review comments

16 files changed, 51 insertions, 67 deletions

diff --git a/src/test/ui/coercion/coerce-expect-unsized-ascribed.stderr b/src/test/ui/coercion/coerce-expect-unsized-ascribed.stderr
index 303d83d3426..44e5c6a99f7 100644
--- a/src/test/ui/coercion/coerce-expect-unsized-ascribed.stderr
+++ b/src/test/ui/coercion/coerce-expect-unsized-ascribed.stderr
@@ -29,7 +29,7 @@ error[E0308]: mismatched types
   --> $DIR/coerce-expect-unsized-ascribed.rs:13:13
    |
 LL |     let _ = box { |x| (x as u8) }: Box<dyn Fn(i32) -> _>;
-   |             ^^^^^^^^^^^^^^^^^^^^^ expected trait `std::ops::Fn`, found closure
+   |             ^^^^^^^^^^^^^^^^^^^^^ expected trait object `dyn std::ops::Fn`, found closure
    |
    = note: expected struct `std::boxed::Box<dyn std::ops::Fn(i32) -> u8>`
               found struct `std::boxed::Box<[closure@$DIR/coerce-expect-unsized-ascribed.rs:13:19: 13:32]>`
@@ -38,7 +38,7 @@ error[E0308]: mismatched types
   --> $DIR/coerce-expect-unsized-ascribed.rs:14:13
    |
 LL |     let _ = box if true { false } else { true }: Box<dyn Debug>;
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected trait `std::fmt::Debug`, found `bool`
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected trait object `dyn std::fmt::Debug`, found `bool`
    |
    = note: expected struct `std::boxed::Box<dyn std::fmt::Debug>`
               found struct `std::boxed::Box<bool>`
@@ -47,7 +47,7 @@ error[E0308]: mismatched types
   --> $DIR/coerce-expect-unsized-ascribed.rs:15:13
    |
 LL |     let _ = box match true { true => 'a', false => 'b' }: Box<dyn Debug>;
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected trait `std::fmt::Debug`, found `char`
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected trait object `dyn std::fmt::Debug`, found `char`
    |
    = note: expected struct `std::boxed::Box<dyn std::fmt::Debug>`
               found struct `std::boxed::Box<char>`
@@ -83,7 +83,7 @@ error[E0308]: mismatched types
   --> $DIR/coerce-expect-unsized-ascribed.rs:21:13
    |
 LL |     let _ = &{ |x| (x as u8) }: &dyn Fn(i32) -> _;
-   |             ^^^^^^^^^^^^^^^^^^ expected trait `std::ops::Fn`, found closure
+   |             ^^^^^^^^^^^^^^^^^^ expected trait object `dyn std::ops::Fn`, found closure
    |
    = note: expected reference `&dyn std::ops::Fn(i32) -> u8`
               found reference `&[closure@$DIR/coerce-expect-unsized-ascribed.rs:21:16: 21:29]`
@@ -92,7 +92,7 @@ error[E0308]: mismatched types
   --> $DIR/coerce-expect-unsized-ascribed.rs:22:13
    |
 LL |     let _ = &if true { false } else { true }: &dyn Debug;
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected trait `std::fmt::Debug`, found `bool`
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected trait object `dyn std::fmt::Debug`, found `bool`
    |
    = note: expected reference `&dyn std::fmt::Debug`
               found reference `&bool`
@@ -101,7 +101,7 @@ error[E0308]: mismatched types
   --> $DIR/coerce-expect-unsized-ascribed.rs:23:13
    |
 LL |     let _ = &match true { true => 'a', false => 'b' }: &dyn Debug;
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected trait `std::fmt::Debug`, found `char`
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected trait object `dyn std::fmt::Debug`, found `char`
    |
    = note: expected reference `&dyn std::fmt::Debug`
               found reference `&char`
@@ -119,7 +119,7 @@ error[E0308]: mismatched types
   --> $DIR/coerce-expect-unsized-ascribed.rs:26:13
    |
 LL |     let _ = Box::new(|x| (x as u8)): Box<dyn Fn(i32) -> _>;
-   |             ^^^^^^^^^^^^^^^^^^^^^^^ expected trait `std::ops::Fn`, found closure
+   |             ^^^^^^^^^^^^^^^^^^^^^^^ expected trait object `dyn std::ops::Fn`, found closure
    |
    = note: expected struct `std::boxed::Box<dyn std::ops::Fn(i32) -> _>`
               found struct `std::boxed::Box<[closure@$DIR/coerce-expect-unsized-ascribed.rs:26:22: 26:35]>`
diff --git a/src/test/ui/const-generics/array-impls/into-iter-no-impls-length-33.stderr b/src/test/ui/const-generics/array-impls/into-iter-no-impls-length-33.stderr
index e615e10bd5f..4869f483634 100644
--- a/src/test/ui/const-generics/array-impls/into-iter-no-impls-length-33.stderr
+++ b/src/test/ui/const-generics/array-impls/into-iter-no-impls-length-33.stderr
@@ -13,7 +13,7 @@ LL | pub fn no_iterator() -> impl Iterator<Item = i32> {
    |                         ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::array::LengthAtMost32` is not implemented for `[i32; 33]`
 LL |
 LL |     IntoIter::new([0i32; 33])
-   |     ------------------------- this returned value is of type `std::array::IntoIter<_, _: usize>`
+   |     ------------------------- this returned value is of type `std::array::IntoIter<i32, 33usize>`
    |
    = note: required because of the requirements on the impl of `std::iter::Iterator` for `std::array::IntoIter<i32, 33usize>`
    = note: the return type of a function must have a statically known size
@@ -33,7 +33,7 @@ LL | pub fn no_double_ended_iterator() -> impl DoubleEndedIterator {
    |                                      ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::array::LengthAtMost32` is not implemented for `[i32; 33]`
 LL |
 LL |     IntoIter::new([0i32; 33])
-   |     ------------------------- this returned value is of type `std::array::IntoIter<_, _: usize>`
+   |     ------------------------- this returned value is of type `std::array::IntoIter<i32, 33usize>`
    |
    = note: required because of the requirements on the impl of `std::iter::DoubleEndedIterator` for `std::array::IntoIter<i32, 33usize>`
    = note: the return type of a function must have a statically known size
@@ -53,7 +53,7 @@ LL | pub fn no_exact_size_iterator() -> impl ExactSizeIterator {
    |                                    ^^^^^^^^^^^^^^^^^^^^^^ the trait `std::array::LengthAtMost32` is not implemented for `[i32; 33]`
 LL |
 LL |     IntoIter::new([0i32; 33])
-   |     ------------------------- this returned value is of type `std::array::IntoIter<_, _: usize>`
+   |     ------------------------- this returned value is of type `std::array::IntoIter<i32, 33usize>`
    |
    = note: required because of the requirements on the impl of `std::iter::ExactSizeIterator` for `std::array::IntoIter<i32, 33usize>`
    = note: the return type of a function must have a statically known size
@@ -73,7 +73,7 @@ LL | pub fn no_fused_iterator() -> impl FusedIterator {
    |                               ^^^^^^^^^^^^^^^^^^ the trait `std::array::LengthAtMost32` is not implemented for `[i32; 33]`
 LL |
 LL |     IntoIter::new([0i32; 33])
-   |     ------------------------- this returned value is of type `std::array::IntoIter<_, _: usize>`
+   |     ------------------------- this returned value is of type `std::array::IntoIter<i32, 33usize>`
    |
    = note: required because of the requirements on the impl of `std::iter::FusedIterator` for `std::array::IntoIter<i32, 33usize>`
    = note: the return type of a function must have a statically known size
@@ -93,7 +93,7 @@ LL | pub fn no_trusted_len() -> impl TrustedLen {
    |                            ^^^^^^^^^^^^^^^ the trait `std::array::LengthAtMost32` is not implemented for `[i32; 33]`
 LL |
 LL |     IntoIter::new([0i32; 33])
-   |     ------------------------- this returned value is of type `std::array::IntoIter<_, _: usize>`
+   |     ------------------------- this returned value is of type `std::array::IntoIter<i32, 33usize>`
    |
    = note: required because of the requirements on the impl of `std::iter::TrustedLen` for `std::array::IntoIter<i32, 33usize>`
    = note: the return type of a function must have a statically known size
@@ -113,7 +113,7 @@ LL | pub fn no_clone() -> impl Clone {
    |                      ^^^^^^^^^^ the trait `std::array::LengthAtMost32` is not implemented for `[i32; 33]`
 LL |
 LL |     IntoIter::new([0i32; 33])
-   |     ------------------------- this returned value is of type `std::array::IntoIter<_, _: usize>`
+   |     ------------------------- this returned value is of type `std::array::IntoIter<i32, 33usize>`
    |
    = note: required because of the requirements on the impl of `std::clone::Clone` for `std::array::IntoIter<i32, 33usize>`
    = note: the return type of a function must have a statically known size
@@ -133,7 +133,7 @@ LL | pub fn no_debug() -> impl Debug {
    |                      ^^^^^^^^^^ the trait `std::array::LengthAtMost32` is not implemented for `[i32; 33]`
 LL |
 LL |     IntoIter::new([0i32; 33])
-   |     ------------------------- this returned value is of type `std::array::IntoIter<_, _: usize>`
+   |     ------------------------- this returned value is of type `std::array::IntoIter<i32, 33usize>`
    |
    = note: required because of the requirements on the impl of `std::fmt::Debug` for `std::array::IntoIter<i32, 33usize>`
    = note: the return type of a function must have a statically known size
diff --git a/src/test/ui/destructure-trait-ref.rs b/src/test/ui/destructure-trait-ref.rs
index fb92196b2bd..34e7cad935a 100644
--- a/src/test/ui/destructure-trait-ref.rs
+++ b/src/test/ui/destructure-trait-ref.rs
@@ -33,12 +33,10 @@ fn main() {
     //~^ ERROR mismatched types
     //~| expected trait object `dyn T`
     //~| found reference `&_`
-    //~| expected trait `T`, found reference
     let &&&x = &(&1isize as &dyn T);
     //~^ ERROR mismatched types
     //~| expected trait object `dyn T`
     //~| found reference `&_`
-    //~| expected trait `T`, found reference
     let box box x = box 1isize as Box<dyn T>;
     //~^ ERROR mismatched types
     //~| expected trait object `dyn T`
diff --git a/src/test/ui/destructure-trait-ref.stderr b/src/test/ui/destructure-trait-ref.stderr
index c78166f411d..f99bf2ffdc9 100644
--- a/src/test/ui/destructure-trait-ref.stderr
+++ b/src/test/ui/destructure-trait-ref.stderr
@@ -22,31 +22,31 @@ error[E0308]: mismatched types
 LL |     let &&x = &1isize as &dyn T;
    |          ^^
    |          |
-   |          expected trait `T`, found reference
+   |          expected trait object `dyn T`, found reference
    |          help: you can probably remove the explicit borrow: `x`
    |
    = note: expected trait object `dyn T`
                  found reference `&_`
 
 error[E0308]: mismatched types
-  --> $DIR/destructure-trait-ref.rs:37:11
+  --> $DIR/destructure-trait-ref.rs:36:11
    |
 LL |     let &&&x = &(&1isize as &dyn T);
    |           ^^
    |           |
-   |           expected trait `T`, found reference
+   |           expected trait object `dyn T`, found reference
    |           help: you can probably remove the explicit borrow: `x`
    |
    = note: expected trait object `dyn T`
                  found reference `&_`
 
 error[E0308]: mismatched types
-  --> $DIR/destructure-trait-ref.rs:42:13
+  --> $DIR/destructure-trait-ref.rs:40:13
    |
 LL |     let box box x = box 1isize as Box<dyn T>;
    |             ^^^^^   ------------------------ this expression has type `std::boxed::Box<dyn T>`
    |             |
-   |             expected trait `T`, found struct `std::boxed::Box`
+   |             expected trait object `dyn T`, found struct `std::boxed::Box`
    |
    = note: expected trait object `dyn T`
                     found struct `std::boxed::Box<_>`
diff --git a/src/test/ui/dst/dst-bad-assign-3.rs b/src/test/ui/dst/dst-bad-assign-3.rs
index e3b621b909a..d05b3937c99 100644
--- a/src/test/ui/dst/dst-bad-assign-3.rs
+++ b/src/test/ui/dst/dst-bad-assign-3.rs
@@ -32,7 +32,7 @@ pub fn main() {
     let z: Box<dyn ToBar> = Box::new(Bar1 {f: 36});
     f5.2 = Bar1 {f: 36};
     //~^ ERROR mismatched types
-    //~| expected trait `ToBar`, found struct `Bar1`
+    //~| expected trait object `dyn ToBar`, found struct `Bar1`
     //~| expected trait object `dyn ToBar`
     //~| found struct `Bar1`
     //~| ERROR the size for values of type
diff --git a/src/test/ui/dst/dst-bad-assign-3.stderr b/src/test/ui/dst/dst-bad-assign-3.stderr
index dc03f38e103..0b6f9df2d83 100644
--- a/src/test/ui/dst/dst-bad-assign-3.stderr
+++ b/src/test/ui/dst/dst-bad-assign-3.stderr
@@ -2,7 +2,7 @@ error[E0308]: mismatched types
   --> $DIR/dst-bad-assign-3.rs:33:12
    |
 LL |     f5.2 = Bar1 {f: 36};
-   |            ^^^^^^^^^^^^ expected trait `ToBar`, found struct `Bar1`
+   |            ^^^^^^^^^^^^ expected trait object `dyn ToBar`, found struct `Bar1`
    |
    = note: expected trait object `dyn ToBar`
                     found struct `Bar1`
diff --git a/src/test/ui/dst/dst-bad-assign.rs b/src/test/ui/dst/dst-bad-assign.rs
index ed94242f5bf..496e01ae005 100644
--- a/src/test/ui/dst/dst-bad-assign.rs
+++ b/src/test/ui/dst/dst-bad-assign.rs
@@ -34,7 +34,7 @@ pub fn main() {
     let z: Box<dyn ToBar> = Box::new(Bar1 {f: 36});
     f5.ptr = Bar1 {f: 36};
     //~^ ERROR mismatched types
-    //~| expected trait `ToBar`, found struct `Bar1`
+    //~| expected trait object `dyn ToBar`, found struct `Bar1`
     //~| expected trait object `dyn ToBar`
     //~| found struct `Bar1`
     //~| ERROR the size for values of type
diff --git a/src/test/ui/dst/dst-bad-assign.stderr b/src/test/ui/dst/dst-bad-assign.stderr
index 8031f162482..434c460759f 100644
--- a/src/test/ui/dst/dst-bad-assign.stderr
+++ b/src/test/ui/dst/dst-bad-assign.stderr
@@ -2,7 +2,7 @@ error[E0308]: mismatched types
   --> $DIR/dst-bad-assign.rs:35:14
    |
 LL |     f5.ptr = Bar1 {f: 36};
-   |              ^^^^^^^^^^^^ expected trait `ToBar`, found struct `Bar1`
+   |              ^^^^^^^^^^^^ expected trait object `dyn ToBar`, found struct `Bar1`
    |
    = note: expected trait object `dyn ToBar`
                     found struct `Bar1`
diff --git a/src/test/ui/error-codes/E0746.stderr b/src/test/ui/error-codes/E0746.stderr
index 1c88ce64749..e7a8fd304ca 100644
--- a/src/test/ui/error-codes/E0746.stderr
+++ b/src/test/ui/error-codes/E0746.stderr
@@ -1,23 +1,23 @@
-error[E0746]: return type cannot have a bare trait because it must be `Sized`
+error[E0746]: return type cannot have an unboxed trait object
   --> $DIR/E0746.rs:8:13
    |
 LL | fn foo() -> dyn Trait { Struct }
    |             ^^^^^^^^^ doesn't have a size known at compile-time
    |
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
-help: you can use the `impl Trait` feature in the return type because all the return paths are of type `Struct`, which implements `dyn Trait`
+help: return `impl Trait` instead, as all return paths are of type `Struct`, which implements `Trait`
    |
 LL | fn foo() -> impl Trait { Struct }
    |             ^^^^^^^^^^
 
-error[E0746]: return type cannot have a bare trait because it must be `Sized`
+error[E0746]: return type cannot have an unboxed trait object
   --> $DIR/E0746.rs:11:13
    |
 LL | fn bar() -> dyn Trait {
    |             ^^^^^^^^^ doesn't have a size known at compile-time
    |
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
-help: you can use the `impl Trait` feature in the return type because all the return paths are of type `{integer}`, which implements `dyn Trait`
+help: return `impl Trait` instead, as all return paths are of type `{integer}`, which implements `Trait`
    |
 LL | fn bar() -> impl Trait {
    |             ^^^^^^^^^^
diff --git a/src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr b/src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr
index ff7438e9aff..3d0707c0916 100644
--- a/src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr
+++ b/src/test/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr
@@ -2,7 +2,7 @@ error[E0308]: mismatched types
   --> $DIR/dyn-trait-return-should-be-impl-trait.rs:7:35
    |
 LL | fn fuz() -> (usize, Trait) { (42, Struct) }
-   |                                   ^^^^^^ expected trait `Trait`, found struct `Struct`
+   |                                   ^^^^^^ expected trait object `dyn Trait`, found struct `Struct`
    |
    = note: expected trait object `(dyn Trait + 'static)`
                     found struct `Struct`
@@ -24,7 +24,7 @@ error[E0308]: mismatched types
   --> $DIR/dyn-trait-return-should-be-impl-trait.rs:10:39
    |
 LL | fn bar() -> (usize, dyn Trait) { (42, Struct) }
-   |                                       ^^^^^^ expected trait `Trait`, found struct `Struct`
+   |                                       ^^^^^^ expected trait object `dyn Trait`, found struct `Struct`
    |
    = note: expected trait object `(dyn Trait + 'static)`
                     found struct `Struct`
@@ -42,26 +42,26 @@ LL | fn bar() -> (usize, dyn Trait) { (42, Struct) }
    = note: required because it appears within the type `(usize, (dyn Trait + 'static))`
    = note: the return type of a function must have a statically known size
 
-error[E0746]: return type cannot have a bare trait because it must be `Sized`
+error[E0746]: return type cannot have an unboxed trait object
   --> $DIR/dyn-trait-return-should-be-impl-trait.rs:13:13
    |
 LL | fn bap() -> Trait { Struct }
    |             ^^^^^ doesn't have a size known at compile-time
    |
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
-help: you can use the `impl Trait` feature in the return type because all the return paths are of type `Struct`, which implements `dyn Trait`
+help: return `impl Trait` instead, as all return paths are of type `Struct`, which implements `Trait`
    |
 LL | fn bap() -> impl Trait { Struct }
    |             ^^^^^^^^^^
 
-error[E0746]: return type cannot have a bare trait because it must be `Sized`
+error[E0746]: return type cannot have an unboxed trait object
   --> $DIR/dyn-trait-return-should-be-impl-trait.rs:15:13
    |
 LL | fn ban() -> dyn Trait { Struct }
    |             ^^^^^^^^^ doesn't have a size known at compile-time
    |
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
-help: you can use the `impl Trait` feature in the return type because all the return paths are of type `Struct`, which implements `dyn Trait`
+help: return `impl Trait` instead, as all return paths are of type `Struct`, which implements `Trait`
    |
 LL | fn ban() -> impl Trait { Struct }
    |             ^^^^^^^^^^
@@ -76,40 +76,26 @@ LL | fn bak() -> dyn Trait { unimplemented!() }
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = note: the return type of a function must have a statically known size
 
-error[E0746]: return type cannot have a bare trait because it must be `Sized`
+error[E0746]: return type cannot have an unboxed trait object
   --> $DIR/dyn-trait-return-should-be-impl-trait.rs:19:13
    |
 LL | fn bal() -> dyn Trait {
    |             ^^^^^^^^^ doesn't have a size known at compile-time
    |
-help: if all the returned values were of the same type you could use `impl Trait` as the return type
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:23:5
-   |
-LL |         return Struct;
-   |                ^^^^^^
-LL |     }
-LL |     42
-   |     ^^
-   = help: alternatively, you can always create a new `enum` with a variant for each returned type
+   = note: if trait `Trait` was object safe, you could return a trait object
+   = note: if all the returned values were of the same type you could use `impl Trait` as the return type
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
    = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
-help: if the performance implications are acceptable, you can return a trait object
-   |
-LL | fn bal() -> Box<dyn Trait> {
-LL |     if true {
-LL |         return Box::new(Struct);
-LL |     }
-LL |     Box::new(42)
-   |
+   = note: you can create a new `enum` with a variant for each returned type
 
-error[E0746]: return type cannot have a bare trait because it must be `Sized`
+error[E0746]: return type cannot have an unboxed trait object
   --> $DIR/dyn-trait-return-should-be-impl-trait.rs:27:13
    |
 LL | fn bat() -> dyn Trait {
    |             ^^^^^^^^^ doesn't have a size known at compile-time
    |
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
-help: you can use the `impl Trait` feature in the return type because all the return paths are of type `{integer}`, which implements `dyn Trait`
+help: return `impl Trait` instead, as all return paths are of type `{integer}`, which implements `Trait`
    |
 LL | fn bat() -> impl Trait {
    |             ^^^^^^^^^^
diff --git a/src/test/ui/impl-trait/equality.stderr b/src/test/ui/impl-trait/equality.stderr
index 215b6d52918..be8653d1689 100644
--- a/src/test/ui/impl-trait/equality.stderr
+++ b/src/test/ui/impl-trait/equality.stderr
@@ -10,7 +10,7 @@ LL |     }
 LL |     0_u32
    |     ^^^^^ expected `i32`, found `u32`
    |
-   = note: `impl Trait` as a return type requires that all the returned values must have the same type
+   = note: to return `impl Trait`, all returned values must be of the same type
    = help: you can instead return a trait object using `Box<dyn Foo>`
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
    = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
diff --git a/src/test/ui/issues/issue-58344.stderr b/src/test/ui/issues/issue-58344.stderr
index 9b07dbd7ab6..e0c196e518b 100644
--- a/src/test/ui/issues/issue-58344.stderr
+++ b/src/test/ui/issues/issue-58344.stderr
@@ -5,7 +5,7 @@ LL | ) -> Either<impl Trait<<u32 as Add<u32>>::Output>, impl Trait<<u32 as Add<u
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Trait<u32>` is not implemented for `impl Trait<<u32 as std::ops::Add>::Output>`
 ...
 LL |     add_generic(value, 1u32)
-   |     ------------------------ this returned value is of type `Either<impl Trait<<_ as std::ops::Add<_>>::Output>, impl Trait<<_ as std::ops::Add<_>>::Output>>`
+   |     ------------------------ this returned value is of type `Either<impl Trait<<u32 as std::ops::Add>::Output>, impl Trait<<u32 as std::ops::Add>::Output>>`
    |
    = note: the return type of a function must have a statically known size
 
@@ -16,7 +16,7 @@ LL | ) -> Either<impl Trait<<u32 as Add<u32>>::Output>, impl Trait<<u32 as Add<u
    |                                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Trait<u32>` is not implemented for `impl Trait<<u32 as std::ops::Add>::Output>`
 ...
 LL |     add_generic(value, 1u32)
-   |     ------------------------ this returned value is of type `Either<impl Trait<<_ as std::ops::Add<_>>::Output>, impl Trait<<_ as std::ops::Add<_>>::Output>>`
+   |     ------------------------ this returned value is of type `Either<impl Trait<<u32 as std::ops::Add>::Output>, impl Trait<<u32 as std::ops::Add>::Output>>`
    |
    = note: the return type of a function must have a statically known size
 
diff --git a/src/test/ui/lifetimes/lifetime-elision-return-type-trait.stderr b/src/test/ui/lifetimes/lifetime-elision-return-type-trait.stderr
index 49fa11c35ae..e43fb6d0edf 100644
--- a/src/test/ui/lifetimes/lifetime-elision-return-type-trait.stderr
+++ b/src/test/ui/lifetimes/lifetime-elision-return-type-trait.stderr
@@ -5,7 +5,7 @@ LL | fn foo() -> impl Future<Item=(), Error=Box<dyn Error>> {
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Future` is not implemented for `std::result::Result<(), _>`
 LL |
 LL |     Ok(())
-   |     ------ this returned value is of type `std::result::Result<_, _>`
+   |     ------ this returned value is of type `std::result::Result<(), _>`
    |
    = note: the return type of a function must have a statically known size
 
diff --git a/src/test/ui/never_type/feature-gate-never_type_fallback.stderr b/src/test/ui/never_type/feature-gate-never_type_fallback.stderr
index 88bfed2b547..77288f1bada 100644
--- a/src/test/ui/never_type/feature-gate-never_type_fallback.stderr
+++ b/src/test/ui/never_type/feature-gate-never_type_fallback.stderr
@@ -5,7 +5,7 @@ LL | fn should_ret_unit() -> impl T {
    |                         ^^^^^^ the trait `T` is not implemented for `()`
 LL |
 LL |     panic!()
-   |     -------- this returned value is of type `_`
+   |     -------- this returned value is of type `()`
    |
    = note: the return type of a function must have a statically known size
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
diff --git a/src/test/ui/point-to-type-err-cause-on-impl-trait-return.stderr b/src/test/ui/point-to-type-err-cause-on-impl-trait-return.stderr
index 9db5250e4d8..2c0425e718a 100644
--- a/src/test/ui/point-to-type-err-cause-on-impl-trait-return.stderr
+++ b/src/test/ui/point-to-type-err-cause-on-impl-trait-return.stderr
@@ -10,7 +10,7 @@ LL |     }
 LL |     1u32
    |     ^^^^ expected `i32`, found `u32`
    |
-   = note: `impl Trait` as a return type requires that all the returned values must have the same type
+   = note: to return `impl Trait`, all returned values must be of the same type
    = help: you can instead return a trait object using `Box<dyn std::fmt::Display>`
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
    = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
@@ -27,7 +27,7 @@ LL |     } else {
 LL |         return 1u32;
    |                ^^^^ expected `i32`, found `u32`
    |
-   = note: `impl Trait` as a return type requires that all the returned values must have the same type
+   = note: to return `impl Trait`, all returned values must be of the same type
    = help: you can instead return a trait object using `Box<dyn std::fmt::Display>`
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
    = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
@@ -44,7 +44,7 @@ LL |     } else {
 LL |         1u32
    |         ^^^^ expected `i32`, found `u32`
    |
-   = note: `impl Trait` as a return type requires that all the returned values must have the same type
+   = note: to return `impl Trait`, all returned values must be of the same type
    = help: you can instead return a trait object using `Box<dyn std::fmt::Display>`
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
    = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
@@ -73,7 +73,7 @@ LL |         0 => return 0i32,
 LL |         _ => 1u32,
    |              ^^^^ expected `i32`, found `u32`
    |
-   = note: `impl Trait` as a return type requires that all the returned values must have the same type
+   = note: to return `impl Trait`, all returned values must be of the same type
    = help: you can instead return a trait object using `Box<dyn std::fmt::Display>`
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
    = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
@@ -92,7 +92,7 @@ LL | |         _ => 2u32,
 LL | |     }
    | |_____^ expected `i32`, found `u32`
    |
-   = note: `impl Trait` as a return type requires that all the returned values must have the same type
+   = note: to return `impl Trait`, all returned values must be of the same type
    = help: you can instead return a trait object using `Box<dyn std::fmt::Display>`
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
    = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
@@ -109,7 +109,7 @@ LL |             return 0i32;
 LL |             1u32
    |             ^^^^ expected `i32`, found `u32`
    |
-   = note: `impl Trait` as a return type requires that all the returned values must have the same type
+   = note: to return `impl Trait`, all returned values must be of the same type
    = help: you can instead return a trait object using `Box<dyn std::fmt::Display>`
    = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
    = note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
diff --git a/src/test/ui/typeck/issue-57673-ice-on-deref-of-boxed-trait.stderr b/src/test/ui/typeck/issue-57673-ice-on-deref-of-boxed-trait.stderr
index 14c09ade7dd..a656b20c23e 100644
--- a/src/test/ui/typeck/issue-57673-ice-on-deref-of-boxed-trait.stderr
+++ b/src/test/ui/typeck/issue-57673-ice-on-deref-of-boxed-trait.stderr
@@ -4,7 +4,7 @@ error[E0308]: mismatched types
 LL | fn ice(x: Box<dyn Iterator<Item=()>>) {
    |                                       - possibly return type missing here?
 LL |     *x
-   |     ^^ expected `()`, found trait `std::iter::Iterator`
+   |     ^^ expected `()`, found trait object `dyn std::iter::Iterator`
    |
    = note: expected unit type `()`
            found trait object `(dyn std::iter::Iterator<Item = ()> + 'static)`