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Auto merge of #99217 - lcnr:implied-bounds-pre-norm, r=lcnr

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consider unnormalized types for implied bounds

extracted, and slightly modified, from #98900

The idea here is that generally, rustc is split into things which can assume its inputs are well formed[^1], and things which have verify that themselves.

Generally most predicates should only deal with well formed inputs, e.g. a `&'a &'b (): Trait` predicate should be able to assume that `'b: 'a` holds. Normalization can loosen wf requirements (see #91068) and must therefore not be used in places which still have to check well formedness. The only such place should hopefully be `WellFormed` predicates

fixes #87748 and #98543

r? `@jackh726` cc `@rust-lang/types`

[^1]: These places may still encounter non-wf inputs and have to deal with them without causing an ICE as we may check for well formedness out of order.
This commit is contained in:
bors 2022-08-09 16:39:43 +00:00
commit 63e4312e6b
28 changed files with 268 additions and 102 deletions
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38
compiler/rustc_borrowck/src/type_check/constraint_conversion.rs
View file

@ -6,7 +6,7 @@ use rustc_infer::infer::region_constraints::{GenericKind, VerifyBound};
use rustc_infer::infer::{self, InferCtxt, SubregionOrigin};
use rustc_middle::mir::ConstraintCategory;
use rustc_middle::ty::subst::GenericArgKind;
use rustc_middle::ty::TypeVisitable;
use rustc_middle::ty::TypeFoldable;
use rustc_middle::ty::{self, TyCtxt};
use rustc_span::{Span, DUMMY_SP};
@ -109,23 +109,11 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
self.add_outlives(r1_vid, r2_vid);
}
GenericArgKind::Type(mut t1) => {
GenericArgKind::Type(t1) => {
// we don't actually use this for anything, but
// the `TypeOutlives` code needs an origin.
let origin = infer::RelateParamBound(DUMMY_SP, t1, None);
// Placeholder regions need to be converted now because it may
// create new region variables, which can't be done later when
// verifying these bounds.
if t1.has_placeholders() {
t1 = tcx.fold_regions(t1, |r, _| match *r {
ty::RePlaceholder(placeholder) => {
self.constraints.placeholder_region(self.infcx, placeholder)
}
_ => r,
});
}
TypeOutlives::new(
&mut *self,
tcx,
@ -143,6 +131,25 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
}
}
/// Placeholder regions need to be converted eagerly because it may
/// create new region variables, which we must not do when verifying
/// our region bounds.
///
/// FIXME: This should get removed once higher ranked region obligations
/// are dealt with during trait solving.
fn replace_placeholders_with_nll<T: TypeFoldable<'tcx>>(&mut self, value: T) -> T {
if value.has_placeholders() {
self.tcx.fold_regions(value, |r, _| match *r {
ty::RePlaceholder(placeholder) => {
self.constraints.placeholder_region(self.infcx, placeholder)
}
_ => r,
})
} else {
value
}
}
fn verify_to_type_test(
&mut self,
generic_kind: GenericKind<'tcx>,
@ -150,7 +157,6 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
verify_bound: VerifyBound<'tcx>,
) -> TypeTest<'tcx> {
let lower_bound = self.to_region_vid(region);
TypeTest { generic_kind, lower_bound, locations: self.locations, verify_bound }
}
@ -198,6 +204,8 @@ impl<'a, 'b, 'tcx> TypeOutlivesDelegate<'tcx> for &'a mut ConstraintConversion<'
a: ty::Region<'tcx>,
bound: VerifyBound<'tcx>,
) {
let kind = self.replace_placeholders_with_nll(kind);
let bound = self.replace_placeholders_with_nll(bound);
let type_test = self.verify_to_type_test(kind, a, bound);
self.add_type_test(type_test);
}

12
compiler/rustc_borrowck/src/type_check/free_region_relations.rs
View file

@ -242,10 +242,9 @@ impl<'tcx> UniversalRegionRelationsBuilder<'_, 'tcx> {
let constraint_sets: Vec<_> = unnormalized_input_output_tys
.flat_map(|ty| {
debug!("build: input_or_output={:?}", ty);
// We only add implied bounds for the normalized type as the unnormalized
// type may not actually get checked by the caller.
//
// Can otherwise be unsound, see #91068.
// We add implied bounds from both the unnormalized and normalized ty.
// See issue #87748
let constraints_implied1 = self.add_implied_bounds(ty);
let TypeOpOutput { output: norm_ty, constraints: constraints1, .. } = self
.param_env
.and(type_op::normalize::Normalize::new(ty))
@ -273,9 +272,10 @@ impl<'tcx> UniversalRegionRelationsBuilder<'_, 'tcx> {
// }
// ```
// Both &Self::Bar and &() are WF
let constraints_implied = self.add_implied_bounds(norm_ty);
let constraints_implied2 =
if ty != norm_ty { self.add_implied_bounds(norm_ty) } else { None };
normalized_inputs_and_output.push(norm_ty);
constraints1.into_iter().chain(constraints_implied)
constraints1.into_iter().chain(constraints_implied1).chain(constraints_implied2)
})
.collect();

13
compiler/rustc_borrowck/src/type_check/mod.rs
View file

@ -1448,9 +1448,14 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
))
});
debug!(?sig);
let sig = self.normalize(sig, term_location);
self.check_call_dest(body, term, &sig, *destination, target, term_location);
// IMPORTANT: We have to prove well formed for the function signature before
// we normalize it, as otherwise types like `<&'a &'b () as Trait>::Assoc`
// get normalized away, causing us to ignore the `'b: 'a` bound used by the function.
//
// Normalization results in a well formed type if the input is well formed, so we
// don't have to check it twice.
//
// See #91068 for an example.
self.prove_predicates(
sig.inputs_and_output
.iter()
@ -1458,6 +1463,8 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
term_location.to_locations(),
ConstraintCategory::Boring,
);
let sig = self.normalize(sig, term_location);
self.check_call_dest(body, term, &sig, *destination, target, term_location);
// The ordinary liveness rules will ensure that all
// regions in the type of the callee are live here. We

2
compiler/rustc_infer/src/infer/region_constraints/mod.rs
View file

@ -187,7 +187,7 @@ pub enum GenericKind<'tcx> {
/// }
/// ```
/// This is described with an `AnyRegion('a, 'b)` node.
#[derive(Debug, Clone)]
#[derive(Debug, Clone, TypeFoldable, TypeVisitable)]
pub enum VerifyBound<'tcx> {
/// See [`VerifyIfEq`] docs
IfEq(ty::Binder<'tcx, VerifyIfEq<'tcx>>),

23
compiler/rustc_middle/src/ty/mod.rs
View file

@ -594,6 +594,29 @@ impl<'tcx> Predicate<'tcx> {
}
self
}
/// Whether this projection can be soundly normalized.
///
/// Wf predicates must not be normalized, as normalization
/// can remove required bounds which would cause us to
/// unsoundly accept some programs. See #91068.
#[inline]
pub fn allow_normalization(self) -> bool {
match self.kind().skip_binder() {
PredicateKind::WellFormed(_) => false,
PredicateKind::Trait(_)
| PredicateKind::RegionOutlives(_)
| PredicateKind::TypeOutlives(_)
| PredicateKind::Projection(_)
| PredicateKind::ObjectSafe(_)
| PredicateKind::ClosureKind(_, _, _)
| PredicateKind::Subtype(_)
| PredicateKind::Coerce(_)
| PredicateKind::ConstEvaluatable(_)
| PredicateKind::ConstEquate(_, _)
| PredicateKind::TypeWellFormedFromEnv(_) => true,
}
}
}
impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for Predicate<'tcx> {

9
compiler/rustc_trait_selection/src/traits/project.rs
View file

@ -619,6 +619,15 @@ impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
constant.eval(self.selcx.tcx(), self.param_env)
}
}
#[inline]
fn fold_predicate(&mut self, p: ty::Predicate<'tcx>) -> ty::Predicate<'tcx> {
if p.allow_normalization() && needs_normalization(&p, self.param_env.reveal()) {
p.super_fold_with(self)
} else {
p
}
}
}
pub struct BoundVarReplacer<'me, 'tcx> {

12
compiler/rustc_trait_selection/src/traits/query/normalize.rs
View file

@ -351,4 +351,16 @@ impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
mir::ConstantKind::Val(_, _) => constant.try_super_fold_with(self)?,
})
}
#[inline]
fn try_fold_predicate(
&mut self,
p: ty::Predicate<'tcx>,
) -> Result<ty::Predicate<'tcx>, Self::Error> {
if p.allow_normalization() && needs_normalization(&p, self.param_env.reveal()) {
p.try_super_fold_with(self)
} else {
Ok(p)
}
}
}

7
compiler/rustc_typeck/src/check/compare_method.rs
View file

@ -264,8 +264,13 @@ fn compare_predicate_entailment<'tcx>(
let trait_sig = tcx.bound_fn_sig(trait_m.def_id).subst(tcx, trait_to_placeholder_substs);
let trait_sig = tcx.liberate_late_bound_regions(impl_m.def_id, trait_sig);
// Next, add all inputs and output as well-formed tys. Importantly,
// we have to do this before normalization, since the normalized ty may
// not contain the input parameters. See issue #87748.
wf_tys.extend(trait_sig.inputs_and_output.iter());
let trait_sig = ocx.normalize(norm_cause, param_env, trait_sig);
// Add the resulting inputs and output as well-formed.
// We also have to add the normalized trait signature
// as we don't normalize during implied bounds computation.
wf_tys.extend(trait_sig.inputs_and_output.iter());
let trait_fty = tcx.mk_fn_ptr(ty::Binder::dummy(trait_sig));

2
src/test/ui/associated-types/issue-59324.rs
View file

@ -15,9 +15,9 @@ pub trait ThriftService<Bug: NotFoo>:
{
fn get_service(
//~^ ERROR the trait bound `Bug: Foo` is not satisfied
//~| ERROR the trait bound `Bug: Foo` is not satisfied
&self,
) -> Self::AssocType;
//~^ the trait bound `Bug: Foo` is not satisfied
}
fn with_factory<H>(factory: dyn ThriftService<()>) {}

9
src/test/ui/associated-types/issue-59324.stderr
View file

@ -20,7 +20,7 @@ LL | |
LL | |
LL | | Service<AssocType = <Bug as Foo>::OnlyFoo>
... |
LL | |
LL | | ) -> Self::AssocType;
LL | | }
| |_^ the trait `Foo` is not implemented for `Bug`
|
@ -34,6 +34,7 @@ error[E0277]: the trait bound `Bug: Foo` is not satisfied
|
LL | / fn get_service(
LL | |
LL | |
LL | | &self,
LL | | ) -> Self::AssocType;
| |_________________________^ the trait `Foo` is not implemented for `Bug`
@ -50,10 +51,10 @@ LL | fn with_factory<H>(factory: dyn ThriftService<()>) {}
| ^^^^^^^^^^^^^^^^^^^^^ the trait `Foo` is not implemented for `()`
error[E0277]: the trait bound `Bug: Foo` is not satisfied
--> $DIR/issue-59324.rs:19:10
--> $DIR/issue-59324.rs:16:8
|
LL | ) -> Self::AssocType;
| ^^^^^^^^^^^^^^^ the trait `Foo` is not implemented for `Bug`
LL | fn get_service(
| ^^^^^^^^^^^ the trait `Foo` is not implemented for `Bug`
|
help: consider further restricting this bound
|

3
src/test/ui/fn/implied-bounds-unnorm-associated-type-2.rs
View file

@ -1,4 +1,4 @@
// check-pass
// check-fail
trait Trait {
type Type;
@ -17,6 +17,7 @@ where
fn g<'a, 'b>() {
f::<'a, 'b>(());
//~^ ERROR lifetime may not live long enough
}
fn main() {}

17
src/test/ui/fn/implied-bounds-unnorm-associated-type-2.stderr Normal file
View file

@ -0,0 +1,17 @@
error: lifetime may not live long enough
--> $DIR/implied-bounds-unnorm-associated-type-2.rs:19:5
|
LL | fn g<'a, 'b>() {
| -- -- lifetime `'b` defined here
| |
| lifetime `'a` defined here
LL | f::<'a, 'b>(());
| ^^^^^^^^^^^^^^^ requires that `'b` must outlive `'a`
|
= help: consider adding the following bound: `'b: 'a`
= note: requirement occurs because of a function pointer to `f`
= note: the function `f` is invariant over the parameter `'a`
= help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
error: aborting due to previous error

5
src/test/ui/fn/implied-bounds-unnorm-associated-type-3.rs
View file

@ -1,6 +1,4 @@
// check-fail
// See issue #91899. If we treat unnormalized args as WF, `Self` can also be a
// source of unsoundness.
// check-pass
pub trait Yokeable<'a>: 'static {
type Output: 'a;
@ -17,7 +15,6 @@ pub trait ZeroCopyFrom<C: ?Sized>: for<'a> Yokeable<'a> {
impl<T> ZeroCopyFrom<[T]> for &'static [T] {
fn zero_copy_from<'b>(cart: &'b [T]) -> &'b [T] {
//~^ the parameter
cart
}
}

14
src/test/ui/fn/implied-bounds-unnorm-associated-type-3.stderr
View file

@ -1,14 +0,0 @@
error[E0310]: the parameter type `T` may not live long enough
--> $DIR/implied-bounds-unnorm-associated-type-3.rs:19:5
|
LL | fn zero_copy_from<'b>(cart: &'b [T]) -> &'b [T] {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ...so that the type `[T]` will meet its required lifetime bounds
|
help: consider adding an explicit lifetime bound...
|
LL | impl<T: 'static> ZeroCopyFrom<[T]> for &'static [T] {
| +++++++++
error: aborting due to previous error
For more information about this error, try `rustc --explain E0310`.

24
src/test/ui/fn/implied-bounds-unnorm-associated-type-4.rs Normal file
View file

@ -0,0 +1,24 @@
// A regression test for #98543
trait Trait {
type Type;
}
impl<T> Trait for T {
type Type = ();
}
fn f<'a, 'b>(s: &'b str, _: <&'a &'b () as Trait>::Type) -> &'a str
where
&'a &'b (): Trait, // <- adding this bound is the change from #91068
{
s
}
fn main() {
let x = String::from("Hello World!");
let y = f(&x, ());
drop(x);
//~^ ERROR cannot move out of `x` because it is borrowed
println!("{}", y);
}

14
src/test/ui/fn/implied-bounds-unnorm-associated-type-4.stderr Normal file
View file

@ -0,0 +1,14 @@
error[E0505]: cannot move out of `x` because it is borrowed
--> $DIR/implied-bounds-unnorm-associated-type-4.rs:21:10
|
LL | let y = f(&x, ());
| -- borrow of `x` occurs here
LL | drop(x);
| ^ move out of `x` occurs here
LL |
LL | println!("{}", y);
| - borrow later used here
error: aborting due to previous error
For more information about this error, try `rustc --explain E0505`.

23
src/test/ui/fn/implied-bounds-unnorm-associated-type-5.rs Normal file
View file

@ -0,0 +1,23 @@
trait Trait<'a>: 'a {
type Type;
}
// if the `T: 'a` bound gets implied we would probably get ub here again
impl<'a, T> Trait<'a> for T {
//~^ ERROR the parameter type `T` may not live long enough
type Type = ();
}
fn f<'a, 'b>(s: &'b str, _: <&'b () as Trait<'a>>::Type) -> &'a str
where
&'b (): Trait<'a>,
{
s
}
fn main() {
let x = String::from("Hello World!");
let y = f(&x, ());
drop(x);
println!("{}", y);
}

19
src/test/ui/fn/implied-bounds-unnorm-associated-type-5.stderr Normal file
View file

@ -0,0 +1,19 @@
error[E0309]: the parameter type `T` may not live long enough
--> $DIR/implied-bounds-unnorm-associated-type-5.rs:6:13
|
LL | impl<'a, T> Trait<'a> for T {
| ^^^^^^^^^ ...so that the type `T` will meet its required lifetime bounds...
|
note: ...that is required by this bound
--> $DIR/implied-bounds-unnorm-associated-type-5.rs:1:18
|
LL | trait Trait<'a>: 'a {
| ^^
help: consider adding an explicit lifetime bound...
|
LL | impl<'a, T: 'a> Trait<'a> for T {
| ++++
error: aborting due to previous error
For more information about this error, try `rustc --explain E0309`.

6
src/test/ui/fn/implied-bounds-unnorm-associated-type.rs
View file

@ -1,6 +1,6 @@
// check-fail
// See issue #91068. Types in the substs of an associated type can't be implied
// to be WF, since they don't actually have to be constructed.
// See issue #91068. We check that the unnormalized associated types in
// function signatures are implied
trait Trait {
type Type;
@ -12,12 +12,12 @@ impl<T> Trait for T {
fn f<'a, 'b>(s: &'b str, _: <&'a &'b () as Trait>::Type) -> &'a str {
s
//~^ ERROR lifetime may not live long enough
}
fn main() {
let x = String::from("Hello World!");
let y = f(&x, ());
drop(x);
//~^ ERROR cannot move out of `x` because it is borrowed
println!("{}", y);
}

20
src/test/ui/fn/implied-bounds-unnorm-associated-type.stderr
View file

@ -1,14 +1,14 @@
error: lifetime may not live long enough
--> $DIR/implied-bounds-unnorm-associated-type.rs:14:5
error[E0505]: cannot move out of `x` because it is borrowed
--> $DIR/implied-bounds-unnorm-associated-type.rs:20:10
|
LL | fn f<'a, 'b>(s: &'b str, _: <&'a &'b () as Trait>::Type) -> &'a str {
| -- -- lifetime `'b` defined here
| |
| lifetime `'a` defined here
LL | s
| ^ function was supposed to return data with lifetime `'a` but it is returning data with lifetime `'b`
|
= help: consider adding the following bound: `'b: 'a`
LL | let y = f(&x, ());
| -- borrow of `x` occurs here
LL | drop(x);
| ^ move out of `x` occurs here
LL |
LL | println!("{}", y);
| - borrow later used here
error: aborting due to previous error
For more information about this error, try `rustc --explain E0505`.

20
src/test/ui/generic-associated-types/bugs/issue-87748.stderr
View file

@ -1,20 +0,0 @@
error[E0478]: lifetime bound not satisfied
--> $DIR/issue-87748.rs:18:5
|
LL | fn do_sth(_: u32) {}
| ^^^^^^^^^^^^^^^^^
|
note: lifetime parameter instantiated with the anonymous lifetime as defined here
--> $DIR/issue-87748.rs:18:5
|
LL | fn do_sth(_: u32) {}
| ^^^^^^^^^^^^^^^^^
note: but lifetime parameter must outlive the anonymous lifetime as defined here
--> $DIR/issue-87748.rs:18:5
|
LL | fn do_sth(_: u32) {}
| ^^^^^^^^^^^^^^^^^
error: aborting due to previous error
For more information about this error, try `rustc --explain E0478`.

10
src/test/ui/generic-associated-types/bugs/issue-87748.rs → src/test/ui/generic-associated-types/issue-87748.rs
View file

@ -1,13 +1,14 @@
// check-fail
// known-bug: #87748
// Checks that we properly add implied bounds from unnormalized projections in
// inputs when typechecking functions.
// This should pass, but unnormalized input args aren't treated as implied.
// check-pass
#![feature(generic_associated_types)]
trait MyTrait {
type Assoc<'a, 'b> where 'b: 'a;
fn do_sth(arg: Self::Assoc<'_, '_>);
fn do_sth2(arg: Self::Assoc<'_, '_>) {}
}
struct Foo;
@ -16,8 +17,7 @@ impl MyTrait for Foo {
type Assoc<'a, 'b> = u32 where 'b: 'a;
fn do_sth(_: u32) {}
// fn do_sth(_: Self::Assoc<'static, 'static>) {}
// fn do_sth(_: Self::Assoc<'_, '_>) {}
fn do_sth2(_: Self::Assoc<'static, 'static>) {}
}
fn main() {}

1
src/test/ui/higher-rank-trait-bounds/normalize-under-binder/issue-85455.rs
View file

@ -7,7 +7,6 @@ trait SomeTrait<'a> {
fn give_me_ice<T>() {
callee::<fn(&()) -> <T as SomeTrait<'_>>::Associated>();
//~^ ERROR the trait bound `for<'r> T: SomeTrait<'r>` is not satisfied [E0277]
//~| ERROR the trait bound `for<'r> T: SomeTrait<'r>` is not satisfied [E0277]
}
fn callee<T: Fn<(&'static (),)>>() {

13
src/test/ui/higher-rank-trait-bounds/normalize-under-binder/issue-85455.stderr
View file

@ -9,17 +9,6 @@ help: consider restricting type parameter `T`
LL | fn give_me_ice<T: for<'r> SomeTrait<'r>>() {
| +++++++++++++++++++++++
error[E0277]: the trait bound `for<'r> T: SomeTrait<'r>` is not satisfied
--> $DIR/issue-85455.rs:8:14
|
LL | callee::<fn(&()) -> <T as SomeTrait<'_>>::Associated>();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `for<'r> SomeTrait<'r>` is not implemented for `T`
|
help: consider restricting type parameter `T`
|
LL | fn give_me_ice<T: for<'r> SomeTrait<'r>>() {
| +++++++++++++++++++++++
error: aborting due to 2 previous errors
error: aborting due to previous error
For more information about this error, try `rustc --explain E0277`.

27
src/test/ui/implied-bounds/assoc-ty-wf-used-to-get-assoc-ty.rs Normal file
View file

@ -0,0 +1,27 @@
// Test for a less than ideal interaction of implied bounds and normalization.
trait Tr {
type Ty;
}
impl<T: 'static> Tr for T {
type Ty = &'static T;
}
// `<&'a u8 as Tr>::Ty` should cause an error because `&'a u8: Tr` doesn't hold for
// all possible 'a. However, we consider normalized types for implied bounds.
//
// We normalize this projection to `&'static &'a u8` and add a nested `&'a u8: 'static`
// bound. This bound is then proven using the implied bounds for `&'static &'a u8` which
// we only get by normalizing in the first place.
fn test<'a>(x: &'a u8, _wf: <&'a u8 as Tr>::Ty) -> &'static u8 { x }
fn main() {
// This works as we have 'static references due to promotion.
let _: &'static u8 = test(&3, &&3);
// This causes an error because the projection requires 'a to be 'static.
// It would be unsound if this compiled.
let x: u8 = 3;
let _: &'static u8 = test(&x, &&3);
//~^ ERROR `x` does not live long enough
}

15
src/test/ui/implied-bounds/assoc-ty-wf-used-to-get-assoc-ty.stderr Normal file
View file

@ -0,0 +1,15 @@
error[E0597]: `x` does not live long enough
--> $DIR/assoc-ty-wf-used-to-get-assoc-ty.rs:24:31
|
LL | let _: &'static u8 = test(&x, &&3);
| -----^^------
| | |
| | borrowed value does not live long enough
| argument requires that `x` is borrowed for `'static`
...
LL | }
| - `x` dropped here while still borrowed
error: aborting due to previous error
For more information about this error, try `rustc --explain E0597`.

1
src/test/ui/nll/relate_tys/impl-fn-ignore-binder-via-bottom.rs
View file

@ -30,4 +30,5 @@ fn main() {
let _x = <fn(&())>::make_f();
//~^ ERROR implementation of `Y` is not general enough
//~| ERROR implementation of `Y` is not general enough
//~| ERROR implementation of `Y` is not general enough
}

11
src/test/ui/nll/relate_tys/impl-fn-ignore-binder-via-bottom.stderr
View file

@ -16,5 +16,14 @@ LL | let _x = <fn(&())>::make_f();
= note: `Y` would have to be implemented for the type `for<'r> fn(&'r ())`
= note: ...but `Y` is actually implemented for the type `fn(&'0 ())`, for some specific lifetime `'0`
error: aborting due to 2 previous errors
error: implementation of `Y` is not general enough
--> $DIR/impl-fn-ignore-binder-via-bottom.rs:30:14
|
LL | let _x = <fn(&())>::make_f();
| ^^^^^^^^^^^^^^^^^^^ implementation of `Y` is not general enough
|
= note: `Y` would have to be implemented for the type `for<'r> fn(&'r ())`
= note: ...but `Y` is actually implemented for the type `fn(&'0 ())`, for some specific lifetime `'0`
error: aborting due to 3 previous errors