non_local_defs: suggest removing leading ref/ptr to make the impl local

compiler/rustc_lint/messages.ftl

@@ -543,11 +543,12 @@ lint_non_local_definitions_cargo_update = the {$macro_kind} `{$macro_name}` may
 lint_non_local_definitions_deprecation = this lint may become deny-by-default in the edition 2024 and higher, see the tracking issue <https://github.com/rust-lang/rust/issues/120363>
 
 lint_non_local_definitions_impl = non-local `impl` definition, `impl` blocks should be written at the same level as their item
-    .help =
+    .move_help =
         move this `impl` block outside of the current {$body_kind_descr} {$depth ->
             [one] `{$body_name}`
            *[other] `{$body_name}` and up {$depth} bodies
         }
+    .remove_help = remove `{$may_remove_part}` to make the `impl` local
     .without_trait = methods and associated constants are still usable outside the current expression, only `impl Local` and `impl dyn Local` can ever be private, and only if the type is nested in the same item as the `impl`
     .with_trait = an `impl` is never scoped, even when it is nested inside an item, as it may impact type checking outside of that item, which can be the case if neither the trait or the self type are at the same nesting level as the `impl`
     .bounds = `impl` may be usable in bounds, etc. from outside the expression, which might e.g. make something constructible that previously wasn't, because it's still on a publicly-visible type

compiler/rustc_lint/src/lints.rs

@@ -1338,6 +1338,7 @@ pub enum NonLocalDefinitionsDiag {
         const_anon: Option<Option<Span>>,
         move_help: Span,
         may_move: Vec<Span>,
+        may_remove: Option<(Span, String)>,
         has_trait: bool,
     },
     MacroRules {
@@ -1361,6 +1362,7 @@ impl<'a> LintDiagnostic<'a, ()> for NonLocalDefinitionsDiag {
                 const_anon,
                 move_help,
                 may_move,
+                may_remove,
                 has_trait,
             } => {
                 diag.primary_message(fluent::lint_non_local_definitions_impl);
@@ -1379,7 +1381,17 @@ impl<'a> LintDiagnostic<'a, ()> for NonLocalDefinitionsDiag {
                 for sp in may_move {
                     ms.push_span_label(sp, fluent::lint_non_local_definitions_may_move);
                 }
-                diag.span_help(ms, fluent::lint_help);
+                diag.span_help(ms, fluent::lint_move_help);
+
+                if let Some((span, part)) = may_remove {
+                    diag.arg("may_remove_part", part);
+                    diag.span_suggestion(
+                        span,
+                        fluent::lint_remove_help,
+                        "",
+                        Applicability::MaybeIncorrect,
+                    );
+                }
 
                 if let Some(cargo_update) = cargo_update {
                     diag.subdiagnostic(&diag.dcx, cargo_update);

compiler/rustc_lint/src/non_local_def.rs

@@ -136,35 +136,8 @@ impl<'tcx> LateLintPass<'tcx> for NonLocalDefinitions {
                 };
 
                 // Part 1: Is the Self type local?
-                let self_ty_has_local_parent = match impl_.self_ty.kind {
-                    TyKind::Path(QPath::Resolved(_, ty_path)) => {
-                        path_has_local_parent(ty_path, cx, parent, parent_parent)
-                    }
-                    TyKind::TraitObject([principle_poly_trait_ref, ..], _, _) => {
-                        path_has_local_parent(
-                            principle_poly_trait_ref.trait_ref.path,
-                            cx,
-                            parent,
-                            parent_parent,
-                        )
-                    }
-                    TyKind::TraitObject([], _, _)
-                    | TyKind::InferDelegation(_, _)
-                    | TyKind::Slice(_)
-                    | TyKind::Array(_, _)
-                    | TyKind::Ptr(_)
-                    | TyKind::Ref(_, _)
-                    | TyKind::BareFn(_)
-                    | TyKind::Never
-                    | TyKind::Tup(_)
-                    | TyKind::Path(_)
-                    | TyKind::Pat(..)
-                    | TyKind::AnonAdt(_)
-                    | TyKind::OpaqueDef(_, _, _)
-                    | TyKind::Typeof(_)
-                    | TyKind::Infer
-                    | TyKind::Err(_) => false,
-                };
+                let self_ty_has_local_parent =
+                    ty_has_local_parent(&impl_.self_ty.kind, cx, parent, parent_parent);
 
                 if self_ty_has_local_parent {
                     return;
@@ -242,6 +215,18 @@ impl<'tcx> LateLintPass<'tcx> for NonLocalDefinitions {
                 let const_anon = matches!(parent_def_kind, DefKind::Const | DefKind::Static { .. })
                     .then_some(span_for_const_anon_suggestion);
 
+                let may_remove = match &impl_.self_ty.kind {
+                    TyKind::Ptr(mut_ty) | TyKind::Ref(_, mut_ty)
+                        if ty_has_local_parent(&mut_ty.ty.kind, cx, parent, parent_parent) =>
+                    {
+                        let type_ =
+                            if matches!(impl_.self_ty.kind, TyKind::Ptr(_)) { "*" } else { "&" };
+                        let part = format!("{}{}", type_, mut_ty.mutbl.prefix_str());
+                        Some((impl_.self_ty.span.shrink_to_lo().until(mut_ty.ty.span), part))
+                    }
+                    _ => None,
+                };
+
                 cx.emit_span_lint(
                     NON_LOCAL_DEFINITIONS,
                     item.span.shrink_to_lo().to(impl_.self_ty.span),
@@ -255,6 +240,7 @@ impl<'tcx> LateLintPass<'tcx> for NonLocalDefinitions {
                         cargo_update: cargo_update(),
                         const_anon,
                         may_move,
+                        may_remove,
                         has_trait: impl_.of_trait.is_some(),
                     },
                 )
@@ -384,6 +370,42 @@ impl<'tcx> Visitor<'tcx> for PathCollector<'tcx> {
     }
 }
 
+/// Given a `Ty` we check if the (outermost) type is local.
+fn ty_has_local_parent(
+    ty_kind: &TyKind<'_>,
+    cx: &LateContext<'_>,
+    impl_parent: DefId,
+    impl_parent_parent: Option<DefId>,
+) -> bool {
+    match ty_kind {
+        TyKind::Path(QPath::Resolved(_, ty_path)) => {
+            path_has_local_parent(ty_path, cx, impl_parent, impl_parent_parent)
+        }
+        TyKind::TraitObject([principle_poly_trait_ref, ..], _, _) => path_has_local_parent(
+            principle_poly_trait_ref.trait_ref.path,
+            cx,
+            impl_parent,
+            impl_parent_parent,
+        ),
+        TyKind::TraitObject([], _, _)
+        | TyKind::InferDelegation(_, _)
+        | TyKind::Slice(_)
+        | TyKind::Array(_, _)
+        | TyKind::Ptr(_)
+        | TyKind::Ref(_, _)
+        | TyKind::BareFn(_)
+        | TyKind::Never
+        | TyKind::Tup(_)
+        | TyKind::Path(_)
+        | TyKind::Pat(..)
+        | TyKind::AnonAdt(_)
+        | TyKind::OpaqueDef(_, _, _)
+        | TyKind::Typeof(_)
+        | TyKind::Infer
+        | TyKind::Err(_) => false,
+    }
+}
+
 /// Given a path and a parent impl def id, this checks if the if parent resolution
 /// def id correspond to the def id of the parent impl definition.
 ///

tests/ui/lint/non-local-defs/exhaustive.stderr

@@ -189,7 +189,9 @@ warning: non-local `impl` definition, `impl` blocks should be written at the sam
   --> $DIR/exhaustive.rs:58:5
    |
 LL |     impl Trait for *mut InsideMain {}
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^-----^^^^^^^^^^
+   |                    |
+   |                    help: remove `*mut ` to make the `impl` local
    |
    = note: `impl` may be usable in bounds, etc. from outside the expression, which might e.g. make something constructible that previously wasn't, because it's still on a publicly-visible type
    = note: an `impl` is never scoped, even when it is nested inside an item, as it may impact type checking outside of that item, which can be the case if neither the trait or the self type are at the same nesting level as the `impl`

tests/ui/lint/non-local-defs/from-local-for-global.stderr

@@ -46,7 +46,9 @@ warning: non-local `impl` definition, `impl` blocks should be written at the sam
   --> $DIR/from-local-for-global.rs:32:5
    |
 LL |     impl StillNonLocal for &Foo {}
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^-^^^
+   |                            |
+   |                            help: remove `&` to make the `impl` local
    |
    = note: `impl` may be usable in bounds, etc. from outside the expression, which might e.g. make something constructible that previously wasn't, because it's still on a publicly-visible type
    = note: an `impl` is never scoped, even when it is nested inside an item, as it may impact type checking outside of that item, which can be the case if neither the trait or the self type are at the same nesting level as the `impl`

tests/ui/lint/non-local-defs/trait-solver-overflow-123573.stderr

@@ -2,7 +2,9 @@ warning: non-local `impl` definition, `impl` blocks should be written at the sam
   --> $DIR/trait-solver-overflow-123573.rs:12:5
    |
 LL |     impl Test for &Local {}
-   |     ^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^-^^^^^
+   |                   |
+   |                   help: remove `&` to make the `impl` local
    |
    = note: `impl` may be usable in bounds, etc. from outside the expression, which might e.g. make something constructible that previously wasn't, because it's still on a publicly-visible type
    = note: an `impl` is never scoped, even when it is nested inside an item, as it may impact type checking outside of that item, which can be the case if neither the trait or the self type are at the same nesting level as the `impl`