More accurate suggestion for `-> Box<dyn Trait>` or `-> impl Trait`

When encountering `-> Trait`, suggest `-> Box<dyn Trait>` (instead of `-> Box<Trait>`.

If there's a single returned type within the `fn`, suggest `-> impl Trait`.

3 files changed, 21 insertions, 35 deletions

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 f25269ca032..9c546659564 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
@@ -48,10 +48,14 @@ error[E0746]: return type cannot have an unboxed trait object
 LL | fn bap() -> Trait { Struct }
    |             ^^^^^ doesn't have a size known at compile-time
    |
-help: box the return type, and wrap all of the returned values in `Box::new`
+help: consider returning an `impl Trait` instead of a `dyn Trait`
+   |
+LL | fn bap() -> impl Trait { Struct }
+   |             ++++
+help: alternatively, box the return type, and wrap all of the returned values in `Box::new`
    |
-LL | fn bap() -> Box<Trait> { Box::new(Struct) }
-   |             ++++     +   +++++++++      +
+LL | fn bap() -> 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:15:13
@@ -59,11 +63,11 @@ 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
    |
-help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
+help: consider returning an `impl Trait` instead of a `dyn Trait`
    |
 LL | fn ban() -> impl Trait { Struct }
    |             ~~~~
-help: box the return type, and wrap all of the returned values in `Box::new`
+help: alternatively, box the return type, and wrap all of the returned values in `Box::new`
    |
 LL | fn ban() -> Box<dyn Trait> { Box::new(Struct) }
    |             ++++         +   +++++++++      +
@@ -74,11 +78,11 @@ 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: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
+help: consider returning an `impl Trait` instead of a `dyn Trait`
    |
 LL | fn bak() -> impl Trait { unimplemented!() }
    |             ~~~~
-help: box the return type, and wrap all of the returned values in `Box::new`
+help: alternatively, box the return type, and wrap all of the returned values in `Box::new`
    |
 LL | fn bak() -> Box<dyn Trait> { Box::new(unimplemented!()) }
    |             ++++         +   +++++++++                +
@@ -89,10 +93,7 @@ error[E0746]: return type cannot have an unboxed trait object
 LL | fn bal() -> dyn Trait {
    |             ^^^^^^^^^ doesn't have a size known at compile-time
    |
-help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
-   |
-LL | fn bal() -> impl Trait {
-   |             ~~~~
+   = help: if there were a single returned type, you could use `impl Trait` instead
 help: box the return type, and wrap all of the returned values in `Box::new`
    |
 LL ~ fn bal() -> Box<dyn Trait> {
@@ -108,10 +109,7 @@ error[E0746]: return type cannot have an unboxed trait object
 LL | fn bax() -> dyn Trait {
    |             ^^^^^^^^^ doesn't have a size known at compile-time
    |
-help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
-   |
-LL | fn bax() -> impl Trait {
-   |             ~~~~
+   = help: if there were a single returned type, you could use `impl Trait` instead
 help: box the return type, and wrap all of the returned values in `Box::new`
    |
 LL ~ fn bax() -> Box<dyn Trait> {
@@ -263,10 +261,7 @@ error[E0746]: return type cannot have an unboxed trait object
 LL | fn bat() -> dyn Trait {
    |             ^^^^^^^^^ doesn't have a size known at compile-time
    |
-help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
-   |
-LL | fn bat() -> impl Trait {
-   |             ~~~~
+   = help: if there were a single returned type, you could use `impl Trait` instead
 help: box the return type, and wrap all of the returned values in `Box::new`
    |
 LL ~ fn bat() -> Box<dyn Trait> {
@@ -282,10 +277,7 @@ error[E0746]: return type cannot have an unboxed trait object
 LL | fn bay() -> dyn Trait {
    |             ^^^^^^^^^ doesn't have a size known at compile-time
    |
-help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
-   |
-LL | fn bay() -> impl Trait {
-   |             ~~~~
+   = help: if there were a single returned type, you could use `impl Trait` instead
 help: box the return type, and wrap all of the returned values in `Box::new`
    |
 LL ~ fn bay() -> Box<dyn Trait> {
diff --git a/tests/ui/impl-trait/object-unsafe-trait-in-return-position-dyn-trait.stderr b/tests/ui/impl-trait/object-unsafe-trait-in-return-position-dyn-trait.stderr
index fc9c30abf13..09a689e6396 100644
--- a/tests/ui/impl-trait/object-unsafe-trait-in-return-position-dyn-trait.stderr
+++ b/tests/ui/impl-trait/object-unsafe-trait-in-return-position-dyn-trait.stderr
@@ -54,10 +54,7 @@ error[E0746]: return type cannot have an unboxed trait object
 LL | fn car() -> dyn NotObjectSafe {
    |             ^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
-help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
-   |
-LL | fn car() -> impl NotObjectSafe {
-   |             ~~~~
+   = help: if there were a single returned type, you could use `impl Trait` instead
 help: box the return type, and wrap all of the returned values in `Box::new`
    |
 LL ~ fn car() -> Box<dyn NotObjectSafe> {
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 9205d74504f..54849c112f5 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
@@ -171,11 +171,11 @@ 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
    |
-help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
+help: consider returning an `impl Trait` instead of a `dyn Trait`
    |
 LL | fn hat() -> impl std::fmt::Display {
    |             ~~~~
-help: box the return type, and wrap all of the returned values in `Box::new`
+help: alternatively, box the return type, and wrap all of the returned values in `Box::new`
    |
 LL ~ fn hat() -> Box<dyn std::fmt::Display> {
 LL |     match 13 {
@@ -192,11 +192,11 @@ 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
    |
-help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
+help: consider returning an `impl Trait` instead of a `dyn Trait`
    |
 LL | fn pug() -> impl std::fmt::Display {
    |             ~~~~
-help: box the return type, and wrap all of the returned values in `Box::new`
+help: alternatively, box the return type, and wrap all of the returned values in `Box::new`
    |
 LL ~ fn pug() -> Box<dyn std::fmt::Display> {
 LL |     match 13 {
@@ -211,10 +211,7 @@ 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
    |
-help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
-   |
-LL | fn man() -> impl std::fmt::Display {
-   |             ~~~~
+   = help: if there were a single returned type, you could use `impl Trait` instead
 help: box the return type, and wrap all of the returned values in `Box::new`
    |
 LL ~ fn man() -> Box<dyn std::fmt::Display> {