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StructuralPartialEq diagnostic inconsistent with code #150298
Description
Code
use std::marker::PhantomData;
#[derive(Debug, Clone, Copy, PartialEq)]
pub struct StructuralType<T: PartialEq> {
ty: PhantomData<T>,
var: u32,
}
impl<T: PartialEq> StructuralType<T> {
const A: Self = StructuralType {
ty: PhantomData,
var: 0,
};
const B: Self = StructuralType {
ty: PhantomData,
var: 0,
};
fn broken(self) {
match self {
Self::A => {}
Self::B => {}
_ => {}
}
}
}Current output
error: constant of non-structural type `StructuralType<T>` in a pattern
--> src/lib.rs:22:13
|
4 | pub struct StructuralType<T: PartialEq> {
| --------------------------------------- `StructuralType<T>` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
...
9 | impl<T: PartialEq> StructuralType<T> {
| ------------------------------------
10 | const A: Self = StructuralType {
| ------------- constant defined here
...
22 | Self::A => {}
| ^^^^^^^ constant of non-structural type
error: constant of non-structural type `StructuralType<T>` in a pattern
--> src/lib.rs:23:13
|
4 | pub struct StructuralType<T: PartialEq> {
| --------------------------------------- `StructuralType<T>` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
...
9 | impl<T: PartialEq> StructuralType<T> {
| ------------------------------------
...
15 | const B: Self = StructuralType {
| ------------- constant defined here
...
23 | Self::B => {}
| ^^^^^^^ constant of non-structural type
error: could not compile `test_structural_ty` (lib) due to 2 previous errorsDesired output
Undecided, however the suggestion to annotate #[derive(PartialEq)] is incorrect.
Rust Version
rustc 1.92.0 (ded5c06cf 2025-12-08)
binary: rustc
commit-hash: ded5c06cf21d2b93bffd5d884aa6e96934ee4234
commit-date: 2025-12-08
host: x86_64-unknown-linux-gnu
release: 1.92.0
LLVM version: 21.1.3Additional Information
- This is reproducable on rust playground.
- This may be the cause of the compiler error here
StructuralPartialEqcomputation error #150296.
Activity
dianne commentedon Jan 10, 2026
Similar to #150296, I think this is because when checking if ADTs shallowly support structural equality, we use an empty
ParamEnv; i.e. we ignore theT: SizedandT: PartialEqin the environment when trying to prove thatStructuralType<T>: StructuralPartialEq. Since those bounds are both required by the definition ofStructuralTypeto useStructuralType<T>as a type at all, we can't prove it'sStructuralPartialEq. Usually using an emptyParamEnvis fine (and better for query caching) since generic params aren't supported in const patterns, but that's caught when evaluating the constant to a valtree; we usually can't do that in the presence of generics. In this case, the generic is effectively unused, so evaluation to a valtree succeeds, meaning we check for structural equality (which fails due to missing clauses in the environment).As with #150296, I think the easy fix here is to be more strict about not supporting params in const patterns' types. I'd personally prefer a simple, consistent rule over carving out special cases where it technically can work. We can always relax it later if we want to support this.
dianne commentedon Jul 24, 2026
On nightly, the errors are now
Closing as fixed by #159504.