335d05aee5
- 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
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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
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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
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LL | impl PartialEq<usize> for MyType {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```
48 lines
1.8 KiB
Rust
48 lines
1.8 KiB
Rust
// RFC 1445 introduced `#[structural_match]`; this attribute must
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// appear on the `struct`/`enum` definition for any `const` used in a
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// pattern.
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//
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// This is our (forever-unstable) way to mark a datatype as having a
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// `PartialEq` implementation that is equivalent to recursion over its
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// substructure. This avoids (at least in the short term) any need to
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// resolve the question of what semantics is used for such matching.
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// (See RFC 1445 for more details and discussion.)
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// Issue 62307 pointed out a case where the structural-match checking
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// was too shallow.
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#[derive(Debug)]
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struct B(i32);
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//~^ NOTE `B` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
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//~| NOTE `B` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
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// Overriding `PartialEq` to use this strange notion of "equality" exposes
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// whether `match` is using structural-equality or method-dispatch
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// under the hood, which is the antithesis of rust-lang/rfcs#1445
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impl PartialEq for B {
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//~^ NOTE the `PartialEq` trait must be derived, manual `impl`s are not sufficient
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//~| NOTE the `PartialEq` trait must be derived, manual `impl`s are not sufficient
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fn eq(&self, other: &B) -> bool { std::cmp::min(self.0, other.0) == 0 }
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}
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fn main() {
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const RR_B0: & & B = & & B(0);
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const RR_B1: & & B = & & B(1);
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//~^ NOTE constant defined here
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//~| NOTE constant defined here
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match RR_B0 {
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RR_B1 => { println!("CLAIM RR0: {:?} matches {:?}", RR_B1, RR_B0); }
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//~^ ERROR constant of non-structural type `B` in a pattern
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//~| NOTE constant of non-structural type
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_ => { }
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}
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match RR_B1 {
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RR_B1 => { println!("CLAIM RR1: {:?} matches {:?}", RR_B1, RR_B1); }
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//~^ ERROR constant of non-structural type `B` in a pattern
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//~| NOTE constant of non-structural type
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_ => { }
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}
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}
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