On object safety error, mention new enum as alternative

When we encounter a `dyn Trait` that isn't object safe, look for its
implementors. If there's one, mention using it directly If there are
less than 9, mention the possibility of creating a new enum and using
that instead.

Fix rust-lang#80194.

Author: estebank

compiler/rustc_infer/src/traits/error_reporting/mod.rs

@@ -5,6 +5,7 @@ use rustc_data_structures::fx::FxIndexSet;
 use rustc_errors::{struct_span_err, DiagnosticBuilder, ErrorGuaranteed, MultiSpan};
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::TyCtxt;
 use rustc_span::Span;
 use std::fmt;
@@ -108,5 +109,41 @@ pub fn report_object_safety_error<'tcx>(
             violation.solution(&mut err);
         }
     }
+
+    let impls_of = tcx.trait_impls_of(trait_def_id);
+    let impls = if impls_of.blanket_impls().is_empty() {
+        impls_of.non_blanket_impls().values().flatten().collect::<Vec<_>>()
+    } else {
+        vec![]
+    };
+    match &impls[..] {
+        [] => {}
+        _ if impls.len() > 9 => {}
+        [only] => {
+            err.help(with_no_trimmed_paths!(format!(
+                "only type `{}` implements the trait, consider using it directly instead",
+                tcx.type_of(*only).instantiate_identity(),
+            )));
+        }
+        impls => {
+            let types = impls
+                .iter()
+                .map(|t| {
+                    with_no_trimmed_paths!(format!(
+                        "\n - {}",
+                        tcx.type_of(*t).instantiate_identity(),
+                    ))
+                })
+                .collect::<Vec<_>>();
+            err.help(format!(
+                "the following types implement the trait, consider defining an enum where each \
+                 variant holds one of these types, implementing `{}` for this new enum and using \
+                 it instead:{}",
+                trait_str,
+                types.join(""),
+            ));
+        }
+    }
+
     err
 }

tests/ui/coherence/coherence-impl-trait-for-trait-object-safe.stderr

@@ -12,6 +12,7 @@ LL | trait NotObjectSafe { fn eq(&self, other: Self); }
    |       |
    |       this trait cannot be made into an object...
    = help: consider moving `eq` to another trait
+   = help: only type `(dyn NotObjectSafe + 'static)` implements the trait, consider using it directly instead
 
 error: aborting due to previous error
 

tests/ui/const-generics/generic_const_exprs/object-safety-err-ret.stderr

@@ -12,6 +12,7 @@ LL | trait Foo {
 LL |     fn test(&self) -> [u8; bar::<Self>()];
    |                       ^^^^^^^^^^^^^^^^^^^ ...because method `test` references the `Self` type in its return type
    = help: consider moving `test` to another trait
+   = help: only type `()` implements the trait, consider using it directly instead
 
 error: aborting due to previous error
 

tests/ui/feature-gates/feature-gate-dispatch-from-dyn-missing-impl.stderr

@@ -14,6 +14,7 @@ LL | trait Trait {
    |       ----- this trait cannot be made into an object...
 LL |     fn ptr(self: Ptr<Self>);
    |                  ^^^^^^^^^ ...because method `ptr`'s `self` parameter cannot be dispatched on
+   = help: only type `i32` implements the trait, consider using it directly instead
 
 error[E0038]: the trait `Trait` cannot be made into an object
   --> $DIR/feature-gate-dispatch-from-dyn-missing-impl.rs:32:5
@@ -31,6 +32,7 @@ LL | trait Trait {
    |       ----- this trait cannot be made into an object...
 LL |     fn ptr(self: Ptr<Self>);
    |                  ^^^^^^^^^ ...because method `ptr`'s `self` parameter cannot be dispatched on
+   = help: only type `i32` implements the trait, consider using it directly instead
    = note: required for the cast from `Ptr<{integer}>` to `Ptr<dyn Trait>`
 
 error: aborting due to 2 previous errors

tests/ui/generic-associated-types/gat-in-trait-path.base.stderr

@@ -12,6 +12,9 @@ LL | trait Foo {
 LL |     type A<'a> where Self: 'a;
    |          ^ ...because it contains the generic associated type `A`
    = help: consider moving `A` to another trait
+   = help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `Foo` for this new enum and using it instead:
+            - Fooy
+            - Fooer<T>
 
 error: aborting due to previous error
 

tests/ui/generic-associated-types/issue-76535.base.stderr

@@ -28,6 +28,7 @@ LL | pub trait SuperTrait {
 LL |     type SubType<'a>: SubTrait where Self: 'a;
    |          ^^^^^^^ ...because it contains the generic associated type `SubType`
    = help: consider moving `SubType` to another trait
+   = help: only type `SuperStruct` implements the trait, consider using it directly instead
 
 error[E0038]: the trait `SuperTrait` cannot be made into an object
   --> $DIR/issue-76535.rs:39:57
@@ -43,6 +44,7 @@ LL | pub trait SuperTrait {
 LL |     type SubType<'a>: SubTrait where Self: 'a;
    |          ^^^^^^^ ...because it contains the generic associated type `SubType`
    = help: consider moving `SubType` to another trait
+   = help: only type `SuperStruct` implements the trait, consider using it directly instead
    = note: required for the cast from `Box<SuperStruct>` to `Box<dyn SuperTrait<SubType = SubStruct<'_>>>`
 
 error: aborting due to 3 previous errors

tests/ui/generic-associated-types/issue-79422.base.stderr

@@ -28,6 +28,9 @@ LL | trait MapLike<K, V> {
 LL |     type VRefCont<'a>: RefCont<'a, V> where Self: 'a;
    |          ^^^^^^^^ ...because it contains the generic associated type `VRefCont`
    = help: consider moving `VRefCont` to another trait
+   = help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `MapLike` for this new enum and using it instead:
+            - std::collections::BTreeMap<K, V>
+            - Source
 
 error[E0038]: the trait `MapLike` cannot be made into an object
   --> $DIR/issue-79422.rs:44:13
@@ -43,6 +46,9 @@ LL | trait MapLike<K, V> {
 LL |     type VRefCont<'a>: RefCont<'a, V> where Self: 'a;
    |          ^^^^^^^^ ...because it contains the generic associated type `VRefCont`
    = help: consider moving `VRefCont` to another trait
+   = help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `MapLike` for this new enum and using it instead:
+            - std::collections::BTreeMap<K, V>
+            - Source
    = note: required for the cast from `Box<BTreeMap<u8, u8>>` to `Box<dyn MapLike<u8, u8, VRefCont = (dyn RefCont<'_, u8> + 'static)>>`
 
 error: aborting due to 3 previous errors

tests/ui/impl-trait/in-trait/foreign-dyn-error.stderr

@@ -9,6 +9,7 @@ note: for a trait to be "object safe" it needs to allow building a vtable to all
    |
 LL |     fn bar(self) -> impl Deref<Target = impl Sized>;
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait cannot be made into an object because method `bar` references an `impl Trait` type in its return type
+   = help: only type `rpitit::Foreign` implements the trait, consider using it directly instead
 
 error: aborting due to previous error
 

tests/ui/impl-trait/in-trait/object-safety.stderr

@@ -12,6 +12,7 @@ LL | trait Foo {
 LL |     fn baz(&self) -> impl Debug;
    |                      ^^^^^^^^^^ ...because method `baz` references an `impl Trait` type in its return type
    = help: consider moving `baz` to another trait
+   = help: only type `u32` implements the trait, consider using it directly instead
 
 error[E0038]: the trait `Foo` cannot be made into an object
   --> $DIR/object-safety.rs:19:15
@@ -27,6 +28,7 @@ LL | trait Foo {
 LL |     fn baz(&self) -> impl Debug;
    |                      ^^^^^^^^^^ ...because method `baz` references an `impl Trait` type in its return type
    = help: consider moving `baz` to another trait
+   = help: only type `u32` implements the trait, consider using it directly instead
 
 error[E0038]: the trait `Foo` cannot be made into an object
   --> $DIR/object-safety.rs:19:13
@@ -42,6 +44,7 @@ LL | trait Foo {
 LL |     fn baz(&self) -> impl Debug;
    |                      ^^^^^^^^^^ ...because method `baz` references an `impl Trait` type in its return type
    = help: consider moving `baz` to another trait
+   = help: only type `u32` implements the trait, consider using it directly instead
 
 error[E0038]: the trait `Foo` cannot be made into an object
   --> $DIR/object-safety.rs:16:13
@@ -57,6 +60,7 @@ LL | trait Foo {
 LL |     fn baz(&self) -> impl Debug;
    |                      ^^^^^^^^^^ ...because method `baz` references an `impl Trait` type in its return type
    = help: consider moving `baz` to another trait
+   = help: only type `u32` implements the trait, consider using it directly instead
    = note: required for the cast from `Box<u32>` to `Box<dyn Foo>`
 
 error: aborting due to 4 previous errors

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

@@ -11,6 +11,9 @@ LL | trait NotObjectSafe {
    |       ------------- this trait cannot be made into an object...
 LL |     fn foo() -> Self;
    |        ^^^ ...because associated function `foo` has no `self` parameter
+   = help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `NotObjectSafe` for this new enum and using it instead:
+            - A
+            - B
 help: consider turning `foo` into a method by giving it a `&self` argument
    |
 LL |     fn foo(&self) -> Self;
@@ -33,6 +36,9 @@ LL | trait NotObjectSafe {
    |       ------------- this trait cannot be made into an object...
 LL |     fn foo() -> Self;
    |        ^^^ ...because associated function `foo` has no `self` parameter
+   = help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `NotObjectSafe` for this new enum and using it instead:
+            - A
+            - B
 help: consider turning `foo` into a method by giving it a `&self` argument
    |
 LL |     fn foo(&self) -> Self;