Better error message when trying to write default impls
Previously, if you tried to write this (using the specialization
feature flag):
default impl PartialEq<MyType> {
...
}
The compiler would give you the mysterious warning "inherent impls
cannot be default". What it really means is that you're trying to
write an impl for a Structure or *Trait Object*, and that cannot
be "default". However, one of the ways to encounter this error
(as shown by the above example) is when you forget to write "for
MyType".
This PR adds a help message that reads "maybe missing a `for`
keyword?" This is useful, actionable advice that will help any user
identify their mistake, and doesn't get in the way or mislead any
user that really meant to use the "default" keyword for this weird
purpose. In particular, this help message will be useful for any
users who don't know the "inherent impl" terminology, and/or users
who forget that inherent impls CAN be written for traits (they apply
to the trait objects). Both of these are somewhat confusing, seldom-
used concepts; a one-line error message without any error number for
longer explanation is NOT the place to introduce these ideas.
I wasn't quite sure what grammar / wording to use. I'm open to suggestions. CC @rust-lang/docs (I hope I'm doing that notation right)
(Apparently not. :( )
Previously, if you tried to write this (using the specialization
feature flag):
default impl PartialEq<MyType> {
...
}
The compiler would give you the mysterious warning "inherent impls
cannot be default". What it really means is that you're trying to
write an impl for a Structure or *Trait Object*, and that cannot
be "default". However, one of the ways to encounter this error
(as shown by the above example) is when you forget to write "for
MyType".
This PR adds a help message that reads "maybe missing a `for`
keyword?" This is useful, actionable advice that will help any user
identify their mistake, and doesn't get in the way or mislead any
user that really meant to use the "default" keyword for this weird
purpose. In particular, this help message will be useful for any
users who don't know the "inherent impl" terminology, and/or users
who forget that inherent impls CAN be written for traits (they apply
to the trait objects). Both of these are somewhat confusing, seldom-
used concepts; a one-line error message without any error number for
longer explanation is NOT the place to introduce these ideas.
Also move the check for not having type parameters into ast_validation.
I was not sure what to do with compile-fail/issue-23046.rs: The issue looks like
maybe the bounds actually played a role in triggering the ICE, but that seems
unlikely given that the compiler seems to entirely ignore them. However, I
couldn't find a testcase without the bounds, so I figured the best I could do is
to just remove the bounds and make sure at least that keeps working.
Properly parse impls for the never type `!`
Recover from missing `for` in `impl Trait for Type`
Prohibit inherent default impls and default impls of auto traits
Change wording in more diagnostics to use "auto traits"
Some minor code cleanups in the parser
No longer parse it.
Remove AutoTrait variant from AST and HIR.
Remove backwards compatibility lint.
Remove coherence checks, they make no sense for the new syntax.
Remove from rustdoc.
The Generics now contain one Vec of an enum for the generic parameters,
rather than two separate Vec's for lifetime and type parameters.
Additionally, places that previously used Vec<LifetimeDef> now use
Vec<GenericParam> instead.
RFC 2008: Future-proofing enums/structs with #[non_exhaustive] attribute
This work-in-progress pull request contains my changes to implement [RFC 2008](https://github.com/rust-lang/rfcs/pull/2008). The related tracking issue is #44109.
As of writing, enum-related functionality is not included and there are some issues related to tuple/unit structs. Enum related tests are currently ignored.
WIP PR requested by @nikomatsakis [in Gitter](https://gitter.im/rust-impl-period/WG-compiler-middle?at=59e90e6297cedeb0482ade3e).
