Refactor variance diagnostics to work with more types

Instead of special-casing mutable pointers/references, we
now support general generic types (currently, we handle
`ty::Ref`, `ty::RawPtr`, and `ty::Adt`)

When a `ty::Adt` is involved, we show an additional note
explaining which of the type's generic parameters is
invariant (e.g. the `T` in `Cell<T>`). Currently, we don't
explain *why* a particular generic parameter ends up becoming
invariant. In the general case, this could require printing
a long 'backtrace' of types, so doing this would be
more suitable for a follow-up PR.

We still only handle the case where our variance switches
to `ty::Invariant`.

20 files changed, 169 insertions, 0 deletions

diff --git a/src/test/ui/associated-types/cache/project-fn-ret-invariant.krisskross.nll.stderr b/src/test/ui/associated-types/cache/project-fn-ret-invariant.krisskross.nll.stderr
index 4fc336122fa..01f800811ab 100644
--- a/src/test/ui/associated-types/cache/project-fn-ret-invariant.krisskross.nll.stderr
+++ b/src/test/ui/associated-types/cache/project-fn-ret-invariant.krisskross.nll.stderr
@@ -10,6 +10,9 @@ LL |     (a, b)
    |     ^^^^^^ function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
    |
    = help: consider adding the following bound: `'a: 'b`
+   = note: requirement occurs because of the type Type<'_>, which makes the generic argument '_ invariant
+   = note: the struct Type<'a> is invariant over the parameter 'a
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/project-fn-ret-invariant.rs:56:5
@@ -23,6 +26,9 @@ LL |     (a, b)
    |     ^^^^^^ 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`
+   = note: requirement occurs because of the type Type<'_>, which makes the generic argument '_ invariant
+   = note: the struct Type<'a> is invariant over the parameter 'a
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 help: `'a` and `'b` must be the same: replace one with the other
 
diff --git a/src/test/ui/associated-types/cache/project-fn-ret-invariant.oneuse.nll.stderr b/src/test/ui/associated-types/cache/project-fn-ret-invariant.oneuse.nll.stderr
index 44850df7b2f..e925a424c37 100644
--- a/src/test/ui/associated-types/cache/project-fn-ret-invariant.oneuse.nll.stderr
+++ b/src/test/ui/associated-types/cache/project-fn-ret-invariant.oneuse.nll.stderr
@@ -10,6 +10,9 @@ LL |     let a = bar(f, x);
    |             ^^^^^^^^^ argument requires that `'a` must outlive `'b`
    |
    = help: consider adding the following bound: `'a: 'b`
+   = note: requirement occurs because of the type Type<'_>, which makes the generic argument '_ invariant
+   = note: the struct Type<'a> is invariant over the parameter 'a
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/project-fn-ret-invariant.rs:40:13
@@ -23,6 +26,9 @@ LL |     let b = bar(f, y);
    |             ^^^^^^^^^ argument requires that `'b` must outlive `'a`
    |
    = help: consider adding the following bound: `'b: 'a`
+   = note: requirement occurs because of the type Type<'_>, which makes the generic argument '_ invariant
+   = note: the struct Type<'a> is invariant over the parameter 'a
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 help: `'a` and `'b` must be the same: replace one with the other
 
diff --git a/src/test/ui/associated-types/cache/project-fn-ret-invariant.transmute.nll.stderr b/src/test/ui/associated-types/cache/project-fn-ret-invariant.transmute.nll.stderr
index c417cdd543e..0457f142e19 100644
--- a/src/test/ui/associated-types/cache/project-fn-ret-invariant.transmute.nll.stderr
+++ b/src/test/ui/associated-types/cache/project-fn-ret-invariant.transmute.nll.stderr
@@ -6,6 +6,10 @@ LL | fn baz<'a, 'b>(x: Type<'a>) -> Type<'static> {
 ...
 LL |     bar(foo, x)
    |     ^^^^^^^^^^^ returning this value requires that `'a` must outlive `'static`
+   |
+   = note: requirement occurs because of the type Type<'_>, which makes the generic argument '_ invariant
+   = note: the struct Type<'a> 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
 
diff --git a/src/test/ui/c-variadic/variadic-ffi-4.stderr b/src/test/ui/c-variadic/variadic-ffi-4.stderr
index cf73403bbae..4b03fe15494 100644
--- a/src/test/ui/c-variadic/variadic-ffi-4.stderr
+++ b/src/test/ui/c-variadic/variadic-ffi-4.stderr
@@ -7,6 +7,10 @@ LL | pub unsafe extern "C" fn no_escape0<'f>(_: usize, ap: ...) -> VaListImpl<'f
    |                                     lifetime `'f` defined here
 LL |     ap
    |     ^^ function was supposed to return data with lifetime `'1` but it is returning data with lifetime `'f`
+   |
+   = note: requirement occurs because of the type VaListImpl<'_>, which makes the generic argument '_ invariant
+   = note: the struct VaListImpl<'f> is invariant over the parameter 'f
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variadic-ffi-4.rs:8:5
@@ -17,6 +21,10 @@ LL | pub unsafe extern "C" fn no_escape0<'f>(_: usize, ap: ...) -> VaListImpl<'f
    |                                     lifetime `'f` defined here
 LL |     ap
    |     ^^ returning this value requires that `'1` must outlive `'f`
+   |
+   = note: requirement occurs because of the type VaListImpl<'_>, which makes the generic argument '_ invariant
+   = note: the struct VaListImpl<'f> is invariant over the parameter 'f
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variadic-ffi-4.rs:14:5
@@ -25,6 +33,10 @@ LL | pub unsafe extern "C" fn no_escape1(_: usize, ap: ...) -> VaListImpl<'stati
    |                                               -- has type `VaListImpl<'1>`
 LL |     ap
    |     ^^ returning this value requires that `'1` must outlive `'static`
+   |
+   = note: requirement occurs because of the type VaListImpl<'_>, which makes the generic argument '_ invariant
+   = note: the struct VaListImpl<'f> is invariant over the parameter 'f
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variadic-ffi-4.rs:18:31
@@ -44,6 +56,10 @@ LL | pub unsafe extern "C" fn no_escape3(_: usize, mut ap0: &mut VaListImpl, mut
    |                                               has type `&mut VaListImpl<'1>`
 LL |     *ap0 = ap1;
    |     ^^^^ assignment requires that `'1` must outlive `'2`
+   |
+   = note: requirement occurs because of the type VaListImpl<'_>, which makes the generic argument '_ invariant
+   = note: the struct VaListImpl<'f> is invariant over the parameter 'f
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variadic-ffi-4.rs:22:5
@@ -54,6 +70,10 @@ LL | pub unsafe extern "C" fn no_escape3(_: usize, mut ap0: &mut VaListImpl, mut
    |                                               has type `&mut VaListImpl<'1>`
 LL |     *ap0 = ap1;
    |     ^^^^ assignment requires that `'2` must outlive `'1`
+   |
+   = note: requirement occurs because of the type VaListImpl<'_>, which makes the generic argument '_ invariant
+   = note: the struct VaListImpl<'f> is invariant over the parameter 'f
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variadic-ffi-4.rs:28:5
@@ -106,6 +126,10 @@ LL | pub unsafe extern "C" fn no_escape5(_: usize, mut ap0: &mut VaListImpl, mut
    |                                               has type `&mut VaListImpl<'1>`
 LL |     *ap0 = ap1.clone();
    |            ^^^^^^^^^^^ argument requires that `'1` must outlive `'2`
+   |
+   = note: requirement occurs because of the type VaListImpl<'_>, which makes the generic argument '_ invariant
+   = note: the struct VaListImpl<'f> is invariant over the parameter 'f
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variadic-ffi-4.rs:35:12
@@ -116,6 +140,10 @@ LL | pub unsafe extern "C" fn no_escape5(_: usize, mut ap0: &mut VaListImpl, mut
    |                                               has type `&mut VaListImpl<'1>`
 LL |     *ap0 = ap1.clone();
    |            ^^^^^^^^^^^ argument requires that `'2` must outlive `'1`
+   |
+   = note: requirement occurs because of the type VaListImpl<'_>, which makes the generic argument '_ invariant
+   = note: the struct VaListImpl<'f> is invariant over the parameter 'f
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: aborting due to 11 previous errors
 
diff --git a/src/test/ui/hr-subtype/hr-subtype.free_inv_x_vs_free_inv_y.nll.stderr b/src/test/ui/hr-subtype/hr-subtype.free_inv_x_vs_free_inv_y.nll.stderr
index e25acebf7f6..b4c54d52e5c 100644
--- a/src/test/ui/hr-subtype/hr-subtype.free_inv_x_vs_free_inv_y.nll.stderr
+++ b/src/test/ui/hr-subtype/hr-subtype.free_inv_x_vs_free_inv_y.nll.stderr
@@ -13,6 +13,9 @@ LL | | fn(Inv<'y>)) }
    | |______________- in this macro invocation
    |
    = help: consider adding the following bound: `'x: 'y`
+   = note: requirement occurs because of the type Inv<'_>, which makes the generic argument '_ invariant
+   = note: the struct Inv<'a> is invariant over the parameter 'a
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
    = note: this error originates in the macro `check` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: lifetime may not live long enough
@@ -30,6 +33,9 @@ LL | | fn(Inv<'y>)) }
    | |______________- in this macro invocation
    |
    = help: consider adding the following bound: `'x: 'y`
+   = note: requirement occurs because of the type Inv<'_>, which makes the generic argument '_ invariant
+   = note: the struct Inv<'a> is invariant over the parameter 'a
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
    = note: this error originates in the macro `check` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 2 previous errors
diff --git a/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-no-bound.stderr b/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-no-bound.stderr
index d77793291c5..cf563072dff 100644
--- a/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-no-bound.stderr
+++ b/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-no-bound.stderr
@@ -51,6 +51,10 @@ LL | |     });
    | |      |
    | |______`cell_a` escapes the function body here
    |        argument requires that `'a` must outlive `'static`
+   |
+   = note: requirement occurs because of the type Cell<&'_#10r u32>, which makes the generic argument &'_#10r u32 invariant
+   = note: the struct Cell<T> is invariant over the parameter T
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-wrong-bound.stderr b/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-wrong-bound.stderr
index cc67270ad20..453f6801d7e 100644
--- a/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-wrong-bound.stderr
+++ b/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-wrong-bound.stderr
@@ -51,6 +51,10 @@ LL | |     });
    | |      |
    | |______`cell_a` escapes the function body here
    |        argument requires that `'a` must outlive `'static`
+   |
+   = note: requirement occurs because of the type Cell<&'_#11r u32>, which makes the generic argument &'_#11r u32 invariant
+   = note: the struct Cell<T> is invariant over the parameter T
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/nll/where_clauses_in_structs.stderr b/src/test/ui/nll/where_clauses_in_structs.stderr
index 8499b00f6f5..952667d518d 100644
--- a/src/test/ui/nll/where_clauses_in_structs.stderr
+++ b/src/test/ui/nll/where_clauses_in_structs.stderr
@@ -9,6 +9,9 @@ LL |     Foo { x, y };
    |           ^ this usage requires that `'a` must outlive `'b`
    |
    = help: consider adding the following bound: `'a: 'b`
+   = note: requirement occurs because of the type Cell<&u32>, which makes the generic argument &u32 invariant
+   = note: the struct Cell<T> is invariant over the parameter T
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/regions/region-invariant-static-error-reporting.nll.stderr b/src/test/ui/regions/region-invariant-static-error-reporting.nll.stderr
index e220cbf5559..376534bf573 100644
--- a/src/test/ui/regions/region-invariant-static-error-reporting.nll.stderr
+++ b/src/test/ui/regions/region-invariant-static-error-reporting.nll.stderr
@@ -11,6 +11,10 @@ LL |         x.unwrap()
    |         |
    |         `x` escapes the function body here
    |         argument requires that `'a` must outlive `'static`
+   |
+   = note: requirement occurs because of the type Invariant<'_>, which makes the generic argument '_ invariant
+   = note: the struct Invariant<'a> 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
 
diff --git a/src/test/ui/regions/regions-bounded-method-type-parameters-cross-crate.nll.stderr b/src/test/ui/regions/regions-bounded-method-type-parameters-cross-crate.nll.stderr
index 47796e50a88..32f3080ea37 100644
--- a/src/test/ui/regions/regions-bounded-method-type-parameters-cross-crate.nll.stderr
+++ b/src/test/ui/regions/regions-bounded-method-type-parameters-cross-crate.nll.stderr
@@ -10,6 +10,9 @@ LL |     a.bigger_region(b)
    |     ^^^^^^^^^^^^^^^^^^ argument requires that `'y` must outlive `'x`
    |
    = help: consider adding the following bound: `'y: 'x`
+   = note: requirement occurs because of the type Inv<'_>, which makes the generic argument '_ invariant
+   = note: the struct Inv<'a> 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
 
diff --git a/src/test/ui/regions/regions-bounded-method-type-parameters-trait-bound.nll.stderr b/src/test/ui/regions/regions-bounded-method-type-parameters-trait-bound.nll.stderr
index 83d6e13dc0a..62032bcb609 100644
--- a/src/test/ui/regions/regions-bounded-method-type-parameters-trait-bound.nll.stderr
+++ b/src/test/ui/regions/regions-bounded-method-type-parameters-trait-bound.nll.stderr
@@ -15,6 +15,9 @@ LL |     f.method(b);
    |     argument requires that `'b` must outlive `'a`
    |
    = help: consider adding the following bound: `'b: 'a`
+   = note: requirement occurs because of the type Inv<'_>, which makes the generic argument '_ invariant
+   = note: the struct Inv<'a> 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
 
diff --git a/src/test/ui/regions/regions-infer-invariance-due-to-decl.nll.stderr b/src/test/ui/regions/regions-infer-invariance-due-to-decl.nll.stderr
index 0c1e3989b23..fede5f2d779 100644
--- a/src/test/ui/regions/regions-infer-invariance-due-to-decl.nll.stderr
+++ b/src/test/ui/regions/regions-infer-invariance-due-to-decl.nll.stderr
@@ -5,6 +5,10 @@ LL | fn to_longer_lifetime<'r>(b_isize: Invariant<'r>) -> Invariant<'static> {
    |                       -- lifetime `'r` defined here
 LL |     b_isize
    |     ^^^^^^^ returning this value requires that `'r` must outlive `'static`
+   |
+   = note: requirement occurs because of the type Invariant<'_>, which makes the generic argument '_ invariant
+   = note: the struct Invariant<'a> 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
 
diff --git a/src/test/ui/regions/regions-infer-invariance-due-to-mutability-3.nll.stderr b/src/test/ui/regions/regions-infer-invariance-due-to-mutability-3.nll.stderr
index 0edeb272399..8f5f3667453 100644
--- a/src/test/ui/regions/regions-infer-invariance-due-to-mutability-3.nll.stderr
+++ b/src/test/ui/regions/regions-infer-invariance-due-to-mutability-3.nll.stderr
@@ -5,6 +5,10 @@ LL | fn to_longer_lifetime<'r>(b_isize: Invariant<'r>) -> Invariant<'static> {
    |                       -- lifetime `'r` defined here
 LL |     b_isize
    |     ^^^^^^^ returning this value requires that `'r` must outlive `'static`
+   |
+   = note: requirement occurs because of the type Invariant<'_>, which makes the generic argument '_ invariant
+   = note: the struct Invariant<'a> 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
 
diff --git a/src/test/ui/regions/regions-infer-invariance-due-to-mutability-4.nll.stderr b/src/test/ui/regions/regions-infer-invariance-due-to-mutability-4.nll.stderr
index 724dd7e3f6d..8079fb0ef0d 100644
--- a/src/test/ui/regions/regions-infer-invariance-due-to-mutability-4.nll.stderr
+++ b/src/test/ui/regions/regions-infer-invariance-due-to-mutability-4.nll.stderr
@@ -5,6 +5,10 @@ LL | fn to_longer_lifetime<'r>(b_isize: Invariant<'r>) -> Invariant<'static> {
    |                       -- lifetime `'r` defined here
 LL |     b_isize
    |     ^^^^^^^ returning this value requires that `'r` must outlive `'static`
+   |
+   = note: requirement occurs because of the type Invariant<'_>, which makes the generic argument '_ invariant
+   = note: the struct Invariant<'a> 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
 
diff --git a/src/test/ui/regions/regions-infer-not-param.nll.stderr b/src/test/ui/regions/regions-infer-not-param.nll.stderr
index fcc2ec31f3e..e211f9d1391 100644
--- a/src/test/ui/regions/regions-infer-not-param.nll.stderr
+++ b/src/test/ui/regions/regions-infer-not-param.nll.stderr
@@ -17,6 +17,9 @@ LL | fn take_indirect2<'a,'b>(p: Indirect2<'a>) -> Indirect2<'b> { p }
    |                   lifetime `'a` defined here
    |
    = help: consider adding the following bound: `'b: 'a`
+   = note: requirement occurs because of the type Indirect2<'_>, which makes the generic argument '_ invariant
+   = note: the struct Indirect2<'a> is invariant over the parameter 'a
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/regions-infer-not-param.rs:19:63
@@ -27,6 +30,9 @@ LL | fn take_indirect2<'a,'b>(p: Indirect2<'a>) -> Indirect2<'b> { p }
    |                   lifetime `'a` defined here
    |
    = help: consider adding the following bound: `'a: 'b`
+   = note: requirement occurs because of the type Indirect2<'_>, which makes the generic argument '_ invariant
+   = note: the struct Indirect2<'a> is invariant over the parameter 'a
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 help: `'b` and `'a` must be the same: replace one with the other
 
diff --git a/src/test/ui/regions/regions-variance-invariant-use-contravariant.nll.stderr b/src/test/ui/regions/regions-variance-invariant-use-contravariant.nll.stderr
index d51db99f81f..8e8ca8e47cc 100644
--- a/src/test/ui/regions/regions-variance-invariant-use-contravariant.nll.stderr
+++ b/src/test/ui/regions/regions-variance-invariant-use-contravariant.nll.stderr
@@ -10,6 +10,9 @@ LL |     let _: Invariant<'short> = c;
    |            ^^^^^^^^^^^^^^^^^ type annotation requires that `'short` must outlive `'long`
    |
    = help: consider adding the following bound: `'short: 'long`
+   = note: requirement occurs because of the type Invariant<'_>, which makes the generic argument '_ invariant
+   = note: the struct Invariant<'a> 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
 
diff --git a/src/test/ui/regions/regions-variance-invariant-use-covariant.nll.stderr b/src/test/ui/regions/regions-variance-invariant-use-covariant.nll.stderr
index 15853e6ca5d..f9a3d727f7a 100644
--- a/src/test/ui/regions/regions-variance-invariant-use-covariant.nll.stderr
+++ b/src/test/ui/regions/regions-variance-invariant-use-covariant.nll.stderr
@@ -6,6 +6,10 @@ LL | fn use_<'b>(c: Invariant<'b>) {
 ...
 LL |     let _: Invariant<'static> = c;
    |            ^^^^^^^^^^^^^^^^^^ type annotation requires that `'b` must outlive `'static`
+   |
+   = note: requirement occurs because of the type Invariant<'_>, which makes the generic argument '_ invariant
+   = note: the struct Invariant<'a> 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
 
diff --git a/src/test/ui/variance/variance-btree-invariant-types.nll.stderr b/src/test/ui/variance/variance-btree-invariant-types.nll.stderr
index 1f06949c033..867d9f8238a 100644
--- a/src/test/ui/variance/variance-btree-invariant-types.nll.stderr
+++ b/src/test/ui/variance/variance-btree-invariant-types.nll.stderr
@@ -5,6 +5,10 @@ LL | fn iter_cov_key<'a, 'new>(v: IterMut<'a, &'static (), ()>) -> IterMut<'a, &
    |                     ---- lifetime `'new` defined here
 LL |     v
    |     ^ returning this value requires that `'new` must outlive `'static`
+   |
+   = note: requirement occurs because of the type std::collections::btree_map::IterMut<'_, &(), ()>, which makes the generic argument &() invariant
+   = note: the struct std::collections::btree_map::IterMut<'a, K, V> is invariant over the parameter K
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variance-btree-invariant-types.rs:7:5
@@ -13,6 +17,10 @@ LL | fn iter_cov_val<'a, 'new>(v: IterMut<'a, (), &'static ()>) -> IterMut<'a, (
    |                     ---- lifetime `'new` defined here
 LL |     v
    |     ^ returning this value requires that `'new` must outlive `'static`
+   |
+   = note: requirement occurs because of the type std::collections::btree_map::IterMut<'_, (), &()>, which makes the generic argument () invariant
+   = note: the struct std::collections::btree_map::IterMut<'a, K, V> is invariant over the parameter K
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variance-btree-invariant-types.rs:10:5
@@ -21,6 +29,10 @@ LL | fn iter_contra_key<'a, 'new>(v: IterMut<'a, &'new (), ()>) -> IterMut<'a, &
    |                        ---- lifetime `'new` defined here
 LL |     v
    |     ^ returning this value requires that `'new` must outlive `'static`
+   |
+   = note: requirement occurs because of the type std::collections::btree_map::IterMut<'_, &(), ()>, which makes the generic argument &() invariant
+   = note: the struct std::collections::btree_map::IterMut<'a, K, V> is invariant over the parameter K
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variance-btree-invariant-types.rs:13:5
@@ -29,6 +41,10 @@ LL | fn iter_contra_val<'a, 'new>(v: IterMut<'a, (), &'new ()>) -> IterMut<'a, (
    |                        ---- lifetime `'new` defined here
 LL |     v
    |     ^ returning this value requires that `'new` must outlive `'static`
+   |
+   = note: requirement occurs because of the type std::collections::btree_map::IterMut<'_, (), &()>, which makes the generic argument () invariant
+   = note: the struct std::collections::btree_map::IterMut<'a, K, V> is invariant over the parameter K
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variance-btree-invariant-types.rs:17:5
@@ -37,6 +53,10 @@ LL | fn range_cov_key<'a, 'new>(v: RangeMut<'a, &'static (), ()>) -> RangeMut<'a
    |                      ---- lifetime `'new` defined here
 LL |     v
    |     ^ returning this value requires that `'new` must outlive `'static`
+   |
+   = note: requirement occurs because of the type RangeMut<'_, &(), ()>, which makes the generic argument &() invariant
+   = note: the struct RangeMut<'a, K, V> is invariant over the parameter K
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variance-btree-invariant-types.rs:20:5
@@ -45,6 +65,10 @@ LL | fn range_cov_val<'a, 'new>(v: RangeMut<'a, (), &'static ()>) -> RangeMut<'a
    |                      ---- lifetime `'new` defined here
 LL |     v
    |     ^ returning this value requires that `'new` must outlive `'static`
+   |
+   = note: requirement occurs because of the type RangeMut<'_, (), &()>, which makes the generic argument () invariant
+   = note: the struct RangeMut<'a, K, V> is invariant over the parameter K
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variance-btree-invariant-types.rs:23:5
@@ -53,6 +77,10 @@ LL | fn range_contra_key<'a, 'new>(v: RangeMut<'a, &'new (), ()>) -> RangeMut<'a
    |                         ---- lifetime `'new` defined here
 LL |     v
    |     ^ returning this value requires that `'new` must outlive `'static`
+   |
+   = note: requirement occurs because of the type RangeMut<'_, &(), ()>, which makes the generic argument &() invariant
+   = note: the struct RangeMut<'a, K, V> is invariant over the parameter K
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variance-btree-invariant-types.rs:26:5
@@ -61,6 +89,10 @@ LL | fn range_contra_val<'a, 'new>(v: RangeMut<'a, (), &'new ()>) -> RangeMut<'a
    |                         ---- lifetime `'new` defined here
 LL |     v
    |     ^ returning this value requires that `'new` must outlive `'static`
+   |
+   = note: requirement occurs because of the type RangeMut<'_, (), &()>, which makes the generic argument () invariant
+   = note: the struct RangeMut<'a, K, V> is invariant over the parameter K
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variance-btree-invariant-types.rs:31:5
@@ -70,6 +102,10 @@ LL | fn occ_cov_key<'a, 'new>(v: OccupiedEntry<'a, &'static (), ()>)
 LL |                          -> OccupiedEntry<'a, &'new (), ()> {
 LL |     v
    |     ^ returning this value requires that `'new` must outlive `'static`
+   |
+   = note: requirement occurs because of the type std::collections::btree_map::OccupiedEntry<'_, &(), ()>, which makes the generic argument &() invariant
+   = note: the struct std::collections::btree_map::OccupiedEntry<'a, K, V> is invariant over the parameter K
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variance-btree-invariant-types.rs:35:5
@@ -79,6 +115,10 @@ LL | fn occ_cov_val<'a, 'new>(v: OccupiedEntry<'a, (), &'static ()>)
 LL |                          -> OccupiedEntry<'a, (), &'new ()> {
 LL |     v
    |     ^ returning this value requires that `'new` must outlive `'static`
+   |
+   = note: requirement occurs because of the type std::collections::btree_map::OccupiedEntry<'_, (), &()>, which makes the generic argument () invariant
+   = note: the struct std::collections::btree_map::OccupiedEntry<'a, K, V> is invariant over the parameter K
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variance-btree-invariant-types.rs:39:5
@@ -88,6 +128,10 @@ LL | fn occ_contra_key<'a, 'new>(v: OccupiedEntry<'a, &'new (), ()>)
 LL |                             -> OccupiedEntry<'a, &'static (), ()> {
 LL |     v
    |     ^ returning this value requires that `'new` must outlive `'static`
+   |
+   = note: requirement occurs because of the type std::collections::btree_map::OccupiedEntry<'_, &(), ()>, which makes the generic argument &() invariant
+   = note: the struct std::collections::btree_map::OccupiedEntry<'a, K, V> is invariant over the parameter K
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variance-btree-invariant-types.rs:43:5
@@ -97,6 +141,10 @@ LL | fn occ_contra_val<'a, 'new>(v: OccupiedEntry<'a, (), &'new ()>)
 LL |                             -> OccupiedEntry<'a, (), &'static ()> {
 LL |     v
    |     ^ returning this value requires that `'new` must outlive `'static`
+   |
+   = note: requirement occurs because of the type std::collections::btree_map::OccupiedEntry<'_, (), &()>, which makes the generic argument () invariant
+   = note: the struct std::collections::btree_map::OccupiedEntry<'a, K, V> is invariant over the parameter K
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variance-btree-invariant-types.rs:48:5
@@ -106,6 +154,10 @@ LL | fn vac_cov_key<'a, 'new>(v: VacantEntry<'a, &'static (), ()>)
 LL |                          -> VacantEntry<'a, &'new (), ()> {
 LL |     v
    |     ^ returning this value requires that `'new` must outlive `'static`
+   |
+   = note: requirement occurs because of the type std::collections::btree_map::VacantEntry<'_, &(), ()>, which makes the generic argument &() invariant
+   = note: the struct std::collections::btree_map::VacantEntry<'a, K, V> is invariant over the parameter K
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variance-btree-invariant-types.rs:52:5
@@ -115,6 +167,10 @@ LL | fn vac_cov_val<'a, 'new>(v: VacantEntry<'a, (), &'static ()>)
 LL |                          -> VacantEntry<'a, (), &'new ()> {
 LL |     v
    |     ^ returning this value requires that `'new` must outlive `'static`
+   |
+   = note: requirement occurs because of the type std::collections::btree_map::VacantEntry<'_, (), &()>, which makes the generic argument () invariant
+   = note: the struct std::collections::btree_map::VacantEntry<'a, K, V> is invariant over the parameter K
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variance-btree-invariant-types.rs:56:5
@@ -124,6 +180,10 @@ LL | fn vac_contra_key<'a, 'new>(v: VacantEntry<'a, &'new (), ()>)
 LL |                             -> VacantEntry<'a, &'static (), ()> {
 LL |     v
    |     ^ returning this value requires that `'new` must outlive `'static`
+   |
+   = note: requirement occurs because of the type std::collections::btree_map::VacantEntry<'_, &(), ()>, which makes the generic argument &() invariant
+   = note: the struct std::collections::btree_map::VacantEntry<'a, K, V> is invariant over the parameter K
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variance-btree-invariant-types.rs:60:5
@@ -133,6 +193,10 @@ LL | fn vac_contra_val<'a, 'new>(v: VacantEntry<'a, (), &'new ()>)
 LL |                             -> VacantEntry<'a, (), &'static ()> {
 LL |     v
    |     ^ returning this value requires that `'new` must outlive `'static`
+   |
+   = note: requirement occurs because of the type std::collections::btree_map::VacantEntry<'_, (), &()>, which makes the generic argument () invariant
+   = note: the struct std::collections::btree_map::VacantEntry<'a, K, V> is invariant over the parameter K
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: aborting due to 16 previous errors
 
diff --git a/src/test/ui/variance/variance-cell-is-invariant.nll.stderr b/src/test/ui/variance/variance-cell-is-invariant.nll.stderr
index 1fcdfc4b5bb..d6a85680141 100644
--- a/src/test/ui/variance/variance-cell-is-invariant.nll.stderr
+++ b/src/test/ui/variance/variance-cell-is-invariant.nll.stderr
@@ -10,6 +10,9 @@ LL |     let _: Foo<'long> = c;
    |            ^^^^^^^^^^ type annotation requires that `'short` must outlive `'long`
    |
    = help: consider adding the following bound: `'short: 'long`
+   = note: requirement occurs because of the type Foo<'_>, which makes the generic argument '_ invariant
+   = note: the struct Foo<'a> 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
 
diff --git a/src/test/ui/variance/variance-use-invariant-struct-1.nll.stderr b/src/test/ui/variance/variance-use-invariant-struct-1.nll.stderr
index 385d83adaf8..6890cb115c3 100644
--- a/src/test/ui/variance/variance-use-invariant-struct-1.nll.stderr
+++ b/src/test/ui/variance/variance-use-invariant-struct-1.nll.stderr
@@ -10,6 +10,9 @@ LL |     v
    |     ^ returning this value requires that `'min` must outlive `'max`
    |
    = help: consider adding the following bound: `'min: 'max`
+   = note: requirement occurs because of the type SomeStruct<&()>, which makes the generic argument &() invariant
+   = note: the struct SomeStruct<T> is invariant over the parameter T
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
   --> $DIR/variance-use-invariant-struct-1.rs:19:5
@@ -23,6 +26,9 @@ LL |     v
    |     ^ returning this value requires that `'min` must outlive `'max`
    |
    = help: consider adding the following bound: `'min: 'max`
+   = note: requirement occurs because of the type SomeStruct<&()>, which makes the generic argument &() invariant
+   = note: the struct SomeStruct<T> is invariant over the parameter T
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: aborting due to 2 previous errors