Auto merge of #86700 - lqd:matthews-nll-hrtb-errors, r=nikomatsakis

Matthew's work on improving NLL's "higher-ranked subtype error"s

This PR rebases `@matthewjasper's` [branch](https://github.com/matthewjasper/rust/tree/nll-hrtb-errors) which has great work to fix the obscure higher-ranked subtype errors that are tracked in #57374.

These are a blocker to turning full NLLs on, and doing some internal cleanups to remove some of the old region code.

The goal is so `@nikomatsakis` can take a look at this early, and I'll then do my best to help do the changes and followup work to land this work, and move closer to turning off the migration mode.

I've only updated the branch and made it compile, removed a warning or two.

r? `@nikomatsakis`

(Here's the [zulip topic to discuss this](https://rust-lang.zulipchat.com/#narrow/stream/122657-t-compiler.2Fwg-nll/topic/.2357374.3A.20improving.20higher-ranked.20subtype.20errors.20via.20.2386700) that Niko wanted)

42 files changed, 341 insertions, 129 deletions

diff --git a/src/test/ui/associated-types/higher-ranked-projection.bad.nll.stderr b/src/test/ui/associated-types/higher-ranked-projection.bad.nll.stderr
index 2e03986a9ed..59e27cd2e7d 100644
--- a/src/test/ui/associated-types/higher-ranked-projection.bad.nll.stderr
+++ b/src/test/ui/associated-types/higher-ranked-projection.bad.nll.stderr
@@ -1,8 +1,12 @@
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/higher-ranked-projection.rs:25:5
    |
 LL |     foo(());
-   |     ^^^^^^^
+   |     ^^^^^^^ one type is more general than the other
+   |
+   = note:   expected type `&'a ()`
+           found reference `&()`
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/closure-expected-type/expect-fn-supply-fn.nll.stderr b/src/test/ui/closure-expected-type/expect-fn-supply-fn.nll.stderr
index 64a0b52a1fa..768dc8e12db 100644
--- a/src/test/ui/closure-expected-type/expect-fn-supply-fn.nll.stderr
+++ b/src/test/ui/closure-expected-type/expect-fn-supply-fn.nll.stderr
@@ -21,23 +21,33 @@ LL |     with_closure_expecting_fn_with_free_region(|x: fn(&'x u32), y| {});
    |
    = help: consider replacing `'x` with `'static`
 
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/expect-fn-supply-fn.rs:32:49
    |
 LL |     with_closure_expecting_fn_with_free_region(|x: fn(&u32), y| {});
-   |                                                 ^
+   |                                                 ^ one type is more general than the other
+   |
+   = note: expected fn pointer `for<'r> fn(&'r u32)`
+              found fn pointer `fn(&u32)`
 
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/expect-fn-supply-fn.rs:39:50
    |
 LL |     with_closure_expecting_fn_with_bound_region(|x: fn(&'x u32), y| {});
-   |                                                  ^
+   |                                                  ^ one type is more general than the other
+   |
+   = note: expected fn pointer `fn(&'x u32)`
+              found fn pointer `for<'r> fn(&'r u32)`
 
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/expect-fn-supply-fn.rs:48:50
    |
 LL |     with_closure_expecting_fn_with_bound_region(|x: Foo<'_>, y| {
-   |                                                  ^
+   |                                                  ^ one type is more general than the other
+   |
+   = note: expected fn pointer `fn(&u32)`
+              found fn pointer `for<'r> fn(&'r u32)`
 
 error: aborting due to 5 previous errors
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/generator/auto-trait-regions.nll.stderr b/src/test/ui/generator/auto-trait-regions.nll.stderr
index 794369a8dc0..76970fb7872 100644
--- a/src/test/ui/generator/auto-trait-regions.nll.stderr
+++ b/src/test/ui/generator/auto-trait-regions.nll.stderr
@@ -24,17 +24,23 @@ LL |         assert_foo(a);
    |
    = note: consider using a `let` binding to create a longer lived value
 
-error: higher-ranked subtype error
+error: implementation of `Foo` is not general enough
   --> $DIR/auto-trait-regions.rs:31:5
    |
 LL |     assert_foo(gen);
-   |     ^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
+   |
+   = note: `&'0 OnlyFooIfStaticRef` must implement `Foo`, for any lifetime `'0`...
+   = note: ...but `Foo` is actually implemented for the type `&'static OnlyFooIfStaticRef`
 
-error: higher-ranked subtype error
+error: implementation of `Foo` is not general enough
   --> $DIR/auto-trait-regions.rs:50:5
    |
 LL |     assert_foo(gen);
-   |     ^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
+   |
+   = note: `Foo` would have to be implemented for the type `A<'0, '1>`, for any two lifetimes `'0` and `'1`...
+   = note: ...but `Foo` is actually implemented for the type `A<'_, '2>`, for some specific lifetime `'2`
 
 error: aborting due to 4 previous errors
 
diff --git a/src/test/ui/generator/resume-arg-late-bound.nll.stderr b/src/test/ui/generator/resume-arg-late-bound.nll.stderr
index 7d712191924..25bc6afc550 100644
--- a/src/test/ui/generator/resume-arg-late-bound.nll.stderr
+++ b/src/test/ui/generator/resume-arg-late-bound.nll.stderr
@@ -1,8 +1,12 @@
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/resume-arg-late-bound.rs:15:5
    |
 LL |     test(gen);
-   |     ^^^^^^^^^
+   |     ^^^^^^^^^ one type is more general than the other
+   |
+   = note: expected type `for<'a> Generator<&'a mut bool>`
+              found type `Generator<&mut bool>`
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/higher-rank-trait-bounds/issue-59311.rs b/src/test/ui/higher-rank-trait-bounds/issue-59311.rs
index 1e1241c7f83..d617571753c 100644
--- a/src/test/ui/higher-rank-trait-bounds/issue-59311.rs
+++ b/src/test/ui/higher-rank-trait-bounds/issue-59311.rs
@@ -14,7 +14,7 @@ pub fn crash<V>(v: &V)
 where
     for<'a> &'a V: T + 'static,
 {
-    v.t(|| {}); //~ ERROR: higher-ranked subtype error
+    v.t(|| {}); //~ ERROR: higher-ranked lifetime error
 }
 
 fn main() {}
diff --git a/src/test/ui/higher-rank-trait-bounds/issue-59311.stderr b/src/test/ui/higher-rank-trait-bounds/issue-59311.stderr
index ca632629267..c16c8206153 100644
--- a/src/test/ui/higher-rank-trait-bounds/issue-59311.stderr
+++ b/src/test/ui/higher-rank-trait-bounds/issue-59311.stderr
@@ -1,8 +1,10 @@
-error: higher-ranked subtype error
+error: higher-ranked lifetime error
   --> $DIR/issue-59311.rs:17:9
    |
 LL |     v.t(|| {});
    |         ^^^^^
+   |
+   = note: could not prove for<'a> &'a V: 'static
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/hr-subtype/hr-subtype.bound_a_b_ret_a_vs_bound_a_ret_a.nll.stderr b/src/test/ui/hr-subtype/hr-subtype.bound_a_b_ret_a_vs_bound_a_ret_a.nll.stderr
index 4cca552d7d4..439a113ef38 100644
--- a/src/test/ui/hr-subtype/hr-subtype.bound_a_b_ret_a_vs_bound_a_ret_a.nll.stderr
+++ b/src/test/ui/hr-subtype/hr-subtype.bound_a_b_ret_a_vs_bound_a_ret_a.nll.stderr
@@ -1,14 +1,17 @@
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/hr-subtype.rs:45:13
    |
 LL |               gimme::<$t1>(None::<$t2>);
-   |               ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
 ...
 LL | / check! { bound_a_b_ret_a_vs_bound_a_ret_a: (for<'a,'b> fn(&'a u32, &'b u32) -> &'a u32,
 LL | | for<'a>    fn(&'a u32, &'a u32) -> &'a u32) }
    | |_____________________________________________- in this macro invocation
    |
+   = note: expected enum `Option<for<'r, 's> fn(&'r u32, &'s u32) -> &'r u32>`
+              found enum `Option<for<'r> fn(&'r u32, &'r u32) -> &'r u32>`
    = note: this error originates in the macro `check` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/hr-subtype/hr-subtype.bound_a_vs_free_x.nll.stderr b/src/test/ui/hr-subtype/hr-subtype.bound_a_vs_free_x.nll.stderr
index 2c1ac126fab..61b3f0ca284 100644
--- a/src/test/ui/hr-subtype/hr-subtype.bound_a_vs_free_x.nll.stderr
+++ b/src/test/ui/hr-subtype/hr-subtype.bound_a_vs_free_x.nll.stderr
@@ -1,14 +1,17 @@
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/hr-subtype.rs:45:13
    |
 LL |               gimme::<$t1>(None::<$t2>);
-   |               ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
 ...
 LL | / check! { bound_a_vs_free_x: (for<'a> fn(&'a u32),
 LL | | fn(&'x u32)) }
    | |______________- in this macro invocation
    |
+   = note: expected enum `Option<for<'r> fn(&'r u32)>`
+              found enum `Option<fn(&u32)>`
    = note: this error originates in the macro `check` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/hr-subtype/hr-subtype.bound_inv_a_b_vs_bound_inv_a.nll.stderr b/src/test/ui/hr-subtype/hr-subtype.bound_inv_a_b_vs_bound_inv_a.nll.stderr
index 816984654f7..75e2ba58f33 100644
--- a/src/test/ui/hr-subtype/hr-subtype.bound_inv_a_b_vs_bound_inv_a.nll.stderr
+++ b/src/test/ui/hr-subtype/hr-subtype.bound_inv_a_b_vs_bound_inv_a.nll.stderr
@@ -1,26 +1,31 @@
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/hr-subtype.rs:45:13
    |
 LL |               gimme::<$t1>(None::<$t2>);
-   |               ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
 ...
 LL | / check! { bound_inv_a_b_vs_bound_inv_a: (for<'a,'b> fn(Inv<'a>, Inv<'b>),
 LL | | for<'a>    fn(Inv<'a>, Inv<'a>)) }
    | |__________________________________- in this macro invocation
    |
+   = note: expected enum `Option<for<'r, 's> fn(Inv<'r>, Inv<'s>)>`
+              found enum `Option<for<'r> fn(Inv<'r>, Inv<'r>)>`
    = note: this error originates in the macro `check` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/hr-subtype.rs:45:13
    |
 LL |               gimme::<$t1>(None::<$t2>);
-   |               ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
 ...
 LL | / check! { bound_inv_a_b_vs_bound_inv_a: (for<'a,'b> fn(Inv<'a>, Inv<'b>),
 LL | | for<'a>    fn(Inv<'a>, Inv<'a>)) }
    | |__________________________________- in this macro invocation
    |
+   = note: expected enum `Option<for<'r, 's> fn(Inv<'r>, Inv<'s>)>`
+              found enum `Option<for<'r> fn(Inv<'r>, Inv<'r>)>`
    = 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
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/hrtb/hrtb-conflate-regions.nll.stderr b/src/test/ui/hrtb/hrtb-conflate-regions.nll.stderr
index f290a93326f..46f5308dd87 100644
--- a/src/test/ui/hrtb/hrtb-conflate-regions.nll.stderr
+++ b/src/test/ui/hrtb/hrtb-conflate-regions.nll.stderr
@@ -1,14 +1,20 @@
-error: higher-ranked subtype error
+error: implementation of `Foo` is not general enough
   --> $DIR/hrtb-conflate-regions.rs:27:10
    |
 LL | fn b() { want_foo2::<SomeStruct>(); }
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
+   |
+   = note: `SomeStruct` must implement `Foo<(&'0 isize, &'1 isize)>`, for any two lifetimes `'0` and `'1`...
+   = note: ...but it actually implements `Foo<(&'2 isize, &'2 isize)>`, for some specific lifetime `'2`
 
-error: higher-ranked subtype error
+error: implementation of `Foo` is not general enough
   --> $DIR/hrtb-conflate-regions.rs:27:10
    |
 LL | fn b() { want_foo2::<SomeStruct>(); }
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
+   |
+   = note: `SomeStruct` must implement `Foo<(&'0 isize, &'1 isize)>`, for any two lifetimes `'0` and `'1`...
+   = note: ...but it actually implements `Foo<(&'2 isize, &'2 isize)>`, for some specific lifetime `'2`
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/hrtb/hrtb-exists-forall-fn.nll.stderr b/src/test/ui/hrtb/hrtb-exists-forall-fn.nll.stderr
index 11390d9e2d2..1ee3c674963 100644
--- a/src/test/ui/hrtb/hrtb-exists-forall-fn.nll.stderr
+++ b/src/test/ui/hrtb/hrtb-exists-forall-fn.nll.stderr
@@ -1,8 +1,12 @@
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/hrtb-exists-forall-fn.rs:17:12
    |
 LL |     let _: for<'b> fn(&'b u32) = foo();
-   |            ^^^^^^^^^^^^^^^^^^^
+   |            ^^^^^^^^^^^^^^^^^^^ one type is more general than the other
+   |
+   = note: expected fn pointer `for<'b> fn(&'b u32)`
+              found fn pointer `fn(&u32)`
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/hrtb/hrtb-exists-forall-trait-contravariant.nll.stderr b/src/test/ui/hrtb/hrtb-exists-forall-trait-contravariant.nll.stderr
index a4c3ffd1f6c..364b613fc77 100644
--- a/src/test/ui/hrtb/hrtb-exists-forall-trait-contravariant.nll.stderr
+++ b/src/test/ui/hrtb/hrtb-exists-forall-trait-contravariant.nll.stderr
@@ -1,8 +1,11 @@
-error: higher-ranked subtype error
+error: implementation of `Trait` is not general enough
   --> $DIR/hrtb-exists-forall-trait-contravariant.rs:34:5
    |
 LL |     foo::<()>();
-   |     ^^^^^^^^^^^
+   |     ^^^^^^^^^^^ implementation of `Trait` is not general enough
+   |
+   = note: `()` must implement `Trait<for<'b> fn(&'b u32)>`
+   = note: ...but it actually implements `Trait<fn(&'0 u32)>`, for some specific lifetime `'0`
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/hrtb/hrtb-exists-forall-trait-invariant.nll.stderr b/src/test/ui/hrtb/hrtb-exists-forall-trait-invariant.nll.stderr
index e2a399b2faa..cb2ce8a4116 100644
--- a/src/test/ui/hrtb/hrtb-exists-forall-trait-invariant.nll.stderr
+++ b/src/test/ui/hrtb/hrtb-exists-forall-trait-invariant.nll.stderr
@@ -1,8 +1,11 @@
-error: higher-ranked subtype error
+error: implementation of `Trait` is not general enough
   --> $DIR/hrtb-exists-forall-trait-invariant.rs:28:5
    |
 LL |     foo::<()>();
-   |     ^^^^^^^^^^^
+   |     ^^^^^^^^^^^ implementation of `Trait` is not general enough
+   |
+   = note: `()` must implement `Trait<for<'b> fn(Cell<&'b u32>)>`
+   = note: ...but it actually implements `Trait<fn(Cell<&'0 u32>)>`, for some specific lifetime `'0`
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/hrtb/hrtb-just-for-static.nll.stderr b/src/test/ui/hrtb/hrtb-just-for-static.nll.stderr
index 8901a1b4681..a812282def9 100644
--- a/src/test/ui/hrtb/hrtb-just-for-static.nll.stderr
+++ b/src/test/ui/hrtb/hrtb-just-for-static.nll.stderr
@@ -1,8 +1,11 @@
-error: higher-ranked subtype error
+error: implementation of `Foo` is not general enough
   --> $DIR/hrtb-just-for-static.rs:24:5
    |
 LL |     want_hrtb::<StaticInt>()
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
+   |
+   = note: `StaticInt` must implement `Foo<&'0 isize>`, for any lifetime `'0`...
+   = note: ...but it actually implements `Foo<&'static isize>`
 
 error: lifetime may not live long enough
   --> $DIR/hrtb-just-for-static.rs:30:5
diff --git a/src/test/ui/hrtb/hrtb-perfect-forwarding.nll.stderr b/src/test/ui/hrtb/hrtb-perfect-forwarding.nll.stderr
index c3dd7949575..aefe3cdfd64 100644
--- a/src/test/ui/hrtb/hrtb-perfect-forwarding.nll.stderr
+++ b/src/test/ui/hrtb/hrtb-perfect-forwarding.nll.stderr
@@ -57,11 +57,14 @@ LL |     foo_hrtb_bar_not(&mut t);
    |
    = help: consider replacing `'b` with `'static`
 
-error: higher-ranked subtype error
+error: implementation of `Bar` is not general enough
   --> $DIR/hrtb-perfect-forwarding.rs:43:5
    |
 LL |     foo_hrtb_bar_not(&mut t);
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Bar` is not general enough
+   |
+   = note: `T` must implement `Bar<&'0 isize>`, for any lifetime `'0`...
+   = note: ...but it actually implements `Bar<&'1 isize>`, for some specific lifetime `'1`
 
 warning: function cannot return without recursing
   --> $DIR/hrtb-perfect-forwarding.rs:48:1
diff --git a/src/test/ui/hrtb/issue-46989.nll.stderr b/src/test/ui/hrtb/issue-46989.nll.stderr
index 6c127b92d97..309e1a676ed 100644
--- a/src/test/ui/hrtb/issue-46989.nll.stderr
+++ b/src/test/ui/hrtb/issue-46989.nll.stderr
@@ -1,8 +1,11 @@
-error: higher-ranked subtype error
+error: implementation of `Foo` is not general enough
   --> $DIR/issue-46989.rs:38:5
    |
 LL |     assert_foo::<fn(&i32)>();
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
+   |
+   = note: `Foo` would have to be implemented for the type `for<'r> fn(&'r i32)`
+   = note: ...but `Foo` is actually implemented for the type `fn(&'0 i32)`, for some specific lifetime `'0`
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/issues/issue-40000.nll.stderr b/src/test/ui/issues/issue-40000.nll.stderr
index 4e2bde06a52..e6f0b5fbfba 100644
--- a/src/test/ui/issues/issue-40000.nll.stderr
+++ b/src/test/ui/issues/issue-40000.nll.stderr
@@ -1,14 +1,21 @@
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/issue-40000.rs:6:9
    |
 LL |     foo(bar);
-   |         ^^^
+   |         ^^^ one type is more general than the other
+   |
+   = note: expected trait object `dyn for<'r> Fn(&'r i32)`
+              found trait object `dyn Fn(&i32)`
 
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/issue-40000.rs:6:9
    |
 LL |     foo(bar);
-   |         ^^^
+   |         ^^^ one type is more general than the other
+   |
+   = note: expected trait object `dyn for<'r> Fn(&'r i32)`
+              found trait object `dyn Fn(&i32)`
 
 error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/issues/issue-54302-cases.nll.stderr b/src/test/ui/issues/issue-54302-cases.nll.stderr
index 7463a3f286f..6e8b69c4bee 100644
--- a/src/test/ui/issues/issue-54302-cases.nll.stderr
+++ b/src/test/ui/issues/issue-54302-cases.nll.stderr
@@ -1,26 +1,38 @@
-error: higher-ranked subtype error
+error: implementation of `Foo` is not general enough
   --> $DIR/issue-54302-cases.rs:63:5
    |
 LL |     <u32 as RefFoo<u32>>::ref_foo(a)
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
+   |
+   = note: `Foo<'static, u32>` would have to be implemented for the type `&'0 u32`, for any lifetime `'0`...
+   = note: ...but `Foo<'_, u32>` is actually implemented for the type `&'1 u32`, for some specific lifetime `'1`
 
-error: higher-ranked subtype error
+error: implementation of `Foo` is not general enough
   --> $DIR/issue-54302-cases.rs:69:5
    |
 LL |     <i32 as RefFoo<i32>>::ref_foo(a)
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
+   |
+   = note: `Foo<'static, i32>` would have to be implemented for the type `&'0 i32`, for any lifetime `'0`...
+   = note: ...but `Foo<'_, i32>` is actually implemented for the type `&'1 i32`, for some specific lifetime `'1`
 
-error: higher-ranked subtype error
+error: implementation of `Foo` is not general enough
   --> $DIR/issue-54302-cases.rs:75:5
    |
 LL |     <u64 as RefFoo<u64>>::ref_foo(a)
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
+   |
+   = note: `Foo<'static, u64>` would have to be implemented for the type `&'0 u64`, for any lifetime `'0`...
+   = note: ...but `Foo<'_, u64>` is actually implemented for the type `&'1 u64`, for some specific lifetime `'1`
 
-error: higher-ranked subtype error
+error: implementation of `Foo` is not general enough
   --> $DIR/issue-54302-cases.rs:81:5
    |
 LL |     <i64 as RefFoo<i64>>::ref_foo(a)
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
+   |
+   = note: `Foo<'static, i64>` would have to be implemented for the type `&'0 i64`, for any lifetime `'0`...
+   = note: ...but `Foo<'_, i64>` is actually implemented for the type `&'1 i64`, for some specific lifetime `'1`
 
 error: aborting due to 4 previous errors
 
diff --git a/src/test/ui/issues/issue-55731.nll.stderr b/src/test/ui/issues/issue-55731.nll.stderr
index dd38bb62912..97fd6678c99 100644
--- a/src/test/ui/issues/issue-55731.nll.stderr
+++ b/src/test/ui/issues/issue-55731.nll.stderr
@@ -1,11 +1,14 @@
-error: higher-ranked subtype error
+error: implementation of `DistributedIteratorMulti` is not general enough
   --> $DIR/issue-55731.rs:48:5
    |
 LL | /     multi(Map {
 LL | |         i: Cloned(PhantomData),
 LL | |         f: X,
 LL | |     });
-   | |______^
+   | |______^ implementation of `DistributedIteratorMulti` is not general enough
+   |
+   = note: `DistributedIteratorMulti<&'0 ()>` would have to be implemented for the type `Cloned<&()>`, for any lifetime `'0`...
+   = note: ...but `DistributedIteratorMulti<&'1 ()>` is actually implemented for the type `Cloned<&'1 ()>`, for some specific lifetime `'1`
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/issues/issue-57843.nll.stderr b/src/test/ui/issues/issue-57843.nll.stderr
index 70d16cc9a1d..2ab49ec61cf 100644
--- a/src/test/ui/issues/issue-57843.nll.stderr
+++ b/src/test/ui/issues/issue-57843.nll.stderr
@@ -1,8 +1,11 @@
-error: higher-ranked subtype error
+error: implementation of `FnOnce` is not general enough
   --> $DIR/issue-57843.rs:25:9
    |
 LL |     Foo(Box::new(|_| ()));
-   |         ^^^^^^^^^^^^^^^^
+   |         ^^^^^^^^^^^^^^^^ implementation of `FnOnce` is not general enough
+   |
+   = note: closure with signature `fn(&'2 bool)` must implement `FnOnce<(&'1 bool,)>`, for any lifetime `'1`...
+   = note: ...but it actually implements `FnOnce<(&'2 bool,)>`, for some specific lifetime `'2`
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/lifetimes/issue-79187-2.nll.stderr b/src/test/ui/lifetimes/issue-79187-2.nll.stderr
index 4970c579e7b..907b43d6762 100644
--- a/src/test/ui/lifetimes/issue-79187-2.nll.stderr
+++ b/src/test/ui/lifetimes/issue-79187-2.nll.stderr
@@ -16,29 +16,47 @@ LL |     take_foo(|a: &i32| -> &i32 { a });
    |                  |        let's call the lifetime of this reference `'2`
    |                  let's call the lifetime of this reference `'1`
 
-error: higher-ranked subtype error
+error: implementation of `FnOnce` is not general enough
   --> $DIR/issue-79187-2.rs:8:5
    |
 LL |     take_foo(|a| a);
-   |     ^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^ implementation of `FnOnce` is not general enough
+   |
+   = note: closure with signature `fn(&'2 i32) -> &i32` must implement `FnOnce<(&'1 i32,)>`, for any lifetime `'1`...
+   = note: ...but it actually implements `FnOnce<(&'2 i32,)>`, for some specific lifetime `'2`
 
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/issue-79187-2.rs:8:5
    |
 LL |     take_foo(|a| a);
-   |     ^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^ one type is more general than the other
+   |
+   = note: expected type `for<'r> Fn<(&'r i32,)>`
+              found type `Fn<(&i32,)>`
+note: this closure does not fulfill the lifetime requirements
+  --> $DIR/issue-79187-2.rs:8:14
+   |
+LL |     take_foo(|a| a);
+   |              ^^^^^
 
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/issue-79187-2.rs:9:5
    |
 LL |     take_foo(|a: &i32| a);
-   |     ^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
+   |
+   = note: expected reference `&i32`
+              found reference `&i32`
 
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/issue-79187-2.rs:10:5
    |
 LL |     take_foo(|a: &i32| -> &i32 { a });
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
+   |
+   = note: expected reference `&i32`
+              found reference `&i32`
 
 error: aborting due to 6 previous errors
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/lifetimes/issue-79187.nll.stderr b/src/test/ui/lifetimes/issue-79187.nll.stderr
index aa8809dbc95..725b132e83a 100644
--- a/src/test/ui/lifetimes/issue-79187.nll.stderr
+++ b/src/test/ui/lifetimes/issue-79187.nll.stderr
@@ -1,14 +1,26 @@
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/issue-79187.rs:5:5
    |
 LL |     thing(f);
-   |     ^^^^^^^^
+   |     ^^^^^^^^ one type is more general than the other
+   |
+   = note: expected type `for<'r> FnOnce<(&'r u32,)>`
+              found type `FnOnce<(&u32,)>`
+note: this closure does not fulfill the lifetime requirements
+  --> $DIR/issue-79187.rs:4:13
+   |
+LL |     let f = |_| ();
+   |             ^^^^^^
 
-error: higher-ranked subtype error
+error: implementation of `FnOnce` is not general enough
   --> $DIR/issue-79187.rs:5:5
    |
 LL |     thing(f);
-   |     ^^^^^^^^
+   |     ^^^^^^^^ implementation of `FnOnce` is not general enough
+   |
+   = note: closure with signature `fn(&'2 u32)` must implement `FnOnce<(&'1 u32,)>`, for any lifetime `'1`...
+   = note: ...but it actually implements `FnOnce<(&'2 u32,)>`, for some specific lifetime `'2`
 
 error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/lub-glb/old-lub-glb-hr-noteq1.nll.stderr b/src/test/ui/lub-glb/old-lub-glb-hr-noteq1.nll.stderr
index b95e247d2a8..3fdc2da9f1e 100644
--- a/src/test/ui/lub-glb/old-lub-glb-hr-noteq1.nll.stderr
+++ b/src/test/ui/lub-glb/old-lub-glb-hr-noteq1.nll.stderr
@@ -1,8 +1,12 @@
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/old-lub-glb-hr-noteq1.rs:11:14
    |
 LL |         _ => y,
-   |              ^
+   |              ^ one type is more general than the other
+   |
+   = note: expected fn pointer `for<'r, 's> fn(&'r u8, &'s u8) -> &'r u8`
+              found fn pointer `for<'r> fn(&'r u8, &'r u8) -> &'r u8`
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/lub-glb/old-lub-glb-object.nll.stderr b/src/test/ui/lub-glb/old-lub-glb-object.nll.stderr
index 51bf96f3233..ad14d6b7521 100644
--- a/src/test/ui/lub-glb/old-lub-glb-object.nll.stderr
+++ b/src/test/ui/lub-glb/old-lub-glb-object.nll.stderr
@@ -1,14 +1,21 @@
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/old-lub-glb-object.rs:10:14
    |
 LL |         _ => y,
-   |              ^
+   |              ^ one type is more general than the other
+   |
+   = note: expected trait object `dyn for<'r, 's> Foo<&'r u8, &'s u8>`
+              found trait object `dyn for<'r> Foo<&'r u8, &'r u8>`
 
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/old-lub-glb-object.rs:10:14
    |
 LL |         _ => y,
-   |              ^
+   |              ^ one type is more general than the other
+   |
+   = note: expected trait object `dyn for<'r, 's> Foo<&'r u8, &'s u8>`
+              found trait object `dyn for<'r> Foo<&'r u8, &'r u8>`
 
 error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/mismatched_types/closure-mismatch.nll.stderr b/src/test/ui/mismatched_types/closure-mismatch.nll.stderr
index 745a61b866e..f29126e6afc 100644
--- a/src/test/ui/mismatched_types/closure-mismatch.nll.stderr
+++ b/src/test/ui/mismatched_types/closure-mismatch.nll.stderr
@@ -1,14 +1,26 @@
-error: higher-ranked subtype error
+error: implementation of `FnOnce` is not general enough
   --> $DIR/closure-mismatch.rs:8:5
    |
 LL |     baz(|_| ());
-   |     ^^^^^^^^^^^
+   |     ^^^^^^^^^^^ implementation of `FnOnce` is not general enough
+   |
+   = note: closure with signature `fn(&'2 ())` must implement `FnOnce<(&'1 (),)>`, for any lifetime `'1`...
+   = note: ...but it actually implements `FnOnce<(&'2 (),)>`, for some specific lifetime `'2`
 
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/closure-mismatch.rs:8:5
    |
 LL |     baz(|_| ());
-   |     ^^^^^^^^^^^
+   |     ^^^^^^^^^^^ one type is more general than the other
+   |
+   = note: expected type `for<'r> Fn<(&'r (),)>`
+              found type `Fn<(&(),)>`
+note: this closure does not fulfill the lifetime requirements
+  --> $DIR/closure-mismatch.rs:8:9
+   |
+LL |     baz(|_| ());
+   |         ^^^^^^
 
 error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/nll/relate_tys/fn-subtype.rs b/src/test/ui/nll/relate_tys/fn-subtype.rs
index ac00627ad00..0730dcc9e49 100644
--- a/src/test/ui/nll/relate_tys/fn-subtype.rs
+++ b/src/test/ui/nll/relate_tys/fn-subtype.rs
@@ -6,5 +6,5 @@
 
 fn main() {
     let x: fn(&'static ()) = |_| {};
-    let y: for<'a> fn(&'a ()) = x; //~ ERROR higher-ranked subtype error
+    let y: for<'a> fn(&'a ()) = x; //~ ERROR mismatched types [E0308]
 }
diff --git a/src/test/ui/nll/relate_tys/fn-subtype.stderr b/src/test/ui/nll/relate_tys/fn-subtype.stderr
index b089b5aaa25..94def690086 100644
--- a/src/test/ui/nll/relate_tys/fn-subtype.stderr
+++ b/src/test/ui/nll/relate_tys/fn-subtype.stderr
@@ -1,8 +1,12 @@
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/fn-subtype.rs:9:33
    |
 LL |     let y: for<'a> fn(&'a ()) = x;
-   |                                 ^
+   |                                 ^ one type is more general than the other
+   |
+   = note: expected fn pointer `for<'r> fn(&'r ())`
+              found fn pointer `fn(&())`
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/nll/relate_tys/hr-fn-aaa-as-aba.rs b/src/test/ui/nll/relate_tys/hr-fn-aaa-as-aba.rs
index fca69b83efe..a6d6ffa0ce3 100644
--- a/src/test/ui/nll/relate_tys/hr-fn-aaa-as-aba.rs
+++ b/src/test/ui/nll/relate_tys/hr-fn-aaa-as-aba.rs
@@ -12,7 +12,7 @@ fn make_it() -> for<'a> fn(&'a u32, &'a u32) -> &'a u32 {
 
 fn foo() {
     let a: for<'a, 'b> fn(&'a u32, &'b u32) -> &'a u32 = make_it();
-    //~^ ERROR higher-ranked subtype error
+    //~^ ERROR mismatched types [E0308]
     drop(a);
 }
 
@@ -20,7 +20,7 @@ fn bar() {
     // The code path for patterns is mildly different, so go ahead and
     // test that too:
     let _: for<'a, 'b> fn(&'a u32, &'b u32) -> &'a u32 = make_it();
-    //~^ ERROR higher-ranked subtype error
+    //~^ ERROR mismatched types [E0308]
 }
 
-fn main() { }
+fn main() {}
diff --git a/src/test/ui/nll/relate_tys/hr-fn-aaa-as-aba.stderr b/src/test/ui/nll/relate_tys/hr-fn-aaa-as-aba.stderr
index 7906dbd37ef..8c1eaeb6aa7 100644
--- a/src/test/ui/nll/relate_tys/hr-fn-aaa-as-aba.stderr
+++ b/src/test/ui/nll/relate_tys/hr-fn-aaa-as-aba.stderr
@@ -1,14 +1,21 @@
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/hr-fn-aaa-as-aba.rs:14:58
    |
 LL |     let a: for<'a, 'b> fn(&'a u32, &'b u32) -> &'a u32 = make_it();
-   |                                                          ^^^^^^^^^
+   |                                                          ^^^^^^^^^ one type is more general than the other
+   |
+   = note: expected fn pointer `for<'r, 's> fn(&'r u32, &'s u32) -> &'r u32`
+              found fn pointer `for<'a> fn(&'a u32, &'a u32) -> &'a u32`
 
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/hr-fn-aaa-as-aba.rs:22:12
    |
 LL |     let _: for<'a, 'b> fn(&'a u32, &'b u32) -> &'a u32 = make_it();
-   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
+   |
+   = note: expected fn pointer `for<'a, 'b> fn(&'a u32, &'b u32) -> &'a u32`
+              found fn pointer `for<'r> fn(&'r u32, &'r u32) -> &'r u32`
 
 error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/nll/relate_tys/impl-fn-ignore-binder-via-bottom.rs b/src/test/ui/nll/relate_tys/impl-fn-ignore-binder-via-bottom.rs
index 44dcd191d1b..37a01f28946 100644
--- a/src/test/ui/nll/relate_tys/impl-fn-ignore-binder-via-bottom.rs
+++ b/src/test/ui/nll/relate_tys/impl-fn-ignore-binder-via-bottom.rs
@@ -30,6 +30,6 @@ impl<T> Y for fn(T) {
 
 fn main() {
     let _x = <fn(&())>::make_f();
-    //~^ higher-ranked subtype error
-    //~| higher-ranked subtype error
+    //~^ ERROR implementation of `Y` is not general enough
+    //~| ERROR implementation of `Y` is not general enough
 }
diff --git a/src/test/ui/nll/relate_tys/impl-fn-ignore-binder-via-bottom.stderr b/src/test/ui/nll/relate_tys/impl-fn-ignore-binder-via-bottom.stderr
index 190b520c678..ed79c7df25e 100644
--- a/src/test/ui/nll/relate_tys/impl-fn-ignore-binder-via-bottom.stderr
+++ b/src/test/ui/nll/relate_tys/impl-fn-ignore-binder-via-bottom.stderr
@@ -1,14 +1,20 @@
-error: higher-ranked subtype error
+error: implementation of `Y` is not general enough
   --> $DIR/impl-fn-ignore-binder-via-bottom.rs:32:14
    |
 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: higher-ranked subtype error
+error: implementation of `Y` is not general enough
   --> $DIR/impl-fn-ignore-binder-via-bottom.rs:32:14
    |
 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 2 previous errors
 
diff --git a/src/test/ui/nll/relate_tys/trait-hrtb.rs b/src/test/ui/nll/relate_tys/trait-hrtb.rs
index 80f31ca6b47..2e94fc5c12d 100644
--- a/src/test/ui/nll/relate_tys/trait-hrtb.rs
+++ b/src/test/ui/nll/relate_tys/trait-hrtb.rs
@@ -12,5 +12,5 @@ fn make_foo<'a>() -> Box<dyn Foo<'a>> {
 
 fn main() {
     let x: Box<dyn Foo<'static>> = make_foo();
-    let y: Box<dyn for<'a> Foo<'a>> = x; //~ ERROR higher-ranked subtype error
+    let y: Box<dyn for<'a> Foo<'a>> = x; //~ ERROR mismatched types [E0308]
 }
diff --git a/src/test/ui/nll/relate_tys/trait-hrtb.stderr b/src/test/ui/nll/relate_tys/trait-hrtb.stderr
index 4df2f352522..60a7f204446 100644
--- a/src/test/ui/nll/relate_tys/trait-hrtb.stderr
+++ b/src/test/ui/nll/relate_tys/trait-hrtb.stderr
@@ -1,8 +1,12 @@
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/trait-hrtb.rs:15:39
    |
 LL |     let y: Box<dyn for<'a> Foo<'a>> = x;
-   |                                       ^
+   |                                       ^ one type is more general than the other
+   |
+   = note: expected trait object `dyn for<'r> Foo<'r>`
+              found trait object `dyn Foo<'_>`
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/nll/relate_tys/universe-violation.rs b/src/test/ui/nll/relate_tys/universe-violation.rs
index d29f8f8af20..8389c8e8377 100644
--- a/src/test/ui/nll/relate_tys/universe-violation.rs
+++ b/src/test/ui/nll/relate_tys/universe-violation.rs
@@ -12,6 +12,6 @@ fn make_it() -> fn(&'static u32) -> &'static u32 {
 
 fn main() {
     let a: fn(_) -> _ = make_it();
-    let b: fn(&u32) -> &u32 = a; //~ ERROR higher-ranked subtype error
+    let b: fn(&u32) -> &u32 = a; //~ ERROR mismatched types [E0308]
     drop(a);
 }
diff --git a/src/test/ui/nll/relate_tys/universe-violation.stderr b/src/test/ui/nll/relate_tys/universe-violation.stderr
index 6dc78789564..ff4c7abc250 100644
--- a/src/test/ui/nll/relate_tys/universe-violation.stderr
+++ b/src/test/ui/nll/relate_tys/universe-violation.stderr
@@ -1,8 +1,12 @@
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/universe-violation.rs:15:31
    |
 LL |     let b: fn(&u32) -> &u32 = a;
-   |                               ^
+   |                               ^ one type is more general than the other
+   |
+   = note: expected fn pointer `for<'r> fn(&'r u32) -> &'r u32`
+              found fn pointer `fn(&u32) -> &u32`
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/regions/region-lifetime-bounds-on-fns-where-clause.nll.stderr b/src/test/ui/regions/region-lifetime-bounds-on-fns-where-clause.nll.stderr
index 4ddea2c27b2..a64ad46ef46 100644
--- a/src/test/ui/regions/region-lifetime-bounds-on-fns-where-clause.nll.stderr
+++ b/src/test/ui/regions/region-lifetime-bounds-on-fns-where-clause.nll.stderr
@@ -27,17 +27,24 @@ LL |     a(x, y);
    = note: mutable references are invariant over their type parameter
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/region-lifetime-bounds-on-fns-where-clause.rs:20:12
    |
 LL |     let _: fn(&mut &isize, &mut &isize) = a;
-   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
+   |
+   = note: expected fn pointer `for<'r, 's, 't0, 't1> fn(&'r mut &'s isize, &'t0 mut &'t1 isize)`
+              found fn pointer `for<'r, 's> fn(&'r mut &isize, &'s mut &isize)`
 
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/region-lifetime-bounds-on-fns-where-clause.rs:20:12
    |
 LL |     let _: fn(&mut &isize, &mut &isize) = a;
-   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
+   |
+   = note: expected fn pointer `for<'r, 's, 't0, 't1> fn(&'r mut &'s isize, &'t0 mut &'t1 isize)`
+              found fn pointer `for<'r, 's> fn(&'r mut &isize, &'s mut &isize)`
 
 error: aborting due to 4 previous errors
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/regions/region-multiple-lifetime-bounds-on-fns-where-clause.nll.stderr b/src/test/ui/regions/region-multiple-lifetime-bounds-on-fns-where-clause.nll.stderr
index a9cf128bb62..ce5e7d01723 100644
--- a/src/test/ui/regions/region-multiple-lifetime-bounds-on-fns-where-clause.nll.stderr
+++ b/src/test/ui/regions/region-multiple-lifetime-bounds-on-fns-where-clause.nll.stderr
@@ -27,23 +27,33 @@ LL |     a(x, y, z);
    = note: mutable references are invariant over their type parameter
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/region-multiple-lifetime-bounds-on-fns-where-clause.rs:22:12
    |
 LL |     let _: fn(&mut &isize, &mut &isize, &mut &isize) = a;
-   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
+   |
+   = note: expected fn pointer `for<'r, 's, 't0, 't1, 't2, 't3> fn(&'r mut &'s isize, &'t0 mut &'t1 isize, &'t2 mut &'t3 isize)`
+              found fn pointer `for<'r, 's, 't0> fn(&'r mut &isize, &'s mut &isize, &'t0 mut &isize)`
 
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/region-multiple-lifetime-bounds-on-fns-where-clause.rs:22:12
    |
 LL |     let _: fn(&mut &isize, &mut &isize, &mut &isize) = a;
-   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
+   |
+   = note: expected fn pointer `for<'r, 's, 't0, 't1, 't2, 't3> fn(&'r mut &'s isize, &'t0 mut &'t1 isize, &'t2 mut &'t3 isize)`
+              found fn pointer `for<'r, 's, 't0> fn(&'r mut &isize, &'s mut &isize, &'t0 mut &isize)`
 
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/region-multiple-lifetime-bounds-on-fns-where-clause.rs:22:12
    |
 LL |     let _: fn(&mut &isize, &mut &isize, &mut &isize) = a;
-   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
+   |
+   = note: expected fn pointer `for<'r, 's, 't0, 't1, 't2, 't3> fn(&'r mut &'s isize, &'t0 mut &'t1 isize, &'t2 mut &'t3 isize)`
+              found fn pointer `for<'r, 's, 't0> fn(&'r mut &isize, &'s mut &isize, &'t0 mut &isize)`
 
 error: aborting due to 5 previous errors
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/regions/regions-fn-subtyping-return-static-fail.nll.stderr b/src/test/ui/regions/regions-fn-subtyping-return-static-fail.nll.stderr
index d762f55f9d5..c2956cd8958 100644
--- a/src/test/ui/regions/regions-fn-subtyping-return-static-fail.nll.stderr
+++ b/src/test/ui/regions/regions-fn-subtyping-return-static-fail.nll.stderr
@@ -1,8 +1,12 @@
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/regions-fn-subtyping-return-static-fail.rs:48:5
    |
 LL |     want_G(baz);
-   |     ^^^^^^^^^^^
+   |     ^^^^^^^^^^^ one type is more general than the other
+   |
+   = note: expected fn pointer `for<'cx> fn(&'cx S) -> &'static S`
+              found fn pointer `for<'r> fn(&'r S) -> &'r S`
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/regions/regions-lifetime-bounds-on-fns.nll.stderr b/src/test/ui/regions/regions-lifetime-bounds-on-fns.nll.stderr
index db86572f1cf..cae692ad2f6 100644
--- a/src/test/ui/regions/regions-lifetime-bounds-on-fns.nll.stderr
+++ b/src/test/ui/regions/regions-lifetime-bounds-on-fns.nll.stderr
@@ -27,17 +27,24 @@ LL |     a(x, y);
    = note: mutable references are invariant over their type parameter
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/regions-lifetime-bounds-on-fns.rs:20:12
    |
 LL |     let _: fn(&mut &isize, &mut &isize) = a;
-   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
+   |
+   = note: expected fn pointer `for<'r, 's, 't0, 't1> fn(&'r mut &'s isize, &'t0 mut &'t1 isize)`
+              found fn pointer `for<'r, 's> fn(&'r mut &isize, &'s mut &isize)`
 
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/regions-lifetime-bounds-on-fns.rs:20:12
    |
 LL |     let _: fn(&mut &isize, &mut &isize) = a;
-   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
+   |
+   = note: expected fn pointer `for<'r, 's, 't0, 't1> fn(&'r mut &'s isize, &'t0 mut &'t1 isize)`
+              found fn pointer `for<'r, 's> fn(&'r mut &isize, &'s mut &isize)`
 
 error: aborting due to 4 previous errors
 
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/rfc1623.nll.stderr b/src/test/ui/rfc1623.nll.stderr
index a3d94679434..cc247bbcb11 100644
--- a/src/test/ui/rfc1623.nll.stderr
+++ b/src/test/ui/rfc1623.nll.stderr
@@ -19,7 +19,7 @@ LL | struct SomeStruct<'x, 'y, 'z: 'x> {
    = note: required because it appears within the type `&SomeStruct`
    = note: shared static variables must have a type that implements `Sync`
 
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/rfc1623.rs:21:35
    |
 LL |   static SOME_STRUCT: &SomeStruct = &SomeStruct {
@@ -29,9 +29,12 @@ LL | |     bar: &Bar { bools: &[true, true] },
 LL | |     f: &id,
 LL | |
 LL | | };
-   | |_^
+   | |_^ one type is more general than the other
+   |
+   = note: expected type `for<'r, 's> Fn<(&'r Foo<'s>,)>`
+              found type `Fn<(&Foo<'_>,)>`
 
-error: higher-ranked subtype error
+error[E0308]: mismatched types
   --> $DIR/rfc1623.rs:21:35
    |
 LL |   static SOME_STRUCT: &SomeStruct = &SomeStruct {
@@ -41,9 +44,12 @@ LL | |     bar: &Bar { bools: &[true, true] },
 LL | |     f: &id,
 LL | |
 LL | | };
-   | |_^
+   | |_^ one type is more general than the other
+   |
+   = note: expected type `for<'r, 's> Fn<(&'r Foo<'s>,)>`
+              found type `Fn<(&Foo<'_>,)>`
 
-error: higher-ranked subtype error
+error: implementation of `FnOnce` is not general enough
   --> $DIR/rfc1623.rs:21:35
    |
 LL |   static SOME_STRUCT: &SomeStruct = &SomeStruct {
@@ -53,9 +59,12 @@ LL | |     bar: &Bar { bools: &[true, true] },
 LL | |     f: &id,
 LL | |
 LL | | };
-   | |_^
+   | |_^ implementation of `FnOnce` is not general enough
+   |
+   = note: `fn(&'2 Foo<'_>) -> &'2 Foo<'_> {id::<&'2 Foo<'_>>}` must implement `FnOnce<(&'1 Foo<'_>,)>`, for any lifetime `'1`...
+   = note: ...but it actually implements `FnOnce<(&'2 Foo<'_>,)>`, for some specific lifetime `'2`
 
-error: higher-ranked subtype error
+error: implementation of `FnOnce` is not general enough
   --> $DIR/rfc1623.rs:21:35
    |
 LL |   static SOME_STRUCT: &SomeStruct = &SomeStruct {
@@ -65,8 +74,12 @@ LL | |     bar: &Bar { bools: &[true, true] },
 LL | |     f: &id,
 LL | |
 LL | | };
-   | |_^
+   | |_^ implementation of `FnOnce` is not general enough
+   |
+   = note: `fn(&Foo<'2>) -> &Foo<'2> {id::<&Foo<'2>>}` must implement `FnOnce<(&Foo<'1>,)>`, for any lifetime `'1`...
+   = note: ...but it actually implements `FnOnce<(&Foo<'2>,)>`, for some specific lifetime `'2`
 
 error: aborting due to 5 previous errors
 
-For more information about this error, try `rustc --explain E0277`.
+Some errors have detailed explanations: E0277, E0308.
+For more information about an error, try `rustc --explain E0277`.
diff --git a/src/test/ui/unboxed-closures/issue-30906.nll.stderr b/src/test/ui/unboxed-closures/issue-30906.nll.stderr
index 2db392e8b8b..147a2097473 100644
--- a/src/test/ui/unboxed-closures/issue-30906.nll.stderr
+++ b/src/test/ui/unboxed-closures/issue-30906.nll.stderr
@@ -1,8 +1,11 @@
-error: higher-ranked subtype error
+error: implementation of `FnOnce` is not general enough
   --> $DIR/issue-30906.rs:18:5
    |
 LL |     test(Compose(f, |_| {}));
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `FnOnce` is not general enough
+   |
+   = note: `fn(&'2 str) -> T` must implement `FnOnce<(&'1 str,)>`, for any lifetime `'1`...
+   = note: ...but it actually implements `FnOnce<(&'2 str,)>`, for some specific lifetime `'2`
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/where-clauses/where-for-self-2.nll.stderr b/src/test/ui/where-clauses/where-for-self-2.nll.stderr
index d0c476dc6ec..f65db78fc89 100644
--- a/src/test/ui/where-clauses/where-for-self-2.nll.stderr
+++ b/src/test/ui/where-clauses/where-for-self-2.nll.stderr
@@ -1,8 +1,11 @@
-error: higher-ranked subtype error
+error: implementation of `Bar` is not general enough
   --> $DIR/where-for-self-2.rs:23:5
    |
 LL |     foo(&X);
-   |     ^^^^^^^
+   |     ^^^^^^^ implementation of `Bar` is not general enough
+   |
+   = note: `&'0 u32` must implement `Bar`, for any lifetime `'0`...
+   = note: ...but `Bar` is actually implemented for the type `&'static u32`
 
 error: aborting due to previous error