- Point at type that should derive `PartialEq` to be structural.
- Point at manual `impl PartialEq`, explaining that it is not sufficient to be structural.
```
error: constant of non-structural type `MyType` in a pattern
--> $DIR/const-partial_eq-fallback-ice.rs:14:12
|
LL | struct MyType;
| ------------- `MyType` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
...
LL | const CONSTANT: &&MyType = &&MyType;
| ------------------------ constant defined here
...
LL | if let CONSTANT = &&MyType {
| ^^^^^^^^ constant of non-structural type
|
note: the `PartialEq` trait must be derived, manual `impl`s are not sufficient; see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
--> $DIR/const-partial_eq-fallback-ice.rs:5:1
|
LL | impl PartialEq<usize> for MyType {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```
Centralize emitting an error in `const_to_pat` so that all errors from that evaluating a `const` in a pattern can add addditional information. With this, now point at the `const` item's definition:
```
error[E0158]: constant pattern depends on a generic parameter
--> $DIR/associated-const-type-parameter-pattern.rs:20:9
|
LL | pub trait Foo {
| -------------
LL | const X: EFoo;
| ------------- constant defined here
...
LL | A::X => println!("A::X"),
| ^^^^
```
