[ICE]: Enum discriminant referring to `Self` causes "... has parameters, but no args were provided in instantiate"

[theemathas]

Code

#![feature(sized_hierarchy)]

use std::marker::PointeeSized;

#[repr(usize)]
enum What<T: PointeeSized> {
    X = size_of::<*mut Self>(),
    Y(*mut T)
}

Meta

Reproducible on the playground with version 1.96.0-nightly (2026-03-11 3b1b0ef4d80d3117924d)

Error output

thread 'rustc' (27) panicked at /rustc-dev/3b1b0ef4d80d3117924d91352c8b6ca528708b3c/compiler/rustc_type_ir/src/binder.rs:658:13:
Val(ZeroSized, FnDef(DefId(2:2522 ~ core[d6e6]::mem::size_of), [*mut What<T/#0>])) has parameters, but no args were provided in instantiate

Backtrace

stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: <rustc_const_eval::interpret::eval_context::InterpCx<rustc_const_eval::const_eval::machine::CompileTimeMachine>>::eval_operand
   3: <rustc_const_eval::interpret::eval_context::InterpCx<rustc_const_eval::const_eval::machine::CompileTimeMachine>>::eval_callee_and_args
   4: rustc_const_eval::const_eval::eval_queries::eval_to_allocation_raw_provider
      [... omitted 1 frame ...]
   5: rustc_const_eval::const_eval::eval_queries::eval_to_const_value_raw_provider
      [... omitted 1 frame ...]
   6: <rustc_middle::ty::context::TyCtxt>::const_eval_poly
   7: <rustc_middle::ty::adt::AdtDef>::eval_explicit_discr
   8: rustc_hir_analysis::check::check::check_item_type
   9: rustc_hir_analysis::check::wfcheck::check_well_formed
      [... omitted 1 frame ...]
  10: rustc_hir_analysis::check::wfcheck::check_type_wf
      [... omitted 1 frame ...]
  11: rustc_hir_analysis::check_crate
  12: rustc_interface::passes::analysis
  13: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::SingleCache<rustc_middle::query::erase::ErasedData<[u8; 0]>>, false>
  14: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: the compiler unexpectedly panicked. This is a bug

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: please make sure that you have updated to the latest nightly

note: please attach the file at `/playground/rustc-ice-2026-03-12T04_25_48-25.txt` to your bug report

note: rustc 1.96.0-nightly (3b1b0ef4d 2026-03-11) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
#0 [eval_to_allocation_raw] const-evaluating + checking `What::X::{constant#0}`
#1 [eval_to_const_value_raw] simplifying constant for the type system `What::X::{constant#0}`
#2 [check_well_formed] checking that `What` is well-formed
#3 [check_type_wf] checking that types are well-formed
#4 [analysis] running analysis passes on crate `playground`
end of query stack

[theemathas]

Some other odd cases:

This errors

#[repr(usize)]
enum What<T: ?Sized> {
    X = foo::<Self>(),
    Y(*mut T)
}
const fn foo<T: ?Sized>() -> usize { 1 }

error[E0277]: the size for values of type `T` cannot be known
 --> src/lib.rs:3:15
  |
3 |     X = foo::<Self>(),
  |               ^^^^ doesn't have a known size
  |
note: required by a bound in `What`
 --> src/lib.rs:2:11
  |
2 | enum What<T: ?Sized> {
  |           ^ required by this bound in `What`
help: consider further restricting type parameter `T` with unstable trait `MetaSized`
  |
2 | enum What<T: ?Sized + std::marker::MetaSized> {
  |                     ++++++++++++++++++++++++

For more information about this error, try `rustc --explain E0277`.

This compiles

#[repr(usize)]
enum What {
    X = foo::<Self>(),
}
const fn foo<T: ?Sized>() -> usize { 1 }

This errors

#[repr(usize)]
enum What {
    X = size_of::<Self>(),
}

error[E0391]: cycle detected when simplifying constant for the type system `What::X::{constant#0}`
    --> src/lib.rs:3:9
     |
   3 |     X = size_of::<Self>(),
     |         ^^^^^^^^^^^^^^^^^
     |
note: ...which requires const-evaluating + checking `What::X::{constant#0}`...
    --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/mem/mod.rs:345:5
     |
 345 |     <T as SizedTypeProperties>::SIZE
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires simplifying constant for the type system `core::mem::SizedTypeProperties::SIZE`...
    --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/mem/mod.rs:1254:5
     |
1254 |     const SIZE: usize = intrinsics::size_of::<Self>();
     |     ^^^^^^^^^^^^^^^^^
note: ...which requires const-evaluating + checking `core::mem::SizedTypeProperties::SIZE`...
    --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/mem/mod.rs:1254:5
     |
1254 |     const SIZE: usize = intrinsics::size_of::<Self>();
     |     ^^^^^^^^^^^^^^^^^
     = note: ...which requires computing layout of `What`...
     = note: ...which again requires simplifying constant for the type system `What::X::{constant#0}`, completing the cycle
note: cycle used when checking that `What` is well-formed
    --> src/lib.rs:2:1
     |
   2 | enum What {
     | ^^^^^^^^^
     = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information

For more information about this error, try `rustc --explain E0391`.

[fmease]

I've only skimmed this. All these errors should be go away if we implement my proposed approach in issue #146496.

This issue might be an instance of #146496.

[fmease]

Lol nvm, I'm too tired. This is an enum discriminant, not a default field value.

[GokhanKabar]

@rustbot claim

[derekblackwood61-lang]

Bump

…

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