Don't require method impls for methods with Self:Sized bounds for impls for unsized types

Author: oli-obk

compiler/rustc_hir_analysis/messages.ftl

@@ -597,6 +597,8 @@ hir_analysis_unused_generic_parameter_adt_no_phantom_data_help =
 hir_analysis_unused_generic_parameter_ty_alias_help =
     consider removing `{$param_name}` or referring to it in the body of the type alias
 
+hir_analysis_useless_impl_item = this item cannot be used as its where bounds are not satisfied for the `Self` type
+
 hir_analysis_value_of_associated_struct_already_specified =
     the value of the associated type `{$item_name}` in trait `{$def_path}` is already specified
     .label = re-bound here

compiler/rustc_hir_analysis/src/check/check.rs

@@ -1035,6 +1035,12 @@ fn check_impl_items_against_trait<'tcx>(
 
     let trait_def = tcx.trait_def(trait_ref.def_id);
 
+    let self_is_guaranteed_unsized =
+        match tcx.struct_tail_raw(trait_ref.self_ty(), |ty| ty, || {}).kind() {
+            ty::Dynamic(_, _, ty::DynKind::Dyn) | ty::Slice(_) | ty::Str => true,
+            _ => false,
+        };
+
     for &impl_item in impl_item_refs {
         let ty_impl_item = tcx.associated_item(impl_item);
         let ty_trait_item = if let Some(trait_item_id) = ty_impl_item.trait_item_def_id {
@@ -1064,6 +1070,15 @@ fn check_impl_items_against_trait<'tcx>(
             }
         }
 
+        if self_is_guaranteed_unsized && tcx.generics_require_sized_self(ty_trait_item.def_id) {
+            tcx.emit_node_span_lint(
+                rustc_lint_defs::builtin::DEAD_CODE,
+                tcx.local_def_id_to_hir_id(ty_impl_item.def_id.expect_local()),
+                tcx.def_span(ty_impl_item.def_id),
+                errors::UselessImplItem,
+            )
+        }
+
         check_specialization_validity(
             tcx,
             trait_def,
@@ -1087,7 +1102,11 @@ fn check_impl_items_against_trait<'tcx>(
                 .as_ref()
                 .is_some_and(|node_item| node_item.item.defaultness(tcx).has_value());
 
-            if !is_implemented && tcx.defaultness(impl_id).is_final() {
+            if !is_implemented
+                && tcx.defaultness(impl_id).is_final()
+                // unsized types don't need to implement methods that have `Self: Sized` bounds.
+                && !(self_is_guaranteed_unsized && tcx.generics_require_sized_self(trait_item_id))
+            {
                 missing_items.push(tcx.associated_item(trait_item_id));
             }
 

compiler/rustc_hir_analysis/src/errors.rs

@@ -947,6 +947,10 @@ pub(crate) enum ImplNotMarkedDefault {
     },
 }
 
+#[derive(LintDiagnostic)]
+#[diag(hir_analysis_useless_impl_item)]
+pub(crate) struct UselessImplItem;
+
 #[derive(Diagnostic)]
 #[diag(hir_analysis_missing_trait_item, code = E0046)]
 pub(crate) struct MissingTraitItem {

tests/ui/traits/trivial_impl_sized.rs

@@ -1,5 +1,5 @@
-//! This test checks that we currently need to implement
-//! members, even if their where bounds don't hold for the impl type.
+//! This test checks that we do not need to implement
+//! members, whose `where Self: Sized` bounds don't hold for the impl type.
 
 trait Foo {
     fn foo()
@@ -15,12 +15,28 @@ impl Foo for () {
 impl Foo for i32 {}
 //~^ ERROR: not all trait items implemented, missing: `foo`
 
-// Should be allowed
 impl Foo for dyn std::fmt::Debug {}
-//~^ ERROR: not all trait items implemented, missing: `foo`
 
+#[deny(dead_code)]
 impl Foo for dyn std::fmt::Display {
     fn foo() {}
+    //~^ ERROR this item cannot be used as its where bounds are not satisfied
+}
+
+struct Struct {
+    i: i32,
+    tail: [u8],
 }
 
+impl Foo for Struct {}
+
+// Ensure we only allow known-unsized types to be skipped
+trait Trait {
+    fn foo(self)
+    where
+        Self: Sized;
+}
+impl<T: ?Sized> Trait for T {}
+//~^ ERROR: not all trait items implemented, missing: `foo`
+
 fn main() {}

tests/ui/traits/trivial_impl_sized.stderr

@@ -9,17 +9,29 @@ LL | |         Self: Sized;
 LL |   impl Foo for i32 {}
    |   ^^^^^^^^^^^^^^^^ missing `foo` in implementation
 
+error: this item cannot be used as its where bounds are not satisfied for the `Self` type
+  --> $DIR/trivial_impl_sized.rs:22:5
+   |
+LL |     fn foo() {}
+   |     ^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/trivial_impl_sized.rs:20:8
+   |
+LL | #[deny(dead_code)]
+   |        ^^^^^^^^^
+
 error[E0046]: not all trait items implemented, missing: `foo`
-  --> $DIR/trivial_impl_sized.rs:19:1
+  --> $DIR/trivial_impl_sized.rs:39:1
    |
-LL | /     fn foo()
+LL | /     fn foo(self)
 LL | |     where
 LL | |         Self: Sized;
    | |____________________- `foo` from trait
-...
-LL |   impl Foo for dyn std::fmt::Debug {}
-   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `foo` in implementation
+LL |   }
+LL |   impl<T: ?Sized> Trait for T {}
+   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `foo` in implementation
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0046`.