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

@@ -30,7 +30,7 @@ use rustc_trait_selection::traits;
 use rustc_trait_selection::traits::outlives_bounds::InferCtxtExt as _;
 use rustc_type_ir::fold::TypeFoldable;
 use tracing::{debug, instrument};
-use ty::TypingMode;
+use ty::{TypingEnv, TypingMode};
 use {rustc_attr_parsing as attr, rustc_hir as hir};
 
 use super::compare_impl_item::check_type_bounds;
@@ -1035,6 +1035,9 @@ fn check_impl_items_against_trait<'tcx>(
 
     let trait_def = tcx.trait_def(trait_ref.def_id);
 
+    let self_is_sized =
+        trait_ref.self_ty().is_sized(tcx, TypingEnv::non_body_analysis(tcx, impl_id));
+
     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 +1067,15 @@ fn check_impl_items_against_trait<'tcx>(
             }
         }
 
+        if !self_is_sized && 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 +1099,11 @@ 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_sized || !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 +15,12 @@ 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
 }
 
 fn main() {}

tests/ui/traits/trivial_impl_sized.stderr

@@ -9,16 +9,17 @@ LL | |         Self: Sized;
 LL |   impl Foo for i32 {}
    |   ^^^^^^^^^^^^^^^^ missing `foo` in implementation
 
-error[E0046]: not all trait items implemented, missing: `foo`
-  --> $DIR/trivial_impl_sized.rs:19:1
+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()
-LL | |     where
-LL | |         Self: Sized;
-   | |____________________- `foo` from trait
-...
-LL |   impl Foo for dyn std::fmt::Debug {}
-   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `foo` in implementation
+LL |     fn foo() {}
+   |     ^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/trivial_impl_sized.rs:20:8
+   |
+LL | #[deny(dead_code)]
+   |        ^^^^^^^^^
 
 error: aborting due to 2 previous errors