Rollup merge of #107339 - aliemjay:covariant, r=lcnr
internally change regions to be covariant
Surprisingly, we consider the reference type `&'a T` to be contravaraint in its lifetime parameter. This is confusing and conflicts with the documentation we have in the reference, rustnomicon, and rustc-dev-guide. This also arguably not the correct use of terminology since we can use `&'static u8` in a place where `&' a u8` is expected, this implies that `&'static u8 <: &' a u8` and consequently `'static <: ' a`, hence covariance.
Because of this, when relating two types, we used to switch the argument positions in a confusing way:
`Subtype(&'a u8 <: &'b u8) => Subtype('b <: 'a) => Outlives('a: 'b) => RegionSubRegion('b <= 'a)`
The reason for the current behavior is probably that we wanted `Subtype('b <: 'a)` and `RegionSubRegion('b <= 'a)` to be equivalent, but I don' t think this is a good reason since these relations are sufficiently different in that the first is a relation in the subtyping lattice and is intrinsic to the type-systems, while the the second relation is an implementation detail of regionck.
This PR changes this behavior to use covariance, so..
`Subtype(&'a u8 <: &'b u8) => Subtype('a <: 'b) => Outlives('a: 'b) => RegionSubRegion('b <= 'a) `
Resolves #103676
r? `@lcnr`
This commit is contained in:
Matthias Krüger
GitHub
commit
a5caa989c9
@ -225,8 +225,7 @@ fn add_constraints_from_ty(
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}
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ty::Ref(region, ty, mutbl) => {
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let contra = self.contravariant(variance);
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self.add_constraints_from_region(current, region, contra);
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self.add_constraints_from_region(current, region, variance);
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self.add_constraints_from_mt(current, &ty::TypeAndMut { ty, mutbl }, variance);
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}
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@ -258,9 +257,8 @@ fn add_constraints_from_ty(
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}
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ty::Dynamic(data, r, _) => {
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// The type `Foo<T+'a>` is contravariant w/r/t `'a`:
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let contra = self.contravariant(variance);
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self.add_constraints_from_region(current, r, contra);
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// The type `dyn Trait<T> +'a` is covariant w/r/t `'a`:
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self.add_constraints_from_region(current, r, variance);
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if let Some(poly_trait_ref) = data.principal() {
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self.add_constraints_from_invariant_substs(
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@ -79,7 +79,8 @@ fn regions(
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debug!("{}.regions({:?}, {:?})", self.tag(), a, b);
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let origin = Subtype(Box::new(self.fields.trace.clone()));
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Ok(self.fields.infcx.inner.borrow_mut().unwrap_region_constraints().glb_regions(
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// GLB(&'static u8, &'a u8) == &RegionLUB('static, 'a) u8 == &'static u8
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Ok(self.fields.infcx.inner.borrow_mut().unwrap_region_constraints().lub_regions(
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self.tcx(),
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origin,
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a,
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@ -79,7 +79,8 @@ fn regions(
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debug!("{}.regions({:?}, {:?})", self.tag(), a, b);
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let origin = Subtype(Box::new(self.fields.trace.clone()));
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Ok(self.fields.infcx.inner.borrow_mut().unwrap_region_constraints().lub_regions(
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// LUB(&'static u8, &'a u8) == &RegionGLB('static, 'a) u8 == &'a u8
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Ok(self.fields.infcx.inner.borrow_mut().unwrap_region_constraints().glb_regions(
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self.tcx(),
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origin,
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a,
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@ -663,13 +663,13 @@ fn regions(
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debug!(?v_b);
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if self.ambient_covariance() {
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// Covariance: a <= b. Hence, `b: a`.
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self.push_outlives(v_b, v_a, self.ambient_variance_info);
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// Covariant: &'a u8 <: &'b u8. Hence, `'a: 'b`.
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self.push_outlives(v_a, v_b, self.ambient_variance_info);
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}
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if self.ambient_contravariance() {
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// Contravariant: b <= a. Hence, `a: b`.
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self.push_outlives(v_a, v_b, self.ambient_variance_info);
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// Contravariant: &'b u8 <: &'a u8. Hence, `'b: 'a`.
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self.push_outlives(v_b, v_a, self.ambient_variance_info);
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}
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Ok(a)
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@ -191,12 +191,13 @@ fn regions(
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// from the "cause" field, we could perhaps give more tailored
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// error messages.
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let origin = SubregionOrigin::Subtype(Box::new(self.fields.trace.clone()));
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// Subtype(&'a u8, &'b u8) => Outlives('a: 'b) => SubRegion('b, 'a)
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self.fields
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.infcx
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.inner
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.borrow_mut()
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.unwrap_region_constraints()
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.make_subregion(origin, a, b);
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.make_subregion(origin, b, a);
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Ok(a)
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}
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@ -443,12 +443,7 @@ pub fn super_relate_tys<'tcx, R: TypeRelation<'tcx>>(
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if a_repr == b_repr =>
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{
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let region_bound = relation.with_cause(Cause::ExistentialRegionBound, |relation| {
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relation.relate_with_variance(
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ty::Contravariant,
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ty::VarianceDiagInfo::default(),
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a_region,
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b_region,
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)
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relation.relate(a_region, b_region)
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})?;
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Ok(tcx.mk_dynamic(relation.relate(a_obj, b_obj)?, region_bound, a_repr))
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}
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@ -497,12 +492,7 @@ pub fn super_relate_tys<'tcx, R: TypeRelation<'tcx>>(
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}
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(&ty::Ref(a_r, a_ty, a_mutbl), &ty::Ref(b_r, b_ty, b_mutbl)) => {
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let r = relation.relate_with_variance(
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ty::Contravariant,
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ty::VarianceDiagInfo::default(),
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a_r,
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b_r,
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)?;
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let r = relation.relate(a_r, b_r)?;
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let a_mt = ty::TypeAndMut { ty: a_ty, mutbl: a_mutbl };
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let b_mt = ty::TypeAndMut { ty: b_ty, mutbl: b_mutbl };
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let mt = relate_type_and_mut(relation, a_mt, b_mt, a)?;
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@ -1,7 +1,7 @@
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#![feature(rustc_attrs)]
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#[rustc_variance]
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struct Foo<'a, T> { //~ ERROR [-, o]
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struct Foo<'a, T> { //~ ERROR [+, o]
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t: &'a mut T,
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}
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@ -1,4 +1,4 @@
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error: [-, o]
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error: [+, o]
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--> $DIR/E0208.rs:4:1
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LL | struct Foo<'a, T> {
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@ -10,7 +10,7 @@ fn method(&'a self) { }
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}
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#[rustc_variance]
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struct Foo<'a, T : Trait<'a>> { //~ ERROR [-, +]
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struct Foo<'a, T : Trait<'a>> { //~ ERROR [+, +]
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field: (T, &'a ())
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}
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@ -1,4 +1,4 @@
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error: [-, +]
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error: [+, +]
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--> $DIR/variance-associated-types.rs:13:1
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LL | struct Foo<'a, T : Trait<'a>> {
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@ -6,7 +6,7 @@
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// Regions that just appear in normal spots are contravariant:
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#[rustc_variance]
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struct Test2<'a, 'b, 'c> { //~ ERROR [-, -, -]
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struct Test2<'a, 'b, 'c> { //~ ERROR [+, +, +]
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x: &'a isize,
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y: &'b [isize],
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c: &'c str
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@ -15,7 +15,7 @@ struct Test2<'a, 'b, 'c> { //~ ERROR [-, -, -]
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// Those same annotations in function arguments become covariant:
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#[rustc_variance]
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struct Test3<'a, 'b, 'c> { //~ ERROR [+, +, +]
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struct Test3<'a, 'b, 'c> { //~ ERROR [-, -, -]
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x: extern "Rust" fn(&'a isize),
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y: extern "Rust" fn(&'b [isize]),
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c: extern "Rust" fn(&'c str),
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@ -24,7 +24,7 @@ struct Test3<'a, 'b, 'c> { //~ ERROR [+, +, +]
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// Mutability induces invariance:
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#[rustc_variance]
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struct Test4<'a, 'b:'a> { //~ ERROR [-, o]
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struct Test4<'a, 'b:'a> { //~ ERROR [+, o]
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x: &'a mut &'b isize,
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}
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@ -32,7 +32,7 @@ struct Test4<'a, 'b:'a> { //~ ERROR [-, o]
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// contravariant context:
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#[rustc_variance]
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struct Test5<'a, 'b:'a> { //~ ERROR [+, o]
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struct Test5<'a, 'b:'a> { //~ ERROR [-, o]
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x: extern "Rust" fn(&'a mut &'b isize),
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}
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@ -42,7 +42,7 @@ struct Test5<'a, 'b:'a> { //~ ERROR [+, o]
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// argument list occurs in an invariant context.
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#[rustc_variance]
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struct Test6<'a, 'b:'a> { //~ ERROR [-, o]
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struct Test6<'a, 'b:'a> { //~ ERROR [+, o]
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x: &'a mut extern "Rust" fn(&'b isize),
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}
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@ -56,7 +56,7 @@ struct Test7<'a> { //~ ERROR [*]
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// Try enums too.
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#[rustc_variance]
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enum Test8<'a, 'b, 'c:'b> { //~ ERROR [+, -, o]
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enum Test8<'a, 'b, 'c:'b> { //~ ERROR [-, +, o]
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Test8A(extern "Rust" fn(&'a isize)),
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Test8B(&'b [isize]),
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Test8C(&'b mut &'c str),
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@ -1,28 +1,28 @@
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error: [-, -, -]
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error: [+, +, +]
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--> $DIR/variance-regions-direct.rs:9:1
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LL | struct Test2<'a, 'b, 'c> {
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| ^^^^^^^^^^^^^^^^^^^^^^^^
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error: [+, +, +]
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error: [-, -, -]
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--> $DIR/variance-regions-direct.rs:18:1
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LL | struct Test3<'a, 'b, 'c> {
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| ^^^^^^^^^^^^^^^^^^^^^^^^
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error: [-, o]
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error: [+, o]
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--> $DIR/variance-regions-direct.rs:27:1
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LL | struct Test4<'a, 'b:'a> {
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| ^^^^^^^^^^^^^^^^^^^^^^^
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error: [+, o]
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error: [-, o]
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--> $DIR/variance-regions-direct.rs:35:1
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LL | struct Test5<'a, 'b:'a> {
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| ^^^^^^^^^^^^^^^^^^^^^^^
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error: [-, o]
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error: [+, o]
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--> $DIR/variance-regions-direct.rs:45:1
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LL | struct Test6<'a, 'b:'a> {
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@ -34,7 +34,7 @@ error: [*]
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LL | struct Test7<'a> {
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| ^^^^^^^^^^^^^^^^
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error: [+, -, o]
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error: [-, +, o]
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--> $DIR/variance-regions-direct.rs:59:1
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LL | enum Test8<'a, 'b, 'c:'b> {
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@ -5,14 +5,14 @@
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#![feature(rustc_attrs)]
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#[rustc_variance]
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enum Base<'a, 'b, 'c:'b, 'd> { //~ ERROR [+, -, o, *]
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enum Base<'a, 'b, 'c:'b, 'd> { //~ ERROR [-, +, o, *]
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Test8A(extern "Rust" fn(&'a isize)),
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Test8B(&'b [isize]),
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Test8C(&'b mut &'c str),
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}
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#[rustc_variance]
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struct Derived1<'w, 'x:'y, 'y, 'z> { //~ ERROR [*, o, -, +]
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struct Derived1<'w, 'x:'y, 'y, 'z> { //~ ERROR [*, o, +, -]
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f: Base<'z, 'y, 'x, 'w>
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}
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@ -22,12 +22,12 @@ struct Derived2<'a, 'b:'a, 'c> { //~ ERROR [o, o, *]
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}
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#[rustc_variance] // Combine + and o to yield o (just pay attention to 'a here)
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struct Derived3<'a:'b, 'b, 'c> { //~ ERROR [o, -, *]
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struct Derived3<'a:'b, 'b, 'c> { //~ ERROR [o, +, *]
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f: Base<'a, 'b, 'a, 'c>
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}
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#[rustc_variance] // Combine + and * to yield + (just pay attention to 'a here)
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struct Derived4<'a, 'b, 'c:'b> { //~ ERROR [+, -, o]
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struct Derived4<'a, 'b, 'c:'b> { //~ ERROR [-, +, o]
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f: Base<'a, 'b, 'c, 'a>
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}
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@ -1,10 +1,10 @@
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error: [+, -, o, *]
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error: [-, +, o, *]
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--> $DIR/variance-regions-indirect.rs:8:1
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LL | enum Base<'a, 'b, 'c:'b, 'd> {
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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error: [*, o, -, +]
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error: [*, o, +, -]
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--> $DIR/variance-regions-indirect.rs:15:1
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LL | struct Derived1<'w, 'x:'y, 'y, 'z> {
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@ -16,13 +16,13 @@ error: [o, o, *]
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LL | struct Derived2<'a, 'b:'a, 'c> {
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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error: [o, -, *]
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error: [o, +, *]
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--> $DIR/variance-regions-indirect.rs:25:1
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LL | struct Derived3<'a:'b, 'b, 'c> {
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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error: [+, -, o]
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error: [-, +, o]
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--> $DIR/variance-regions-indirect.rs:30:1
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LL | struct Derived4<'a, 'b, 'c:'b> {
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@ -11,7 +11,7 @@
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trait T { fn foo(&self); }
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#[rustc_variance]
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struct TOption<'a> { //~ ERROR [-]
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struct TOption<'a> { //~ ERROR [+]
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v: Option<Box<dyn T + 'a>>,
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}
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@ -1,4 +1,4 @@
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error: [-]
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error: [+]
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--> $DIR/variance-trait-object-bound.rs:14:1
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LL | struct TOption<'a> {
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@ -7,7 +7,7 @@
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// not considered bivariant.
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#[rustc_variance]
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struct InvariantMut<'a,A:'a,B:'a> { //~ ERROR [-, o, o]
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struct InvariantMut<'a,A:'a,B:'a> { //~ ERROR [+, o, o]
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t: &'a mut (A,B)
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}
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@ -1,4 +1,4 @@
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error: [-, o, o]
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error: [+, o, o]
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--> $DIR/variance-types.rs:10:1
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LL | struct InvariantMut<'a,A:'a,B:'a> {
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