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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
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new_with_diagnostics
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Use parenthetical notation for `Fn` traits
Always use the `Fn(T) -> R` format when printing closure traits instead of `Fn<(T,), Output = R>`.
Address #67100:
```
error[E0277]: expected a `Fn()` closure, found `F`
--> file.rs:6:13
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6 | call_fn(f)
| ------- ^ expected an `Fn()` closure, found `F`
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| required by a bound introduced by this call
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= note: wrap the `F` in a closure with no arguments: `|| { /* code */ }`
note: required by a bound in `call_fn`
--> file.rs:1:15
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1 | fn call_fn<F: Fn() -> ()>(f: &F) {
| ^^^^^^^^^^ required by this bound in `call_fn`
help: consider further restricting this bound
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5 | fn call_any<F: std::any::Any + Fn()>(f: &F) {
| ++++++
```
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Always use the `Fn(T) -> R` format when printing closure traits instead of `Fn<(T,), Output = R>`.
Fix #67100:
```
error[E0277]: expected a `Fn()` closure, found `F`
--> file.rs:6:13
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6 | call_fn(f)
| ------- ^ expected an `Fn()` closure, found `F`
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| required by a bound introduced by this call
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= note: wrap the `F` in a closure with no arguments: `|| { /* code */ }`
note: required by a bound in `call_fn`
--> file.rs:1:15
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1 | fn call_fn<F: Fn() -> ()>(f: &F) {
| ^^^^^^^^^^ required by this bound in `call_fn`
help: consider further restricting this bound
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5 | fn call_any<F: std::any::Any + Fn()>(f: &F) {
| ++++++
```
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to be uncovered
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These types are always generic only over their own generic parameters with no inference variables involved.
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Rollup of 8 pull requests
Successful merges:
- #114009 (compiler: allow transmute of ZST arrays with generics)
- #122195 (Note that the caller chooses a type for type param)
- #122651 (Suggest `_` for missing generic arguments in turbofish)
- #122784 (Add `tag_for_variant` query)
- #122839 (Split out `PredicatePolarity` from `ImplPolarity`)
- #122873 (Merge my contributor emails into one using mailmap)
- #122885 (Adjust better spastorino membership to triagebot's adhoc_groups)
- #122888 (add a couple more tests)
r? `@ghost`
`@rustbot` modify labels: rollup
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Expose the Freeze trait again (unstably) and forbid implementing it manually
non-emoji version of https://github.com/rust-lang/rust/pull/121501
cc #60715
This trait is useful for generic constants (associated consts of generic traits). See the test (`tests/ui/associated-consts/freeze.rs`) added in this PR for a usage example. The builtin `Freeze` trait is the only way to do it, users cannot work around this issue.
It's also a useful trait for building some very specific abstrations, as shown by the usage by the `zerocopy` crate: https://github.com/google/zerocopy/issues/941
cc ```@RalfJung```
T-lang signed off on reexposing this unstably: https://github.com/rust-lang/rust/pull/121501#issuecomment-1969827742
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Implement RFC 3373: Avoid non-local definitions in functions
This PR implements [RFC 3373: Avoid non-local definitions in functions](https://github.com/rust-lang/rust/issues/120363).
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Add newtypes for bool fields/params/return types
Fixed all the cases of this found with some simple searches for `*/ bool` and `bool /*`; probably many more
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Properly deal with weak alias types as self types of impls
Fixes #114216.
Fixes #116100.
Not super happy about the two ad hoc “normalization” implementations for weak alias types:
1. In `inherent_impls`: The “peeling”, normalization to [“WHNF”][whnf]: Semantically that's exactly what we want (neither proper normalization nor shallow normalization would be correct here). Basically a weak alias type is “nominal” (well...^^) if the WHNF is nominal. [#97974](https://github.com/rust-lang/rust/pull/97974) followed the same approach.
2. In `constrained_generic_params`: Generic parameters are constrained by a weak alias type if the corresp. “normalized” type constrains them (where we only normalize *weak* alias types not arbitrary ones). Weak alias types are injective if the corresp. “normalized” type is injective.
Both have ad hoc overflow detection mechanisms.
**Coherence** is handled in #117164.
r? `@oli-obk` or types
[whnf]: https://en.wikipedia.org/wiki/Lambda_calculus_definition#Weak_head_normal_form
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Clean up potential_query_instability with FxIndexMap and UnordMap
From https://github.com/rust-lang/rust/pull/120485#issuecomment-1916437191
r? `@michaelwoerister`
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For some cases where it's clear that an error has already occurred,
e.g.:
- there's a comment stating exactly that, or
- things like HIR lowering, where we are lowering an error kind
The commit also tweaks some comments around delayed bug sites.
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the header
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This may be a small performance boost as we have to hash less to lookup the value
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Dejargonize `subst`
In favor of #110793, replace almost every occurence of `subst` and `substitution` from rustc codes, but they still remains in subtrees under `src/tools/` like clippy and test codes (I'd like to replace them after this)
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Avoid accessing the HIR in the happy path of `coherent_trait`
Unfortunately the hir is still used in unsafety checks, and we do not have a way to avoid that. An impl's unsafety is not part of any query other than hir.
So this PR does not affect perf, but could still be considered a cleanup
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A trait's local impls are trivially coherent if there are no impls.
This avoids creating a dependency edge on the hir or the specialization graph
This may resolve part of the performance issue of https://github.com/rust-lang/rust/pull/120558
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Implementing traits marked with `#[rustc_specialization_trait]` normally
requires (min-)specialization to be enabled for the enclosing crate.
With this change, that permission can also be granted by an
`allow_internal_unstable` attribute on the macro that generates the impl.
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