.

S-bug-has-testStatus: This bug is tracked inside the repo by a `known-bug` test.Status: This bug is tracked inside the repo by a `known-bug` test.

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.

#130104 In rust-lang/rust;

· matthiaskrgr opened on Sep 8, 2024