Rollup merge of #111704 - compiler-errors:sized-return-cleanup, r=oli-obk

Remove return type sized check hack from hir typeck

Remove a bunch of special-cased suggestions when someone returns `-> dyn Trait` that checks for type equality, etc.

This was a pretty complex piece of code that also relied on a hack in hir typeck (see changes to `compiler/rustc_hir_typeck/src/check.rs`), and I'm not convinced that it's necessary to maintain, when all we really need to tell the user is that they should return `-> impl Trait` or `-> Box<dyn Trait>`, depending on their specific use-case.

This is necessary because we may need to move the "return type is sized" check from hir typeck to wfcheck, which does not have access to typeck results. This is a prerequisite for that, and I'm fairly confident that the diagnostics "regressions" here are not a big deal.

12 files changed, 131 insertions, 207 deletions

diff --git a/tests/ui/error-codes/E0746.fixed b/tests/ui/error-codes/E0746.fixed
deleted file mode 100644
index ca8319aa020..00000000000
--- a/tests/ui/error-codes/E0746.fixed
+++ /dev/null
@@ -1,18 +0,0 @@
-// run-rustfix
-#![allow(dead_code)]
-struct Struct;
-trait Trait {}
-impl Trait for Struct {}
-impl Trait for u32 {}
-
-fn foo() -> impl Trait { Struct }
-//~^ ERROR E0746
-
-fn bar() -> impl Trait { //~ ERROR E0746
-    if true {
-        return 0;
-    }
-    42
-}
-
-fn main() {}
diff --git a/tests/ui/error-codes/E0746.rs b/tests/ui/error-codes/E0746.rs
index bf5ba8fff56..86b5b7444d1 100644
--- a/tests/ui/error-codes/E0746.rs
+++ b/tests/ui/error-codes/E0746.rs
@@ -1,5 +1,5 @@
-// run-rustfix
 #![allow(dead_code)]
+
 struct Struct;
 trait Trait {}
 impl Trait for Struct {}
diff --git a/tests/ui/error-codes/E0746.stderr b/tests/ui/error-codes/E0746.stderr
index 2153b59ad18..9fe90ab7bec 100644
--- a/tests/ui/error-codes/E0746.stderr
+++ b/tests/ui/error-codes/E0746.stderr
@@ -4,11 +4,14 @@ error[E0746]: return type cannot have an unboxed trait object
 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: use `impl Trait` as the return type, as all return paths are of type `Struct`, which implements `Trait`
+help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
    |
 LL | fn foo() -> impl Trait { Struct }
-   |             ~~~~~~~~~~
+   |             ~~~~
+help: box the return type, and wrap all of the returned values in `Box::new`
+   |
+LL | fn foo() -> Box<dyn Trait> { Box::new(Struct) }
+   |             ++++         +   +++++++++      +
 
 error[E0746]: return type cannot have an unboxed trait object
   --> $DIR/E0746.rs:11:13
@@ -16,11 +19,18 @@ error[E0746]: return type cannot have an unboxed trait object
 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: use `impl Trait` as the return type, as all return paths are of type `{integer}`, which implements `Trait`
+help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
    |
 LL | fn bar() -> impl Trait {
-   |             ~~~~~~~~~~
+   |             ~~~~
+help: box the return type, and wrap all of the returned values in `Box::new`
+   |
+LL ~ fn bar() -> Box<dyn Trait> {
+LL |     if true {
+LL ~         return Box::new(0);
+LL |     }
+LL ~     Box::new(42)
+   |
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/impl-trait/dyn-trait-return-should-be-impl-trait.rs b/tests/ui/impl-trait/dyn-trait-return-should-be-impl-trait.rs
index cbf1daabe2b..af368203de0 100644
--- a/tests/ui/impl-trait/dyn-trait-return-should-be-impl-trait.rs
+++ b/tests/ui/impl-trait/dyn-trait-return-should-be-impl-trait.rs
@@ -26,7 +26,7 @@ fn bax() -> dyn Trait { //~ ERROR E0746
     if true {
         Struct
     } else {
-        42 //~ ERROR `if` and `else` have incompatible types
+        42
     }
 }
 fn bam() -> Box<dyn Trait> {
diff --git a/tests/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr b/tests/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr
index dc1e40ea560..ed9261d0de5 100644
--- a/tests/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr
+++ b/tests/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr
@@ -46,11 +46,10 @@ error[E0746]: return type cannot have an unboxed trait object
 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: use `impl Trait` as the return type, as all return paths are of type `Struct`, which implements `Trait`
+help: box the return type, and wrap all of the returned values in `Box::new`
    |
-LL | fn bap() -> impl Trait { Struct }
-   |             ~~~~~~~~~~
+LL | fn bap() -> Box<Trait> { Box::new(Struct) }
+   |             ++++     +   +++++++++      +
 
 error[E0746]: return type cannot have an unboxed trait object
   --> $DIR/dyn-trait-return-should-be-impl-trait.rs:15:13
@@ -58,11 +57,14 @@ error[E0746]: return type cannot have an unboxed trait object
 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: use `impl Trait` as the return type, as all return paths are of type `Struct`, which implements `Trait`
+help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
    |
 LL | fn ban() -> impl Trait { Struct }
-   |             ~~~~~~~~~~
+   |             ~~~~
+help: box the return type, and wrap all of the returned values in `Box::new`
+   |
+LL | fn ban() -> Box<dyn Trait> { Box::new(Struct) }
+   |             ++++         +   +++++++++      +
 
 error[E0746]: return type cannot have an unboxed trait object
   --> $DIR/dyn-trait-return-should-be-impl-trait.rs:17:13
@@ -70,14 +72,14 @@ error[E0746]: return type cannot have an unboxed trait object
 LL | fn bak() -> dyn Trait { unimplemented!() }
    |             ^^^^^^^^^ doesn't have a size known at compile-time
    |
-help: use `impl Trait` as the return type if all return paths have the same type but you want to expose only the trait in the signature
+help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
    |
 LL | fn bak() -> impl Trait { unimplemented!() }
-   |             ~~~~~~~~~~
-help: use a boxed trait object if all return paths implement trait `Trait`
+   |             ~~~~
+help: box the return type, and wrap all of the returned values in `Box::new`
    |
-LL | fn bak() -> Box<dyn Trait> { unimplemented!() }
-   |             ++++         +
+LL | fn bak() -> Box<dyn Trait> { Box::new(unimplemented!()) }
+   |             ++++         +   +++++++++                +
 
 error[E0746]: return type cannot have an unboxed trait object
   --> $DIR/dyn-trait-return-should-be-impl-trait.rs:19:13
@@ -85,34 +87,18 @@ error[E0746]: return type cannot have an unboxed trait object
 LL | fn bal() -> dyn Trait {
    |             ^^^^^^^^^ doesn't have a size known at compile-time
    |
-   = 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>
-   = 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: you can create a new `enum` with a variant for each returned type
-help: return a boxed trait object instead
+help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
    |
-LL | fn bal() -> Box<dyn Trait> {
-   |             ++++         +
-help: ... and box this value
+LL | fn bal() -> impl Trait {
+   |             ~~~~
+help: box the return type, and wrap all of the returned values in `Box::new`
    |
-LL |         return Box::new(Struct);
-   |                +++++++++      +
-help: ... and box this value
+LL ~ fn bal() -> Box<dyn Trait> {
+LL |     if true {
+LL ~         return Box::new(Struct);
+LL |     }
+LL ~     Box::new(42)
    |
-LL |     Box::new(42)
-   |     +++++++++  +
-
-error[E0308]: `if` and `else` have incompatible types
-  --> $DIR/dyn-trait-return-should-be-impl-trait.rs:29:9
-   |
-LL | /     if true {
-LL | |         Struct
-   | |         ------ expected because of this
-LL | |     } else {
-LL | |         42
-   | |         ^^ expected `Struct`, found integer
-LL | |     }
-   | |_____- `if` and `else` have incompatible types
 
 error[E0746]: return type cannot have an unboxed trait object
   --> $DIR/dyn-trait-return-should-be-impl-trait.rs:25:13
@@ -120,22 +106,18 @@ error[E0746]: return type cannot have an unboxed trait object
 LL | fn bax() -> dyn Trait {
    |             ^^^^^^^^^ doesn't have a size known at compile-time
    |
-   = 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>
-   = 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: you can create a new `enum` with a variant for each returned type
-help: return a boxed trait object instead
+help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
    |
-LL | fn bax() -> Box<dyn Trait> {
-   |             ++++         +
-help: ... and box this value
+LL | fn bax() -> impl Trait {
+   |             ~~~~
+help: box the return type, and wrap all of the returned values in `Box::new`
    |
-LL |         Box::new(Struct)
-   |         +++++++++      +
-help: ... and box this value
+LL ~ fn bax() -> Box<dyn Trait> {
+LL |     if true {
+LL ~         Box::new(Struct)
+LL |     } else {
+LL ~         Box::new(42)
    |
-LL |         Box::new(42)
-   |         +++++++++  +
 
 error[E0308]: mismatched types
   --> $DIR/dyn-trait-return-should-be-impl-trait.rs:34:16
@@ -279,11 +261,18 @@ error[E0746]: return type cannot have an unboxed trait object
 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: use `impl Trait` as the return type, as all return paths are of type `{integer}`, which implements `Trait`
+help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
    |
 LL | fn bat() -> impl Trait {
-   |             ~~~~~~~~~~
+   |             ~~~~
+help: box the return type, and wrap all of the returned values in `Box::new`
+   |
+LL ~ fn bat() -> Box<dyn Trait> {
+LL |     if true {
+LL ~         return Box::new(0);
+LL |     }
+LL ~     Box::new(42)
+   |
 
 error[E0746]: return type cannot have an unboxed trait object
   --> $DIR/dyn-trait-return-should-be-impl-trait.rs:66:13
@@ -291,13 +280,20 @@ error[E0746]: return type cannot have an unboxed trait object
 LL | fn bay() -> 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: use `impl Trait` as the return type, as all return paths are of type `{integer}`, which implements `Trait`
+help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
    |
 LL | fn bay() -> impl Trait {
-   |             ~~~~~~~~~~
+   |             ~~~~
+help: box the return type, and wrap all of the returned values in `Box::new`
+   |
+LL ~ fn bay() -> Box<dyn Trait> {
+LL |     if true {
+LL ~         Box::new(0)
+LL |     } else {
+LL ~         Box::new(42)
+   |
 
-error: aborting due to 20 previous errors
+error: aborting due to 19 previous errors
 
 Some errors have detailed explanations: E0277, E0308, E0746.
 For more information about an error, try `rustc --explain E0277`.
diff --git a/tests/ui/impl-trait/point-to-type-err-cause-on-impl-trait-return.rs b/tests/ui/impl-trait/point-to-type-err-cause-on-impl-trait-return.rs
index fa7664a83ee..a8a6288eb56 100644
--- a/tests/ui/impl-trait/point-to-type-err-cause-on-impl-trait-return.rs
+++ b/tests/ui/impl-trait/point-to-type-err-cause-on-impl-trait-return.rs
@@ -77,7 +77,7 @@ fn hat() -> dyn std::fmt::Display { //~ ERROR return type cannot have an unboxed
 fn pug() -> dyn std::fmt::Display { //~ ERROR return type cannot have an unboxed trait object
     match 13 {
         0 => 0i32,
-        1 => 1u32, //~ ERROR `match` arms have incompatible types
+        1 => 1u32,
         _ => 2u32,
     }
 }
@@ -86,7 +86,7 @@ fn man() -> dyn std::fmt::Display { //~ ERROR return type cannot have an unboxed
     if false {
         0i32
     } else {
-        1u32 //~ ERROR `if` and `else` have incompatible types
+        1u32
     }
 }
 
diff --git a/tests/ui/impl-trait/point-to-type-err-cause-on-impl-trait-return.stderr b/tests/ui/impl-trait/point-to-type-err-cause-on-impl-trait-return.stderr
index 3c65fd998c5..9205d74504f 100644
--- a/tests/ui/impl-trait/point-to-type-err-cause-on-impl-trait-return.stderr
+++ b/tests/ui/impl-trait/point-to-type-err-cause-on-impl-trait-return.stderr
@@ -171,39 +171,20 @@ error[E0746]: return type cannot have an unboxed trait object
 LL | fn hat() -> dyn std::fmt::Display {
    |             ^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
-   = 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>
-   = note: if all the returned values were of the same type you could use `impl std::fmt::Display` 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: you can create a new `enum` with a variant for each returned type
-help: return a boxed trait object instead
-   |
-LL | fn hat() -> Box<dyn std::fmt::Display> {
-   |             ++++                     +
-help: ... and box this value
-   |
-LL |             return Box::new(0i32);
-   |                    +++++++++    +
-help: ... and box this value
-   |
-LL |             Box::new(1u32)
-   |             +++++++++    +
-
-error[E0308]: `match` arms have incompatible types
-  --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:80:14
+help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
    |
-LL | /     match 13 {
-LL | |         0 => 0i32,
-   | |              ---- this is found to be of type `i32`
-LL | |         1 => 1u32,
-   | |              ^^^^ expected `i32`, found `u32`
-LL | |         _ => 2u32,
-LL | |     }
-   | |_____- `match` arms have incompatible types
+LL | fn hat() -> impl std::fmt::Display {
+   |             ~~~~
+help: box the return type, and wrap all of the returned values in `Box::new`
    |
-help: change the type of the numeric literal from `u32` to `i32`
+LL ~ fn hat() -> Box<dyn std::fmt::Display> {
+LL |     match 13 {
+LL |         0 => {
+LL ~             return Box::new(0i32);
+LL |         }
+LL |         _ => {
+LL ~             Box::new(1u32)
    |
-LL |         1 => 1i32,
-   |               ~~~
 
 error[E0746]: return type cannot have an unboxed trait object
   --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:77:13
@@ -211,43 +192,18 @@ error[E0746]: return type cannot have an unboxed trait object
 LL | fn pug() -> dyn std::fmt::Display {
    |             ^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
-   = 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>
-   = note: if all the returned values were of the same type you could use `impl std::fmt::Display` 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: you can create a new `enum` with a variant for each returned type
-help: return a boxed trait object instead
-   |
-LL | fn pug() -> Box<dyn std::fmt::Display> {
-   |             ++++                     +
-help: ... and box this value
+help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
    |
-LL |         0 => Box::new(0i32),
-   |              +++++++++    +
-help: ... and box this value
-   |
-LL |         1 => Box::new(1u32),
-   |              +++++++++    +
-help: ... and box this value
-   |
-LL |         _ => Box::new(2u32),
-   |              +++++++++    +
-
-error[E0308]: `if` and `else` have incompatible types
-  --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:89:9
+LL | fn pug() -> impl std::fmt::Display {
+   |             ~~~~
+help: box the return type, and wrap all of the returned values in `Box::new`
    |
-LL | /     if false {
-LL | |         0i32
-   | |         ---- expected because of this
-LL | |     } else {
-LL | |         1u32
-   | |         ^^^^ expected `i32`, found `u32`
-LL | |     }
-   | |_____- `if` and `else` have incompatible types
-   |
-help: change the type of the numeric literal from `u32` to `i32`
+LL ~ fn pug() -> Box<dyn std::fmt::Display> {
+LL |     match 13 {
+LL ~         0 => Box::new(0i32),
+LL ~         1 => Box::new(1u32),
+LL ~         _ => Box::new(2u32),
    |
-LL |         1i32
-   |          ~~~
 
 error[E0746]: return type cannot have an unboxed trait object
   --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:85:13
@@ -255,24 +211,20 @@ error[E0746]: return type cannot have an unboxed trait object
 LL | fn man() -> dyn std::fmt::Display {
    |             ^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
-   = 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>
-   = note: if all the returned values were of the same type you could use `impl std::fmt::Display` 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: you can create a new `enum` with a variant for each returned type
-help: return a boxed trait object instead
+help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
    |
-LL | fn man() -> Box<dyn std::fmt::Display> {
-   |             ++++                     +
-help: ... and box this value
+LL | fn man() -> impl std::fmt::Display {
+   |             ~~~~
+help: box the return type, and wrap all of the returned values in `Box::new`
    |
-LL |         Box::new(0i32)
-   |         +++++++++    +
-help: ... and box this value
+LL ~ fn man() -> Box<dyn std::fmt::Display> {
+LL |     if false {
+LL ~         Box::new(0i32)
+LL |     } else {
+LL ~         Box::new(1u32)
    |
-LL |         Box::new(1u32)
-   |         +++++++++    +
 
-error: aborting due to 14 previous errors
+error: aborting due to 12 previous errors
 
 Some errors have detailed explanations: E0308, E0746.
 For more information about an error, try `rustc --explain E0308`.
diff --git a/tests/ui/issues/issue-18107.stderr b/tests/ui/issues/issue-18107.stderr
index 1669b550a9b..cf4e06316a7 100644
--- a/tests/ui/issues/issue-18107.stderr
+++ b/tests/ui/issues/issue-18107.stderr
@@ -4,14 +4,18 @@ error[E0746]: return type cannot have an unboxed trait object
 LL |     dyn AbstractRenderer
    |     ^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
-help: use `impl AbstractRenderer` as the return type if all return paths have the same type but you want to expose only the trait in the signature
+help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
    |
 LL |     impl AbstractRenderer
+   |     ~~~~
+help: box the return type, and wrap all of the returned values in `Box::new`
    |
-help: use a boxed trait object if all return paths implement trait `AbstractRenderer`
+LL ~     Box<dyn AbstractRenderer>
+LL |
+LL | {
+LL |     match 0 {
+LL ~         _ => Box::new(unimplemented!())
    |
-LL |     Box<dyn AbstractRenderer>
-   |     ++++                    +
 
 error: aborting due to previous error
 
diff --git a/tests/ui/unsized/box-instead-of-dyn-fn.rs b/tests/ui/unsized/box-instead-of-dyn-fn.rs
index 2fa741bc1c5..321c2ebf5a1 100644
--- a/tests/ui/unsized/box-instead-of-dyn-fn.rs
+++ b/tests/ui/unsized/box-instead-of-dyn-fn.rs
@@ -8,7 +8,6 @@ fn print_on_or_the_other<'a>(a: i32, b: &'a String) -> dyn Fn() + 'a {
         move || println!("{a}")
     } else {
         Box::new(move || println!("{}", b))
-        //~^ ERROR `if` and `else` have incompatible types
     }
 }
 
diff --git a/tests/ui/unsized/box-instead-of-dyn-fn.stderr b/tests/ui/unsized/box-instead-of-dyn-fn.stderr
index bfb7c3957f4..6087f5c5465 100644
--- a/tests/ui/unsized/box-instead-of-dyn-fn.stderr
+++ b/tests/ui/unsized/box-instead-of-dyn-fn.stderr
@@ -1,42 +1,21 @@
-error[E0308]: `if` and `else` have incompatible types
-  --> $DIR/box-instead-of-dyn-fn.rs:10:9
-   |
-LL | /     if a % 2 == 0 {
-LL | |         move || println!("{a}")
-   | |         -----------------------
-   | |         |
-   | |         the expected closure
-   | |         expected because of this
-LL | |     } else {
-LL | |         Box::new(move || println!("{}", b))
-   | |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected closure, found `Box<[closure@box-instead-of-dyn-fn.rs:10:18]>`
-LL | |
-LL | |     }
-   | |_____- `if` and `else` have incompatible types
-   |
-   = note: expected closure `[closure@$DIR/box-instead-of-dyn-fn.rs:8:9: 8:16]`
-               found struct `Box<[closure@$DIR/box-instead-of-dyn-fn.rs:10:18: 10:25]>`
-
 error[E0746]: return type cannot have an unboxed trait object
   --> $DIR/box-instead-of-dyn-fn.rs:5:56
    |
 LL | fn print_on_or_the_other<'a>(a: i32, b: &'a String) -> dyn Fn() + 'a {
    |                                                        ^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
-   = 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>
-   = note: if all the returned values were of the same type you could use `impl Fn() + 'a` 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: you can create a new `enum` with a variant for each returned type
-help: return a boxed trait object instead
+help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
+   |
+LL | fn print_on_or_the_other<'a>(a: i32, b: &'a String) -> impl Fn() + 'a {
+   |                                                        ~~~~
+help: box the return type, and wrap all of the returned values in `Box::new`
    |
-LL | fn print_on_or_the_other<'a>(a: i32, b: &'a String) -> Box<dyn Fn() + 'a> {
-   |                                                        ++++             +
-help: ... and box this value
+LL ~ fn print_on_or_the_other<'a>(a: i32, b: &'a String) -> Box<dyn Fn() + 'a> {
+LL |
+LL |     if a % 2 == 0 {
+LL ~         Box::new(move || println!("{a}"))
    |
-LL |         Box::new(move || println!("{a}"))
-   |         +++++++++                       +
 
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
-Some errors have detailed explanations: E0308, E0746.
-For more information about an error, try `rustc --explain E0308`.
+For more information about this error, try `rustc --explain E0746`.
diff --git a/tests/ui/unsized/issue-91801.stderr b/tests/ui/unsized/issue-91801.stderr
index 8795aa1687f..da5c4322403 100644
--- a/tests/ui/unsized/issue-91801.stderr
+++ b/tests/ui/unsized/issue-91801.stderr
@@ -4,11 +4,10 @@ error[E0746]: return type cannot have an unboxed trait object
 LL | fn or<'a>(first: &'static Validator<'a>, second: &'static Validator<'a>) -> Validator<'a> {
    |                                                                             ^^^^^^^^^^^^^ 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: use `impl Fn(&'a Something) -> Result<(), ()> + Send + Sync + 'a` as the return type, as all return paths are of type `Box<[closure@$DIR/issue-91801.rs:10:21: 10:70]>`, which implements `Fn(&'a Something) -> Result<(), ()> + Send + Sync + 'a`
+help: box the return type, and wrap all of the returned values in `Box::new`
    |
-LL | fn or<'a>(first: &'static Validator<'a>, second: &'static Validator<'a>) -> impl Fn(&'a Something) -> Result<(), ()> + Send + Sync + 'a {
-   |                                                                             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+LL | fn or<'a>(first: &'static Validator<'a>, second: &'static Validator<'a>) -> Box<Validator<'a>> {
+   |                                                                             ++++             +
 
 error: aborting due to previous error
 
diff --git a/tests/ui/unsized/issue-91803.stderr b/tests/ui/unsized/issue-91803.stderr
index 2dad9e89294..a43b8d0741f 100644
--- a/tests/ui/unsized/issue-91803.stderr
+++ b/tests/ui/unsized/issue-91803.stderr
@@ -4,11 +4,14 @@ error[E0746]: return type cannot have an unboxed trait object
 LL | fn or<'a>(first: &'static dyn Foo<'a>) -> dyn Foo<'a> {
    |                                           ^^^^^^^^^^^ 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: use `impl Foo<'a>` as the return type, as all return paths are of type `Box<_>`, which implements `Foo<'a>`
+help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
    |
 LL | fn or<'a>(first: &'static dyn Foo<'a>) -> impl Foo<'a> {
-   |                                           ~~~~~~~~~~~~
+   |                                           ~~~~
+help: box the return type, and wrap all of the returned values in `Box::new`
+   |
+LL | fn or<'a>(first: &'static dyn Foo<'a>) -> Box<dyn Foo<'a>> {
+   |                                           ++++           +
 
 error: aborting due to previous error