Restrict constants in patterns #32199

to careful use of the span from deriving, we can permit it in stable code if it derives from deriving (not-even-a-pun intended)

this was required to preserve the span from the #[structural_match] attribute -- but honestly I am not 100% sure if it makes sense.

This is a [breaking-change]: according to RFC rust-lang#1445, constants used as patterns must be of a type that *derives* `Eq`. If you encounter a problem, you are most likely using a constant in an expression where the type of the constant is some struct that does not currently implement `Eq`. Something like the following: ```rust struct SomeType { ... } const SOME_CONST: SomeType = ...; match foo { SOME_CONST => ... } ``` The easiest and most future compatible fix is to annotate the type in question with `#[derive(Eq)]` (note that merely *implementing* `Eq` is not enough, it must be *derived*): ```rust struct SomeType { ... } const SOME_CONST: SomeType = ...; match foo { SOME_CONST => ... } ``` Another good option is to rewrite the match arm to use an `if` condition (this is also particularly good for floating point types, which implement `PartialEq` but not `Eq`): ```rust match foo { c if c == SOME_CONST => ... } ``` Finally, a third alternative is to tag the type with `#[structural_match]`; but this is not recommended, as the attribute is never expected to be stabilized. Please see RFC rust-lang#1445 for more details.

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Restrict constants in patterns #32199

Restrict constants in patterns #32199

Commits on Mar 25, 2016