Store `do_not_recommend`-ness in impl header
Alternative to #128674
It's less flexible, but also less invasive. Hopefully it's also performant. I'd recommend we think separately about the design for how to gate arbitrary diagnostic attributes moving forward.
doing so requires overwriting global cache entries and
generally adds significant complexity to the solver. This is
also only ever done for root goals, so it feels easier to wrap
the `evaluate_canonical_goal` in an ordinary query if
necessary.
Uplift elaboration into `rustc_type_ir`
Allows us to deduplicate and consolidate elaboration (including these stupid elaboration duplicate fns i added for pretty printing like 3 years ago) so I'm pretty hyped about this change :3
r? lcnr
Only compute vtable information during codegen
This PR removes vtable information from the `Object` and `TraitUpcasting` candidate sources in the trait solvers, and defers the computation of relevant information to `Instance::resolve`. This is because vtables really aren't a thing in the trait world -- they're an implementation detail in codegen.
Previously it was just easiest to tangle this information together since we were already doing the work of looking at all the supertraits in the trait solver, and specifically because we use traits to represent when it's possible to call a method via a vtable (`Object` candidate) and do upcasting (`Unsize` candidate). but I am somewhat suspicious we're doing a *lot* of extra work, especially in polymorphic contexts, so let's see what perf says.
Make proof tree probing and `Candidate`/`CandidateSource` generic over interner
`<TyCtxt<'tcx>>` is ugly, but will become `<I>` when things actually become generic.
r? lcnr
Check that alias-relate terms are WF if reporting an error in alias-relate
Check that each of the left/right term is WF when deriving a best error obligation for an alias-relate goal. This will make sure that given `<i32 as NotImplemented>::Assoc = ()` will drill down into `i32: NotImplemented` since we currently treat the projection as rigid.
r? lcnr
Use `Variance` glob imported variants everywhere
Fully commit to using the globbed variance. Could be convinced the other way, and change this PR to not use the globbed variants anywhere, but I'd rather we do one or the other.
r? lcnr
Align `Term` methods with `GenericArg` methods, add `Term::expect_*`
* `Term::ty` -> `Term::as_type`.
* `Term::ct` -> `Term::as_const`.
* Adds `Term::expect_type` and `Term::expect_const`, and uses them in favor of `.ty().unwrap()`, etc.
I could also shorten these to `as_ty` and then do `GenericArg::as_ty` as well, but I do think the `as_` is important to signal that this is a conversion method, and not a getter, like `Const::ty` is.
r? types
Make `WHERE_CLAUSES_OBJECT_SAFETY` a regular object safety violation
#### The issue
In #50781, we have known about unsound `where` clauses in function arguments:
```rust
trait Impossible {}
trait Foo {
fn impossible(&self)
where
Self: Impossible;
}
impl Foo for &() {
fn impossible(&self)
where
Self: Impossible,
{}
}
// `where` clause satisfied for the object, meaning that the function now *looks* callable.
impl Impossible for dyn Foo {}
fn main() {
let x: &dyn Foo = &&();
x.impossible();
}
```
... which currently segfaults at runtime because we try to call a method in the vtable that doesn't exist. :(
#### What did u change
This PR removes the `WHERE_CLAUSES_OBJECT_SAFETY` lint and instead makes it a regular object safety violation. I choose to make this into a hard error immediately rather than a `deny` because of the time that has passed since this lint was authored, and the single (1) regression (see below).
That means that it's OK to mention `where Self: Trait` where clauses in your trait, but making such a trait into a `dyn Trait` object will report an object safety violation just like `where Self: Sized`, etc.
```rust
trait Impossible {}
trait Foo {
fn impossible(&self)
where
Self: Impossible; // <~ This definition is valid, just not object-safe.
}
impl Foo for &() {
fn impossible(&self)
where
Self: Impossible,
{}
}
fn main() {
let x: &dyn Foo = &&(); // <~ THIS is where we emit an error.
}
```
#### Regressions
From a recent crater run, there's only one crate that relies on this behavior: https://github.com/rust-lang/rust/pull/124305#issuecomment-2122381740. The crate looks unmaintained and there seems to be no dependents.
#### Further
We may later choose to relax this (e.g. when the where clause is implied by the supertraits of the trait or something), but this is not something I propose to do in this FCP.
For example, given:
```
trait Tr {
fn f(&self) where Self: Blanket;
}
impl<T: ?Sized> Blanket for T {}
```
Proving that some placeholder `S` implements `S: Blanket` would be sufficient to prove that the same (blanket) impl applies for both `Concerete: Blanket` and `dyn Trait: Blanket`.
Repeating here that I don't think we need to implement this behavior right now.
----
r? lcnr
