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move implicit `Sized` predicate to end of list
In `Bounds::predicates()`, move the implicit `Sized` predicate to the
end of the generated list. This means that if there is an explicit
`Sized` bound, it will be checked first, and any resulting
diagnostics will have a more useful span.
Fixes #85998, at least partially. ~~Based on #85979, but only the last 2 commits are new for this pull request.~~ (edit: rebased) A full fix would need to deal with where-clauses, and that seems difficult. Basically, predicates are being collected in multiple stages, and there are two places where implicit `Sized` predicates can be inserted: once for generic parameters, and once for where-clauses. I think this insertion is happening too early, and we should actually do it only at points where we collect all of the relevant trait bounds for a type parameter.
I could use some help interpreting the changes to the stderr output. It looks like reordering the predicates changed some diagnostics that don't obviously have anything to do with `Sized` bounds. Possibly some error reporting code is making assumptions about ordering of predicates? The diagnostics for src/test/ui/derives/derives-span-Hash-*.rs seem to have improved, no longer pointing at the type parameter identifier, but src/test/ui/type-alias-impl-trait/generic_duplicate_param_use9.rs became less verbose for some reason.
I also ran into an instance of #84970 while working on this, but I kind of expected that could happen, because I'm reordering predicates. I can open a separate issue on that if it would be helpful.
``@estebank`` this seems likely to conflict (slightly?) with your work on #85947; how would you like to resolve that?
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Follow the diagnostic output style guide
Detected by #89455.
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Coerce const FnDefs to implement const Fn traits
You can now pass a FnDef to a function expecting `F` where `F: ~const FnTrait`.
r? ``@oli-obk``
``@rustbot`` label T-compiler F-const_trait_impl
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Const drop
The changes are pretty primitive at this point. But at least it works. ^-^
Problems with the current change that I can think of now:
- [x] `~const Drop` shouldn't change anything in the non-const world.
- [x] types that do not have drop glues shouldn't fail to satisfy `~const Drop` in const contexts. `struct S { a: u8, b: u16 }` This might not fail for `needs_non_const_drop`, but it will fail in `rustc_trait_selection`.
- [x] The current change accepts types that have `const Drop` impls but have non-const `Drop` glue.
Fixes #88424.
Significant Changes:
- `~const Drop` is no longer treated as a normal trait bound. In non-const contexts, this bound has no effect, but in const contexts, this restricts the input type and all of its transitive fields to either a) have a `const Drop` impl or b) can be trivially dropped (i.e. no drop glue)
- `T: ~const Drop` will not be linted like `T: Drop`.
- Instead of recursing and iterating through the type in `rustc_mir::transform::check_consts`, we use the trait system to special case `~const Drop`. See [`rustc_trait_selection::...::candidate_assembly#assemble_const_drop_candidates`](https://github.com/fee1-dead/rust/blob/const-drop/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs#L817) and others.
Changes not related to `const Drop`ping and/or changes that are insignificant:
- `Node.constness_for_typeck` no longer returns `hir::Constness::Const` for type aliases in traits. This was previously used to hack how we determine default bound constness for items. But because we now use an explicit opt-in, it is no longer needed.
- Removed `is_const_impl_raw` query. We have `impl_constness`, and the only existing use of that query uses `HirId`, which means we can just operate it with hir.
- `ty::Destructor` now has a field `constness`, which represents the constness of the destructor.
r? `@oli-obk`
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- [x] Removed `?const` and change uses of `?const`
- [x] Added `~const` to the AST. It is gated behind const_trait_impl.
- [x] Validate `~const` in ast_validation.
- [ ] Add enum `BoundConstness` to the HIR. (With variants `NotConst` and
`ConstIfConst` allowing future extensions)
- [ ] Adjust trait selection and pre-existing code to use `BoundConstness`.
- [ ] Optional steps (*for this PR, obviously*)
- [ ] Fix #88155
- [ ] Do something with constness bounds in chalk
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constified implementations of `Default`
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Use note for pointing at bound introducing requirement
Modify output for pointing where a trait bound obligation is introduced in an E0277 from using a span label to using a note in order to always preserve order of the output:
Before:
```
error[E0277]: `<<Self as Case1>::A as Iterator>::Item` doesn't implement `Debug`
--> $DIR/bounds-on-assoc-in-trait.rs:18:28
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LL | type A: Iterator<Item: Debug>;
| ^^^^^ `<<Self as Case1>::A as Iterator>::Item` cannot be formatted using `{:?}` because it doesn't implement `Debug`
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::: $SRC_DIR/core/src/fmt/mod.rs:LL:COL
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LL | pub trait Debug {
| --------------- required by this bound in `Debug`
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= help: the trait `Debug` is not implemented for `<<Self as Case1>::A as Iterator>::Item`
```
After:
```
error[E0277]: `<<Self as Case1>::A as Iterator>::Item` doesn't implement `Debug`
--> $DIR/bounds-on-assoc-in-trait.rs:18:28
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LL | type A: Iterator<Item: Debug>;
| ^^^^^ `<<Self as Case1>::A as Iterator>::Item` cannot be formatted using `{:?}` because it doesn't implement `Debug`
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= help: the trait `Debug` is not implemented for `<<Self as Case1>::A as Iterator>::Item`
note: required by a bound in `Debug`
--> $SRC_DIR/core/src/fmt/mod.rs:LL:COL
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LL | pub trait Debug {
| ^^^^^^^^^^^^^^^ required by this bound in `Debug`
```
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* On suggestions that include deletions, use a diff inspired output format
* When suggesting addition, use `+` as underline
* Color highlight modified span
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deny using default function in impl const Trait
Fixes #79450.
I don't know if my implementation is correct:
- The check is in `rustc_passes::check_const`, should I put it somewhere else instead?
- Is my approach (to checking the impl) optimal? It works for the current tests, but it might have some issues or there might be a better way of doing this.
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