`StructuralPartialEq` diagnostic inconsistent with code

[cedtwo]

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 errors

Desired 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: 21.1.3

Additional Information

• This is reproducable on rust playground.
• This may be the cause of the compiler error here StructuralPartialEq computation error #150296.

[dianne]

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 the T: Sized and T: PartialEq in the environment when trying to prove that StructuralType<T>: StructuralPartialEq. Since those bounds are both required by the definition of StructuralType to use StructuralType<T> as a type at all, we can't prove it's StructuralPartialEq. Usually using an empty ParamEnv is 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]

On nightly, the errors are now

error[E0158]: constant pattern cannot depend on generic parameters
  --> src/lib.rs:22:13
   |
 9 | impl<T: PartialEq> StructuralType<T> {
   | ------------------------------------
   |      |
   |      constant depends on this generic parameter
10 |     const A: Self = StructuralType {
   |     ------------- constant defined here
...
20 |     fn broken(self) {
   |        ------
21 |         match self {
22 |             Self::A => {}
   |             ^^^^^^^ `const` depends on a generic parameter

error[E0158]: constant pattern cannot depend on generic parameters
  --> src/lib.rs:23:13
   |
 9 | impl<T: PartialEq> StructuralType<T> {
   | ------------------------------------
   |      |
   |      constant depends on this generic parameter
...
15 |     const B: Self = StructuralType {
   |     ------------- constant defined here
...
20 |     fn broken(self) {
   |        ------
...
23 |             Self::B => {}
   |             ^^^^^^^ `const` depends on a generic parameter

Closing as fixed by #159504.