Reject relaxed bounds inside associated type bounds (ATB)
**Reject** relaxed bounds — most notably `?Sized` — inside associated type bounds `TraitRef<AssocTy: …>`.
This was previously accepted without warning despite being incorrect: ATBs are *not* a place where we perform *sized elaboration*, meaning `TraitRef<AssocTy: …>` does *not* elaborate to `TraitRef<AssocTy: Sized + …>` if `…` doesn't contain `?Sized`. Therefore `?Sized` is meaningless. In no other (stable) place do we (intentionally) allow relaxed bounds where we don't also perform sized elab, this is highly inconsistent and confusing! Another point of comparison: For the desugared `$SelfTy: TraitRef, $SelfTy::AssocTy: …` we don't do sized elab either (and thus also don't allow relaxed bounds).
Moreover — as I've alluded to back in https://github.com/rust-lang/rust/pull/135841#pullrequestreview-2619462717 — some later validation steps only happen during sized elaboration during HIR ty lowering[^1]. Namely, rejecting duplicates (e.g., `?Trait + ?Trait`) and ensuring that `Trait` in `?Trait` is equal to `Sized`[^2]. As you can probably guess, on stable/master we don't run these checks for ATBs (so we allow even more nonsensical bounds like `Iterator<Item: ?Copy>` despite T-types's ruling established in the FCP'ed rust-lang/rust#135841).
This PR rectifies all of this. I cratered this back in 2025-01-10 with (allegedly) no regressions found ([report](https://github.com/rust-lang/rust/pull/135331#issuecomment-2585330783), [its analysis](https://github.com/rust-lang/rust/pull/135331#issuecomment-2585356422)). [However a contributor manually found two occurrences](https://github.com/rust-lang/rust/issues/135229#issuecomment-2581832852) of `TraitRef<AssocTy: ?Sized>` in small hobby projects (presumably via GH code search). I immediately sent downstream PRs: https://github.com/Gui-Yom/turbo-metrics/pull/14, https://github.com/ireina7/summon/pull/1 (however, the owners have showed no reaction so far).
I'm leaning towards banning these forms **without a FCW** because a FCW isn't worth the maintenance cost[^3]. Note that associated type bounds were stabilized in 1.79.0 (released 2024-06-13 which is 13 months ago), so the proliferation of ATBs shouldn't be that high yet. If you think we should do another crater run since the last one was 6 months ago, I'm fine with that.
Fixesrust-lang/rust#135229.
[^1]: I consider this a flaw in the implementation and [I've already added a huge FIXME](https://github.com/rust-lang/rust/blob/82a02aefe07092c737c852daccebf49ca25507e3/compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs#L195-L207).
[^2]: To be more precise, if the internal flag `-Zexperimental-default-bounds` is provided other "default traits" (needs internal feature `lang_items`) are permitted as well (cc closely related internal feature: `more_maybe_bounds`).
[^3]: Having to track this and adding an entire lint whose remnants would remain in the code base forever (we never *fully* remove lints).
Having multiple relaxed bounds like `?Sized + ?Iterator` is actually *fine*.
We actually want to reject *duplicate* relaxed bounds like `?Sized + ?Sized`
because these most certainly represent a user error.
Note that this doesn't mean that we accept more code because a bound like
`?Iterator` is still invalid as it's not relaxing a *default* trait and
the only way to define / use more default bounds is under the experimental
and internal feature `more_maybe_bounds` plus `lang_items` plus unstable
flag `-Zexperimental-default-bounds` (historical context: for the longest
time, bounds like `?Iterator` were actually allowed and lead to a hard
warning).
Ultimately, this simply *reframes* the diagnostic. The scope of
`more_maybe_bounds` / `-Zexperimental-default-bounds` remains unchanged
as well.
* The phrasing "only does something for" made sense back when this
diagnostic was a (hard) warning. Now however, it's simply a hard
error and thus completely rules out "doing something".
* The primary message was way too long
* The new wording more closely mirrors the wording we use for applying
other bound modifiers (like `const` and `async`) to incompatible
traits.
* "all other traits are not bound by default" is no longer accurate
under Sized Hierarchy. E.g., traits and assoc tys are (currently)
bounded by `MetaSized` by default but can't be relaxed using
`?MetaSized` (instead, you relax it by adding `PointeeSized`).
* I've decided against adding any diagnositic notes or suggestions
for now like "trait `Trait` can't be relaxed as it's not bound by
default" which would be incorrect for `MetaSized` and assoc tys
as mentioned above) or "consider changing `?MetaSized` to
`PointeeSized`" as the Sized Hierarchy impl is still WIP)
```
error[E0610]: `{integer}` is a primitive type and therefore doesn't have fields
--> $DIR/attempted-access-non-fatal.rs:7:15
|
LL | let _ = 2.l;
| ^
|
help: if intended to be a floating point literal, consider adding a `0` after the period and a `f64` suffix
|
LL - let _ = 2.l;
LL + let _ = 2.0f64;
|
```
Remove the "which is required by `{root_obligation}`" post-script in
"the trait `X` is not implemented for `Y`" explanation in E0277. This
information is already conveyed in the notes explaining requirements,
making it redundant while making the text (particularly in labels)
harder to read.
```
error[E0277]: the trait bound `NotCopy: Copy` is not satisfied
--> $DIR/wf-static-type.rs:10:13
|
LL | static FOO: IsCopy<Option<NotCopy>> = IsCopy { t: None };
| ^^^^^^^^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `NotCopy`
|
= note: required for `Option<NotCopy>` to implement `Copy`
note: required by a bound in `IsCopy`
--> $DIR/wf-static-type.rs:7:17
|
LL | struct IsCopy<T:Copy> { t: T }
| ^^^^ required by this bound in `IsCopy`
```
vs the prior
```
error[E0277]: the trait bound `NotCopy: Copy` is not satisfied
--> $DIR/wf-static-type.rs:10:13
|
LL | static FOO: IsCopy<Option<NotCopy>> = IsCopy { t: None };
| ^^^^^^^^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `NotCopy`, which is required by `Option<NotCopy>: Copy`
|
= note: required for `Option<NotCopy>` to implement `Copy`
note: required by a bound in `IsCopy`
--> $DIR/wf-static-type.rs:7:17
|
LL | struct IsCopy<T:Copy> { t: T }
| ^^^^ required by this bound in `IsCopy`
```
Instead of
```
error[E0277]: the size for values of type `(dyn ThriftService<(), AssocType = _> + 'static)` cannot be known at compilation time
--> $DIR/issue-59324.rs:23:20
|
LL | fn with_factory<H>(factory: dyn ThriftService<()>) {}
| ^^^^^^^ doesn't have a size known at compile-time
```
output
```
error[E0277]: the size for values of type `(dyn ThriftService<(), AssocType = _> + 'static)` cannot be known at compilation time
--> $DIR/issue-59324.rs:23:29
|
LL | fn with_factory<H>(factory: dyn ThriftService<()>) {}
| ^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
```
```
error[E0277]: the size for values of type `str` cannot be known at compilation time
--> $DIR/unsized-str-in-return-expr-arg-and-local.rs:15:9
|
LL | let x = *"";
| ^ doesn't have a size known at compile-time
|
= help: the trait `Sized` is not implemented for `str`
= note: all local variables must have a statically known size
= help: unsized locals are gated as an unstable feature
help: references are always `Sized`, even if they point to unsized data; consider not dereferencing the expression
|
LL - let x = *"";
LL + let x = "";
|
```
When encountering `-> Trait`, suggest `-> Box<dyn Trait>` (instead of `-> Box<Trait>`.
If there's a single returned type within the `fn`, suggest `-> impl Trait`.
```
error[E0277]: the size for values of type `[i32]` cannot be known at compilation time
--> f100.rs:2:33
|
2 | let _ = std::mem::size_of::<[i32]>();
| ^^^^^ doesn't have a size known at compile-time
|
= help: the trait `Sized` is not implemented for `[i32]`
note: required by an implicit `Sized` bound in `std::mem::size_of`
--> /home/gh-estebank/rust/library/core/src/mem/mod.rs:312:22
|
312 | pub const fn size_of<T>() -> usize {
| ^ required by the implicit `Sized` requirement on this bound in `size_of`
```
Fix#120178.
Expand the primary span of E0277 when the immediate unmet bound is not what the user wrote:
```
error[E0277]: the trait bound `i32: Bar` is not satisfied
--> f100.rs:6:6
|
6 | <i32 as Foo>::foo();
| ^^^ the trait `Bar` is not implemented for `i32`, which is required by `i32: Foo`
|
help: this trait has no implementations, consider adding one
--> f100.rs:2:1
|
2 | trait Bar {}
| ^^^^^^^^^
note: required for `i32` to implement `Foo`
--> f100.rs:3:14
|
3 | impl<T: Bar> Foo for T {}
| --- ^^^ ^
| |
| unsatisfied trait bound introduced here
```
Fix#40120.
Add `implement_via_object` to `rustc_deny_explicit_impl` to control object candidate assembly
Some built-in traits are special, since they are used to prove facts about the program that are important for later phases of compilation such as codegen and CTFE. For example, the `Unsize` trait is used to assert to the compiler that we are able to unsize a type into another type. It doesn't have any methods because it doesn't actually *instruct* the compiler how to do this unsizing, but this is later used (alongside an exhaustive match of combinations of unsizeable types) during codegen to generate unsize coercion code.
Due to this, these built-in traits are incompatible with the type erasure provided by object types. For example, the existence of `dyn Unsize<T>` does not mean that the compiler is able to unsize `Box<dyn Unsize<T>>` into `Box<T>`, since `Unsize` is a *witness* to the fact that a type can be unsized, and it doesn't actually encode that unsizing operation in its vtable as mentioned above.
The old trait solver gets around this fact by having complex control flow that never considers object bounds for certain built-in traits:
https://github.com/rust-lang/rust/blob/2f896da247e0ee8f0bef7cd7c54cfbea255b9f68/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs#L61-L132
However, candidate assembly in the new solver is much more lovely, and I'd hate to add this list of opt-out cases into the new solver. Instead of maintaining this complex and hard-coded control flow, instead we can make this a property of the trait via a built-in attribute. We already have such a build attribute that's applied to every single trait that we care about: `rustc_deny_explicit_impl`. This PR adds `implement_via_object` as a meta-item to that attribute that allows us to opt a trait out of object-bound candidate assembly as well.
r? `@lcnr`
Stuart Cook