bjoernager/rust
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avoid creating an Instance only to immediately disassemble it again

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This commit is contained in:
Ralf Jung 2024-07-13 18:32:10 +02:00
parent 86ce911f90
commit 67c99d6338
13 changed files with 54 additions and 114 deletions
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17
compiler/rustc_error_codes/src/error_codes/E0158.md
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@ -1,5 +1,4 @@
An associated `const`, `const` parameter or `static` has been referenced
in a pattern.
A generic parameter or `static` has been referenced in a pattern.
Erroneous code example:
@ -15,25 +14,25 @@ trait Bar {
fn test<A: Bar>(arg: Foo) {
match arg {
A::X => println!("A::X"), // error: E0158: associated consts cannot be
// referenced in patterns
A::X => println!("A::X"), // error: E0158: constant pattern depends
// on a generic parameter
Foo::Two => println!("Two")
}
}
```
Associated `const`s cannot be referenced in patterns because it is impossible
Generic parameters cannot be referenced in patterns because it is impossible
for the compiler to prove exhaustiveness (that some pattern will always match).
Take the above example, because Rust does type checking in the *generic*
method, not the *monomorphized* specific instance. So because `Bar` could have
theoretically infinite implementations, there's no way to always be sure that
theoretically arbitrary implementations, there's no way to always be sure that
`A::X` is `Foo::One`. So this code must be rejected. Even if code can be
proven exhaustive by a programmer, the compiler cannot currently prove this.
The same holds true of `const` parameters and `static`s.
The same holds true of `static`s.
If you want to match against an associated `const`, `const` parameter or
`static` consider using a guard instead:
If you want to match against a `const` that depends on a generic parameter or a
`static`, consider using a guard instead:
```
trait Trait {