On fn with no borrowed argument, explain that &dyn Trait could have been an option

Author: estebank

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -2002,6 +2002,11 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
  },
  Applicability::MachineApplicable,
  );
+ } else {
+ err.help(
+ "if the returned value came from a borrowed argument that implements the trait, \
+ then you could return `&dyn Trait`",
+ );
  }
 
  true

tests/ui/error-codes/E0746.stderr

@@ -4,6 +4,7 @@ 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
  |
+ = help: if the returned value came from a borrowed argument that implements the trait, then you could return `&dyn Trait`
 help: return an `impl Trait` instead of a `dyn Trait`
  |
 LL | fn foo() -> impl Trait { Struct }
@@ -19,6 +20,7 @@ error[E0746]: return type cannot have an unboxed trait object
 LL | fn bar() -> dyn Trait {
  | ^^^^^^^^^ doesn't have a size known at compile-time
  |
+ = help: if the returned value came from a borrowed argument that implements the trait, then you could return `&dyn Trait`
 help: return an `impl Trait` instead of a `dyn Trait`
  |
 LL | fn bar() -> impl Trait {

tests/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr

@@ -48,6 +48,7 @@ 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: if the returned value came from a borrowed argument that implements the trait, then you could return `&dyn Trait`
 help: return an `impl Trait` instead of a `dyn Trait`
  |
 LL | fn bap() -> impl Trait { Struct }
@@ -63,6 +64,7 @@ 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: if the returned value came from a borrowed argument that implements the trait, then you could return `&dyn Trait`
 help: return an `impl Trait` instead of a `dyn Trait`
  |
 LL | fn ban() -> impl Trait { Struct }
@@ -78,6 +80,7 @@ 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: if the returned value came from a borrowed argument that implements the trait, then you could return `&dyn Trait`
 help: return an `impl Trait` instead of a `dyn Trait`
  |
 LL | fn bak() -> impl Trait { unimplemented!() }
@@ -101,6 +104,7 @@ LL | return Struct;
 LL | }
 LL | 42
  | ^^ this returned value has type `{integer}`
+ = help: if the returned value came from a borrowed argument that implements the trait, then you could return `&dyn Trait`
 help: box the return type to make a boxed trait object, and wrap all of the returned values in `Box::new`
  |
 LL ~ fn bal() -> Box<dyn Trait> {
@@ -124,6 +128,7 @@ LL | Struct
 LL | } else {
 LL | 42
  | ^^ this returned value has type `{integer}`
+ = help: if the returned value came from a borrowed argument that implements the trait, then you could return `&dyn Trait`
 help: box the return type to make a boxed trait object, and wrap all of the returned values in `Box::new`
  |
 LL ~ fn bax() -> Box<dyn Trait> {
@@ -275,6 +280,7 @@ 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: if the returned value came from a borrowed argument that implements the trait, then you could return `&dyn Trait`
 help: return an `impl Trait` instead of a `dyn Trait`
  |
 LL | fn bat() -> impl Trait {
@@ -294,6 +300,7 @@ 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: if the returned value came from a borrowed argument that implements the trait, then you could return `&dyn Trait`
 help: return an `impl Trait` instead of a `dyn Trait`
  |
 LL | fn bay() -> impl Trait {

tests/ui/impl-trait/object-unsafe-trait-in-return-position-dyn-trait.stderr

@@ -92,6 +92,7 @@ help: trait `NotObjectSafe` is not object safe, so you can't return a boxed trai
  |
 LL | fn car() -> dyn NotObjectSafe {
  | ^^^^^^^^^^^^^
+ = help: if the returned value came from a borrowed argument that implements the trait, then you could return `&dyn Trait`
 
 error[E0038]: the trait `NotObjectSafe` cannot be made into an object
  --> $DIR/object-unsafe-trait-in-return-position-dyn-trait.rs:31:16

tests/ui/impl-trait/point-to-type-err-cause-on-impl-trait-return.stderr

@@ -171,6 +171,7 @@ 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: if the returned value came from a borrowed argument that implements the trait, then you could return `&dyn Trait`
 help: return an `impl Trait` instead of a `dyn Trait`
  |
 LL | fn hat() -> impl std::fmt::Display {
@@ -192,6 +193,7 @@ 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: if the returned value came from a borrowed argument that implements the trait, then you could return `&dyn Trait`
 help: return an `impl Trait` instead of a `dyn Trait`
  |
 LL | fn pug() -> impl std::fmt::Display {
@@ -219,6 +221,7 @@ LL | 0i32
 LL | } else {
 LL | 1u32
  | ^^^^ this returned value has type `u32`
+ = help: if the returned value came from a borrowed argument that implements the trait, then you could return `&dyn Trait`
 help: box the return type to make a boxed trait object, and wrap all of the returned values in `Box::new`
  |
 LL ~ fn man() -> Box<dyn std::fmt::Display> {

tests/ui/inference/ice-ifer-var-leaked-out-of-rollback-122098.stderr

@@ -36,10 +36,10 @@ LL | struct Query<'q> {}
  = help: consider removing `'q`, referring to it in a field, or using a marker such as `PhantomData`
 
 error[E0277]: the size for values of type `Self` cannot be known at compilation time
- --> $DIR/ice-ifer-var-leaked-out-of-rollback-122098.rs:7:17
+ --> $DIR/ice-ifer-var-leaked-out-of-rollback-122098.rs:7:21
  |
 LL | fn for_each(mut self, mut f: Box<dyn FnMut(Self::Item<'_>) + 'static>) {}
- | ^^^^^^^^ doesn't have a size known at compile-time
+ |  ^^^^ doesn't have a size known at compile-time
  |
  = help: unsized fn params are gated as an unstable feature
 help: consider further restricting `Self`

tests/ui/traits/bound/not-on-bare-trait-2021.stderr

@@ -60,6 +60,7 @@ error[E0746]: return type cannot have an unboxed trait object
 LL | fn qux() -> Foo {
  | ^^^ doesn't have a size known at compile-time
  |
+ = help: if the returned value came from a borrowed argument that implements the trait, then you could return `&dyn Trait`
 help: return an `impl Trait` instead of a `dyn Trait`
  |
 LL | fn qux() -> impl Foo {

tests/ui/traits/bound/not-on-bare-trait.stderr

@@ -154,6 +154,7 @@ error[E0746]: return type cannot have an unboxed trait object
 LL | fn qux() -> Foo {
  | ^^^ doesn't have a size known at compile-time
  |
+ = help: if the returned value came from a borrowed argument that implements the trait, then you could return `&dyn Trait`
 help: return an `impl Trait` instead of a `dyn Trait`
  |
 LL | fn qux() -> impl Foo {