Refactor dyn-compatibility error and suggestions

This CL makes a number of small changes to dyn compatibility errors:
- "object safety" has been renamed to "dyn-compatibility" throughout
- "Convert to enum" suggestions are no longer generated when there
  exists a type-generic impl of the trait or an impl for `dyn OtherTrait`
- Several error messages are reorganized for user readability

Additionally, the dyn compatibility error creation code has been
split out into functions.

cc #132713
cc #133267

compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs

@@ -103,17 +103,12 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         // most importantly, that the supertraits don't contain `Self`,
         // to avoid ICEs.
         for item in &regular_traits {
-            let violations =
-                hir_ty_lowering_dyn_compatibility_violations(tcx, item.trait_ref().def_id());
+            let item_def_id = item.trait_ref().def_id();
+            let violations = hir_ty_lowering_dyn_compatibility_violations(tcx, item_def_id);
             if !violations.is_empty() {
-                let reported = report_dyn_incompatibility(
-                    tcx,
-                    span,
-                    Some(hir_id),
-                    item.trait_ref().def_id(),
-                    &violations,
-                )
-                .emit();
+                let reported =
+                    report_dyn_incompatibility(tcx, span, Some(hir_id), item_def_id, &violations)
+                        .emit();
                 return Ty::new_error(tcx, reported);
             }
         }
@@ -276,7 +271,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         self.dcx(),
                         i.bottom().1,
                         E0038,
-                        "the {} `{}` cannot be made into an object",
+                        "the {} `{}` is not dyn-compatible",
                         tcx.def_descr(def_id),
                         tcx.item_name(def_id),
                     )

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

@@ -432,33 +432,18 @@ pub fn report_dyn_incompatibility<'tcx>(
         hir::Node::Item(item) => Some(item.ident.span),
         _ => None,
     });
+
     let mut err = struct_span_code_err!(
         tcx.dcx(),
         span,
         E0038,
-        "the trait `{}` cannot be made into an object",
+        "the trait `{}` is not dyn-compatible",
         trait_str
     );
-    err.span_label(span, format!("`{trait_str}` cannot be made into an object"));
-
-    if let Some(hir_id) = hir_id
-        && let hir::Node::Ty(ty) = tcx.hir_node(hir_id)
-        && let hir::TyKind::TraitObject([trait_ref, ..], ..) = ty.kind
-    {
-        let mut hir_id = hir_id;
-        while let hir::Node::Ty(ty) = tcx.parent_hir_node(hir_id) {
-            hir_id = ty.hir_id;
-        }
-        if tcx.parent_hir_node(hir_id).fn_sig().is_some() {
-            // Do not suggest `impl Trait` when dealing with things like super-traits.
-            err.span_suggestion_verbose(
-                ty.span.until(trait_ref.span),
-                "consider using an opaque type instead",
-                "impl ",
-                Applicability::MaybeIncorrect,
-            );
-        }
-    }
+    err.span_label(span, format!("`{trait_str}` is not dyn-compatible"));
+
+    attempt_dyn_to_impl_suggestion(tcx, hir_id, &mut err);
+
     let mut reported_violations = FxIndexSet::default();
     let mut multi_span = vec![];
     let mut messages = vec![];
@@ -473,7 +458,7 @@ pub fn report_dyn_incompatibility<'tcx>(
         if reported_violations.insert(violation.clone()) {
             let spans = violation.spans();
             let msg = if trait_span.is_none() || spans.is_empty() {
-                format!("the trait cannot be made into an object because {}", violation.error_msg())
+                format!("the trait is not dyn-compatible because {}", violation.error_msg())
             } else {
                 format!("...because {}", violation.error_msg())
             };
@@ -490,24 +475,20 @@ pub fn report_dyn_incompatibility<'tcx>(
     let has_multi_span = !multi_span.is_empty();
     let mut note_span = MultiSpan::from_spans(multi_span.clone());
     if let (Some(trait_span), true) = (trait_span, has_multi_span) {
-        note_span.push_span_label(trait_span, "this trait cannot be made into an object...");
+        note_span.push_span_label(trait_span, "this trait is not dyn-compatible...");
     }
     for (span, msg) in iter::zip(multi_span, messages) {
         note_span.push_span_label(span, msg);
     }
     // FIXME(dyn_compat_renaming): Update the URL.
     err.span_note(
         note_span,
-        "for a trait to be \"dyn-compatible\" it needs to allow building a vtable to allow the call \
-         to be resolvable dynamically; for more information visit \
-         <https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
+        "for a trait to be dyn-compatible it needs to allow building a vtable\n\
+        for more information, visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
     );
 
     // Only provide the help if its a local trait, otherwise it's not actionable.
     if trait_span.is_some() {
-        let mut reported_violations: Vec<_> = reported_violations.into_iter().collect();
-        reported_violations.sort();
-
         let mut potential_solutions: Vec<_> =
             reported_violations.into_iter().map(|violation| violation.solution()).collect();
         potential_solutions.sort();
@@ -518,68 +499,124 @@ pub fn report_dyn_incompatibility<'tcx>(
         }
     }
 
+    attempt_dyn_to_enum_suggestion(tcx, trait_def_id, &*trait_str, &mut err);
+
+    err
+}
+
+/// Attempt to suggest converting the `dyn Trait` argument to an enumeration
+/// over the types that implement `Trait`.
+fn attempt_dyn_to_enum_suggestion(
+    tcx: TyCtxt<'_>,
+    trait_def_id: DefId,
+    trait_str: &str,
+    err: &mut Diag<'_>,
+) {
     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()
-            .filter(|def_id| {
-                !matches!(tcx.type_of(*def_id).instantiate_identity().kind(), ty::Dynamic(..))
-            })
-            .collect::<Vec<_>>()
-    } else {
-        vec![]
+
+    // Don't suggest converting to an enum if there are any blanket impls.
+    if !impls_of.blanket_impls().is_empty() {
+        return;
+    }
+
+    let concrete_impls = {
+        let mut concrete_impls = Vec::new();
+        for impl_id in impls_of.non_blanket_impls().values().flatten() {
+            // Don't suggest converting to enum if there are any non-lifetime generics.
+            if has_non_lifetime_generics(tcx, *impl_id) {
+                return;
+            }
+
+            let impl_type = tcx.type_of(*impl_id).instantiate_identity();
+
+            // Don't suggest converting to enum if there are any
+            // `impl Trait for dyn OtherTrait`
+            if let ty::Dynamic(..) = impl_type.kind() {
+                return;
+            }
+
+            concrete_impls.push(impl_type);
+        }
+        concrete_impls
     };
-    let externally_visible = if !impls.is_empty()
-        && let Some(def_id) = trait_def_id.as_local()
+    if concrete_impls.is_empty() {
+        return;
+    }
+
+    const MAX_IMPLS_TO_SUGGEST_CONVERTING_TO_ENUM: usize = 9;
+    if concrete_impls.len() > MAX_IMPLS_TO_SUGGEST_CONVERTING_TO_ENUM {
+        return;
+    }
+
+    let externally_visible = if let Some(def_id) = trait_def_id.as_local() {
         // We may be executing this during typeck, which would result in cycle
         // if we used effective_visibilities query, which looks into opaque types
         // (and therefore calls typeck).
-        && tcx.resolutions(()).effective_visibilities.is_exported(def_id)
-    {
-        true
+        tcx.resolutions(()).effective_visibilities.is_exported(def_id)
     } else {
         false
     };
-    match &impls[..] {
-        [] => {}
-        _ if impls.len() > 9 => {}
-        [only] if externally_visible => {
-            err.help(with_no_trimmed_paths!(format!(
-                "only type `{}` is seen to implement the trait in this crate, consider using it \
-                 directly instead",
-                tcx.type_of(*only).instantiate_identity(),
-            )));
-        }
-        [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!("  {}", 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:\n{}",
-                trait_str,
-                types.join("\n"),
-            ));
-        }
+
+    if let [only_impl] = &concrete_impls[..] {
+        let within = if externally_visible { " within this crate" } else { "" };
+        err.help(with_no_trimmed_paths!(format!(
+            "only type `{only_impl}` implements `{trait_str}`{within}. \
+            Consider using it directly instead."
+        )));
+    } else {
+        let types = concrete_impls
+            .iter()
+            .map(|t| with_no_trimmed_paths!(format!("  {}", t)))
+            .collect::<Vec<String>>()
+            .join("\n");
+
+        err.help(format!(
+            "the following types implement `{trait_str}`:\n\
+             {types}\n\
+             Consider defining an enum where each variant holds one of these types,\n\
+             implementing `{trait_str}` for this new enum and using it instead.",
+        ));
     }
+
     if externally_visible {
         err.note(format!(
-            "`{trait_str}` can be implemented in other crates; if you want to support your users \
+            "`{trait_str}` may be implemented in other crates; if you want to support your users \
              passing their own types here, you can't refer to a specific type",
         ));
     }
+}
 
-    err
+fn has_non_lifetime_generics(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
+    tcx.generics_of(def_id).own_params.iter().any(|param| match param.kind {
+        ty::GenericParamDefKind::Type { .. } | ty::GenericParamDefKind::Const { .. } => true,
+        ty::GenericParamDefKind::Lifetime => false,
+    })
+}
+
+/// Attempt to suggest that a `dyn Trait` argument or return type be converted
+/// to use `impl Trait`.
+fn attempt_dyn_to_impl_suggestion(tcx: TyCtxt<'_>, hir_id: Option<hir::HirId>, err: &mut Diag<'_>) {
+    let Some(hir_id) = hir_id else { return };
+    let hir::Node::Ty(ty) = tcx.hir_node(hir_id) else { return };
+    let hir::TyKind::TraitObject([trait_ref, ..], ..) = ty.kind else { return };
+
+    // Only suggest converting `dyn` to `impl` if we're in a function signature.
+    // This ensures that we don't suggest converting e.g.
+    //   `type Alias = Box<dyn DynIncompatibleTrait>;` to
+    //   `type Alias = Box<impl DynIncompatibleTrait>;`
+    let Some((_id, first_non_type_parent_node)) =
+        tcx.hir().parent_iter(hir_id).find(|(_id, node)| !matches!(node, hir::Node::Ty(_)))
+    else {
+        return;
+    };
+    if first_non_type_parent_node.fn_sig().is_none() {
+        return;
+    }
+
+    err.span_suggestion_verbose(
+        ty.span.until(trait_ref.span),
+        "consider using an opaque type instead",
+        "impl ",
+        Applicability::MaybeIncorrect,
+    );
 }

compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs

@@ -52,7 +52,6 @@ fn dyn_compatibility_violations(
 ) -> &'_ [DynCompatibilityViolation] {
     debug_assert!(tcx.generics_of(trait_def_id).has_self);
     debug!("dyn_compatibility_violations: {:?}", trait_def_id);
-
     tcx.arena.alloc_from_iter(
         tcx.supertrait_def_ids(trait_def_id)
             .flat_map(|def_id| dyn_compatibility_violations_for_trait(tcx, def_id)),

tests/rustdoc-ui/invalid_const_in_lifetime_position.rs

@@ -8,4 +8,4 @@ fn f<'a>(arg : Box<dyn X<Y<1> = &'a ()>>) {}
 //~| ERROR associated type takes 0 generic arguments but 1 generic argument
 //~| ERROR associated type takes 1 lifetime argument but 0 lifetime arguments
 //~| ERROR associated type takes 0 generic arguments but 1 generic argument
-//~| ERROR trait `X` cannot be made into an object
+//~| ERROR trait `X` is not dyn-compatible

tests/rustdoc-ui/invalid_const_in_lifetime_position.stderr

@@ -92,17 +92,17 @@ LL |     type Y<'a>;
    |          ^
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
-error[E0038]: the trait `X` cannot be made into an object
+error[E0038]: the trait `X` is not dyn-compatible
   --> $DIR/invalid_const_in_lifetime_position.rs:4:20
    |
 LL | fn f<'a>(arg : Box<dyn X<Y<1> = &'a ()>>) {}
-   |                    ^^^^^^^^^^^^^^^^^^^^ `X` cannot be made into an object
+   |                    ^^^^^^^^^^^^^^^^^^^^ `X` is not dyn-compatible
    |
 note: for a trait to be "dyn-compatible" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
   --> $DIR/invalid_const_in_lifetime_position.rs:2:10
    |
 LL | trait X {
-   |       - this trait cannot be made into an object...
+   |       - this trait is not dyn-compatible...
 LL |     type Y<'a>;
    |          ^ ...because it contains the generic associated type `Y`
    = help: consider moving `Y` to another trait

tests/rustdoc-ui/issues/issue-105742.rs

@@ -4,8 +4,8 @@ use std::ops::Index;
 pub fn next<'a, T>(s: &'a mut dyn SVec<Item = T, Output = T>) {
     //~^ expected 1 lifetime argument
     //~| expected 1 generic argument
-    //~| the trait `SVec` cannot be made into an object
-    //~| `SVec` cannot be made into an object
+    //~| the trait `SVec` is not dyn-compatible
+    //~| `SVec` is not dyn-compatible
     //~| missing generics for associated type `SVec::Item`
     //~| missing generics for associated type `SVec::Item`
     let _ = s;

tests/rustdoc-ui/issues/issue-105742.stderr

@@ -294,19 +294,19 @@ help: add missing generic argument
 LL |     Output = <Self as SVec>::Item> as SVec>::Item<T>,
    |                                                  +++
 
-error[E0038]: the trait `SVec` cannot be made into an object
+error[E0038]: the trait `SVec` is not dyn-compatible
   --> $DIR/issue-105742.rs:4:31
    |
 LL | pub fn next<'a, T>(s: &'a mut dyn SVec<Item = T, Output = T>) {
-   |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `SVec` cannot be made into an object
+   |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `SVec` is not dyn-compatible
    |
 note: for a trait to be "dyn-compatible" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
   --> $DIR/issue-105742.rs:14:17
    |
 LL |    pub trait SVec: Index<
    |  ____________----__^
    | |            |
-   | |            this trait cannot be made into an object...
+   | |            this trait is not dyn-compatible...
 LL | |      <Self as SVec>::Item,
 LL | |
 LL | |

tests/ui/associated-consts/associated-const-in-trait.rs

@@ -5,9 +5,9 @@ trait Trait {
 }
 
 impl dyn Trait {
-    //~^ ERROR the trait `Trait` cannot be made into an object [E0038]
+    //~^ ERROR the trait `Trait` is not dyn-compatible [E0038]
     const fn n() -> usize { Self::N }
-    //~^ ERROR the trait `Trait` cannot be made into an object [E0038]
+    //~^ ERROR the trait `Trait` is not dyn-compatible [E0038]
 }
 
 fn main() {}