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Make it crystal clear what lint type_alias_bounds
actually signifies
#126575
Conversation
.suggestion = the clause will not be checked when the type alias is used, and should be removed | ||
lint_builtin_type_alias_bounds_enable_feat_help = add `#![feature(lazy_type_alias)]` to the crate attributes to enable the desired semantics | ||
lint_builtin_type_alias_bounds_label = will not be checked at usage sites of the type alias | ||
lint_builtin_type_alias_bounds_limitation_note = this is a known limitation of the type checker that may be lifted in a future edition. |
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Type checker / type system (implementation vs specification), it's not as clear-cut in Rust.
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could just leave out the of the type checker
part
// Bounds of the form `T: 'a` with `T` type param affect object lifetime defaults. | ||
if let hir::WherePredicate::BoundPredicate(pred) = pred | ||
&& pred.bounds.iter().any(|bound| matches!(bound, hir::GenericBound::Outlives(_))) | ||
&& pred.bound_generic_params.is_empty() // indeed, even if absent from the RHS |
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This does look a bit wrong but as a matter of fact while T: 'a
does
associate T
with the object lifetime default 'a
, for<'x> T: 'a
does not even though 'x
does not occur inside 'a
!
let ty = cx.tcx.type_of(item.owner_id).instantiate_identity(); | ||
if ty.has_type_flags(ty::TypeFlags::HAS_CT_PROJECTION) | ||
&& cx.tcx.features().generic_const_exprs | ||
{ | ||
return; |
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That's just what I have concluded from extensively playing around without
actually reading through the source, so corrections and clarifications are
welcome.
Please check out tests/ui/const-generics/generic_const_exprs/type-alias-bounds.rs
for an elaboration.
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Your explanation in the test seems correct to me 👍 what a funny interaction. Does a similar thing happen with feature(type_alias_impl_trait)
and type Foo<T: Trait> = Bar<T::Assoc, impl Sized>;
🤔
HAS_CT_PROJECTIONS
is a bit sus since really the check you want is "does this have unevaluated const arg of DefKind::AnonConst
" and we might wind up having unevaluated const args that arent anon consts one day. I don't think it really matters though since gce is kinda on rocky ground anyway so complicating this over some hypothetical potential future (that probably wont happen) seems not worth it to me.
👍 to adding this special case if you want it. It'd also just be fine to not have it, gce is unstable enough (and generally broken enough) that I expect we'll wind up fixing bounds on type aliases long before we stabilize gce lol.
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Does a similar thing happen with
feature(type_alias_impl_trait)
andtype Foo<T: Trait> = Bar<T::Assoc, impl Sized>;
All TAITs are implicitly lazy — TyCtxt::type_alias_is_lazy
is also true if the aliased type contains opaque types and therefore TAIT references get HIR-ty-lowered to ty::Weak
.
the check you want is "does this have unevaluated const arg of DefKind::AnonConst"
So basically visiting the Ty
and check if it has any UnevaluatedConst
s where tcx.def_kind($.def.0) == DefKind::AnonConst
? Sure I can do that.
Yeah, I figured that the way (M)GCEs are resolved and dealt with will likely change significantly. Hence the FIXME(generic_const_exprs): Revisit this before stabilization
. I just really wanted to add a test for this relatively surprising behavior just so ppl working on (M)GCE have something tangible to work with. I don't particularly care right now how the final behavior will look like as long as various edge cases are considered :^)
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I just really wanted to add a test for this relatively surprising behavior
E.g., the fact that type A<T> = (T, I32<{ 0 }>);
doesn't get checked for well-formedness under GCE but type A<T> = (T, I32<{{ 0 }}>);
or type A<T> = (T, I32<{ 0; 0 }>);
does :P Ofc, I know why that happens atm but I hope that that particular behavior won't stay as is ^^'
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So basically visiting the
Ty
and check if it has anyUnevaluatedConst
s wheretcx.def_kind($.def.0) == DefKind::AnonConst
? Sure I can do that.
Yeah but it's really not a big deal, I'd be perfectly comfortable landing with the current handling of gce stuff.
but I hope that that particular behavior won't stay as is ^^'
It'll definitely get fixed when stabilizing lazy type aliases. It'll also be fixed by min_generic_const_exprs
since that wont use anon consts
*[other] these bounds | ||
} | ||
lint_builtin_type_alias_bounds_qualify_assoc_tys_sugg = fully qualify this associated type | ||
lint_builtin_type_alias_bounds_where_clause = where clauses on type aliases are not enforced |
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Moving "in[/on] type aliases" before "not enforced" was not a stylistic decision.
The previous wording could be read as "not enforced within the body or definition of type aliases
which — while not entirely untrue (lack of item wfchecking) — would miss the point and might confuse
users who realize that trait bounds can indeed define/enable shorthand projections
(and are thus "enforced" for some unusual interpretation of the word like "to give strength or force to").
#[subdiagnostic] | ||
pub sub: Option<SuggestChangingAssocTypes<'a, 'b>>, | ||
} | ||
let affect_object_lifetime_defaults = self |
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I went back and forth on whether to display any sort of note or span note (à la "however, these bounds do affect the lifetime default of trait objects passed to the type alias") or help message ("annotate trait object types passed to the type alias with an explicit lifetime") but nothing seemed right (and I couldn't get it to show below the removal suggestion due to a limitation of Diag
infra)
// See also `tests/ui/const-generics/generic_const_exprs/type-alias-bounds.rs`. | ||
let ty = cx.tcx.type_of(item.owner_id).instantiate_identity(); | ||
if ty.has_type_flags(ty::TypeFlags::HAS_CT_PROJECTION) | ||
&& cx.tcx.features().generic_const_exprs |
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This should be correct in cross-crate scenarios, too. I've tested GCE const projections from external crates used inside of a type alias of the local non-GCE crate.
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…ing instead of restricting their self ty
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.suggestion = the clause will not be checked when the type alias is used, and should be removed | ||
lint_builtin_type_alias_bounds_enable_feat_help = add `#![feature(lazy_type_alias)]` to the crate attributes to enable the desired semantics | ||
lint_builtin_type_alias_bounds_label = will not be checked at usage sites of the type alias | ||
lint_builtin_type_alias_bounds_limitation_note = this is a known limitation of the type checker that may be lifted in a future edition. |
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nit .
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oh, well actually this has a newline... hm.... i don't know how to best render this then
Thanks for making this so clear. Sorry for taking so long to approve. Can you double check the messages don't have a |
In that case, whatevs @bors r+ |
Rollup of 7 pull requests Successful merges: - rust-lang#126575 (Make it crystal clear what lint `type_alias_bounds` actually signifies) - rust-lang#127017 (Extend rules of dead code analysis for impls for adts to impls for types refer to adts) - rust-lang#127523 (Migrate `dump-ice-to-disk` and `panic-abort-eh_frame` `run-make` tests to rmake) - rust-lang#127557 (Add a label to point to the lacking macro name definition) - rust-lang#127989 (Migrate `interdependent-c-libraries`, `compiler-rt-works-on-mingw` and `incr-foreign-head-span` `run-make` tests to rmake) - rust-lang#128099 (migrate tests/run-make/extern-flag-disambiguates to rmake) - rust-lang#128170 (Make Clone::clone a lang item) r? `@ghost` `@rustbot` modify labels: rollup
Rollup merge of rust-lang#126575 - fmease:update-lint-type_alias_bounds, r=compiler-errors Make it crystal clear what lint `type_alias_bounds` actually signifies This is part of my work on https://github.com/rust-lang/rust/labels/F-lazy_type_alias ([tracking issue](rust-lang#112792)). --- To recap, the lint `type_alias_bounds` detects bounds on generic parameters and where clauses on (eager) type aliases. These bounds should've never been allowed because they are currently neither enforced[^1] at usage sites of type aliases nor thoroughly checked for correctness at definition sites due to the way type aliases are represented in the compiler. Allowing them was an oversight. Explicitly label this as a known limitation of the type checker/system and establish the experimental feature `lazy_type_alias` as its eventual proper solution. Where this becomes a bit tricky (for me as a rustc dev) are the "secondary effects" of these bounds whose existence I sadly can't deny. As a matter of fact, type alias bounds do play some small roles during type checking. However, after a lot of thinking over the last two weeks I've come to the conclusion (not without second-guessing myself though) that these use cases should not trump the fact that these bounds are currently *inherently broken*. Therefore the lint `type_alias_bounds` should and will continue to flag bounds that may have subordinate uses. The two *known* secondary effects are: 1. They may enable the use of "shorthand" associated type paths `T::Assoc` (as opposed to fully qualified paths `<T as Trait>::Assoc`) where `T` is a type param bounded by some trait `Trait` which defines that assoc ty. 2. They may affect the default lifetime of trait object types passed as a type argument to the type alias. That concept is called (trait) object lifetime default. The second one is negligible, no question asked. The first one however is actually "kinda nice" (for writability) and comes up in practice from time to time. So why don't I just special-case trait bounds that "define" shorthand assoc type paths as originally planned in rust-lang#125709? 1. Starting to permit even a tiny subset of bounds would already be enough to send a signal to users that bounds in type aliases have been legitimized and that they can expect to see type alias bounds in the wild from now on (proliferation). This would be actively misleading and dangerous because those bounds don't behave at all like one would expect, they are *not real*[^2]! 1. Let's take `type A<T: Trait> = T::Proj;` for example. Everywhere else in the language `T: Trait` means `T: Trait + Sized`. For type aliases, that's not the case though: `T: Trait` and `T: Trait + ?Sized` for that matter do neither mean `T: Trait + Sized` nor `T: Trait + ?Sized` (for both!). Instead, whether `T` requires `Sized` or not entirely depends on the definition of `Trait`[^2]. Namely, whether or not it is bounded by `Sized`. 2. Given `type A<T: Trait<AssocA = ()>> = T::AssocB;`, while `X: Trait` gets checked given `A<X>` (by virtue of projection wfchecking post alias expansion[^2]), the associated type constraint `AssocA = ()` gets dropped entirely! While we could choose to warn on such cases, it would inevitably lead to a huge pile of special cases. 3. While it's common knowledge that the body / aliased type / RHS of an (eager) type alias does not get checked for well-formedness, I'm not sure if people would realize that that extends to bounds as well. Namely, `type A<T: Trait<[u8]>> = T::Proj;` compiles even if `Trait`'s generic parameter requires `Sized`. Of course, at usage sites `[u8]: Sized` would still end up getting checked[^2], so it's not a huge problem if you have full control over `A`. However, imagine that `A` was actually part of a public API and was never used inside the defining crate (not unreasonable). In such a scenario, downstream users would be presented with an impossible to use type alias! Remember, bounds may grow arbitrarily complex and nuanced in practice. 4. Even if we allowed trait bounds that "define" shorthand assoc type paths, we would still need to continue to warn in cases where the assoc ty comes from a supertrait despite the fact that the shorthand syntax can be used: `type A<T: Sub> = T::Assoc;` does compile given `trait Sub: Super {}` and `trait Super { type Assoc; }`. However, `A<X>` does not enforce `X: Sub`, only `X: Super`[^2]. All that to say, type alias bounds are simply not real and we shouldn't pretend they are! 5. Summarizing the points above, we would be legitimizing bounds that are completely broken! 2. It's infeasible to implement: Due to the lack of `TypeckResults` in `ItemCtxt` (and a way to propagate it to other parts of the compiler), the resolution of type-dependent paths in non-`Body` items (most notably type aliases) is not recoverable from the HIR alone which would be necessary because the information of whether an associated type path (projection) is a shorthand is only present pre&in-HIR and doesn't survive HIR ty lowering. Of course, I could rerun parts of HIR ty lowering inside the lint `type_alias_bounds` (namely, `probe_single_ty_param_bound_for_assoc_ty` which would need to be exposed or alternatively a stripped-down version of it). This likely has a performance impact and introduces complexity. In short, the "benefits" are not worth the costs. --- * 3rd commit: Update a diagnostic to avoid suggesting type alias bounds * 4th commit: Flag type alias bounds even if the RHS contains inherent associated types. * I started to allow them at some point in the past which was not correct (see commit for details) * 5th commit: Allow type alias bounds if the RHS contains const projections and GCEs are enabled * (and add a `FIXME(generic_const_exprs)` to be revisited before (M)GCE's stabilization) * As a matter of fact type alias bounds are enforced in this case because the contained AnonConsts do get checked for well-formedness and crucially they inherit the generics and predicates of their parent item (here: the type alias) * Remaining commits: Improve the lint `type_alias_bounds` itself --- Fixes rust-lang#125789 (sugg diag fix). Fixes rust-lang#125709 (wontfix, acknowledgement, sugg diag applic fix). Fixes rust-lang#104918 (sugg diag applic fix). Fixes rust-lang#100270 (wontfix, acknowledgement, sugg diag applic fix). Fixes rust-lang#94398 (true fix). r? `@compiler-errors` `@oli-obk` [^1]: From the perspective of the trait solver. [^2]: Given `type A<T: Trait> = T::Proj;`, the reason why the trait bound "`T: Trait`" gets *seemingly* enforced at usage sites of the type alias `A` is simply because `A<X>` gets expanded to "`<X as Trait>::Proj`" very early on and it's the *expansion* that gets checked for well-formedness, not the type alias reference.
This is part of my work on F-lazy_type_alias`#![feature(lazy_type_alias)]`
(tracking issue).
To recap, the lint
type_alias_bounds
detects bounds on generic parameters and where clauses on (eager) type aliases. These bounds should've never been allowed because they are currently neither enforced1 at usage sites of type aliases nor thoroughly checked for correctness at definition sites due to the way type aliases are represented in the compiler. Allowing them was an oversight.Explicitly label this as a known limitation of the type checker/system and establish the experimental feature
lazy_type_alias
as its eventual proper solution.Where this becomes a bit tricky (for me as a rustc dev) are the "secondary effects" of these bounds whose existence I sadly can't deny. As a matter of fact, type alias bounds do play some small roles during type checking. However, after a lot of thinking over the last two weeks I've come to the conclusion (not without second-guessing myself though) that these use cases should not trump the fact that these bounds are currently inherently broken. Therefore the lint
type_alias_bounds
should and will continue to flag bounds that may have subordinate uses.The two known secondary effects are:
T::Assoc
(as opposed to fully qualified paths<T as Trait>::Assoc
) whereT
is a type param bounded by some traitTrait
which defines that assoc ty.The second one is negligible, no question asked. The first one however is actually "kinda nice" (for writability) and comes up in practice from time to time.
So why don't I just special-case trait bounds that "define" shorthand assoc type paths as originally planned in #125709?
type A<T: Trait> = T::Proj;
for example. Everywhere else in the languageT: Trait
meansT: Trait + Sized
. For type aliases, that's not the case though:T: Trait
andT: Trait + ?Sized
for that matter do neither meanT: Trait + Sized
norT: Trait + ?Sized
(for both!). Instead, whetherT
requiresSized
or not entirely depends on the definition ofTrait
2. Namely, whether or not it is bounded bySized
.type A<T: Trait<AssocA = ()>> = T::AssocB;
, whileX: Trait
gets checked givenA<X>
(by virtue of projection wfchecking post alias expansion2), the associated type constraintAssocA = ()
gets dropped entirely! While we could choose to warn on such cases, it would inevitably lead to a huge pile of special cases.type A<T: Trait<[u8]>> = T::Proj;
compiles even ifTrait
's generic parameter requiresSized
. Of course, at usage sites[u8]: Sized
would still end up getting checked2, so it's not a huge problem if you have full control overA
. However, imagine thatA
was actually part of a public API and was never used inside the defining crate (not unreasonable). In such a scenario, downstream users would be presented with an impossible to use type alias! Remember, bounds may grow arbitrarily complex and nuanced in practice.type A<T: Sub> = T::Assoc;
does compile giventrait Sub: Super {}
andtrait Super { type Assoc; }
. However,A<X>
does not enforceX: Sub
, onlyX: Super
2. All that to say, type alias bounds are simply not real and we shouldn't pretend they are!TypeckResults
inItemCtxt
(and a way to propagate it to other parts of the compiler), the resolution of type-dependent paths in non-Body
items (most notably type aliases) is not recoverable from the HIR alone which would be necessary because the information of whether an associated type path (projection) is a shorthand is only present pre&in-HIR and doesn't survive HIR ty lowering. Of course, I could rerun parts of HIR ty lowering inside the linttype_alias_bounds
(namely,probe_single_ty_param_bound_for_assoc_ty
which would need to be exposed or alternatively a stripped-down version of it). This likely has a performance impact and introduces complexity. In short, the "benefits" are not worth the costs.FIXME(generic_const_exprs)
to be revisited before (M)GCE's stabilization)type_alias_bounds
itselfFixes #125789 (sugg diag fix).
Fixes #125709 (wontfix, acknowledgement, sugg diag applic fix).
Fixes #104918 (sugg diag applic fix).
Fixes #100270 (wontfix, acknowledgement, sugg diag applic fix).
Fixes #94398 (true fix).
r? @compiler-errors @oli-obk
Footnotes
From the perspective of the trait solver. ↩
Given
type A<T: Trait> = T::Proj;
, the reason why the trait bound "T: Trait
" gets seemingly enforced at usage sites of the type aliasA
is simply becauseA<X>
gets expanded to "<X as Trait>::Proj
" very early on and it's the expansion that gets checked for well-formedness, not the type alias reference. ↩ ↩2 ↩3 ↩4 ↩5