Lint now-unnecessary associated type bounds

compiler/rustc_hir_analysis/messages.ftl

@@ -288,6 +288,11 @@ hir_analysis_unrecognized_intrinsic_function =
     unrecognized intrinsic function: `{$name}`
     .label = unrecognized intrinsic
 
+hir_analysis_unused_associated_type_bounds =
+    unnecessary associated type bound for not object safe associated type
+    .note = this associated type has a `where Self: Sized` bound. Thus, while the associated type can be specified, it cannot be used in any way, because trait objects are not `Sized`.
+    .suggestion = remove this bound
+
 hir_analysis_value_of_associated_struct_already_specified =
     the value of the associated type `{$item_name}` (from trait `{$def_path}`) is already specified
     .label = re-bound here

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -29,6 +29,7 @@ use rustc_hir::intravisit::{walk_generics, Visitor as _};
 use rustc_hir::{GenericArg, GenericArgs, OpaqueTyOrigin};
 use rustc_infer::infer::{InferCtxt, InferOk, TyCtxtInferExt};
 use rustc_infer::traits::ObligationCause;
+use rustc_lint_defs::builtin::UNUSED_ASSOCIATED_TYPE_BOUNDS;
 use rustc_middle::middle::stability::AllowUnstable;
 use rustc_middle::ty::subst::{self, GenericArgKind, InternalSubsts, SubstsRef};
 use rustc_middle::ty::DynKind;
@@ -929,6 +930,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
     fn conv_object_ty_poly_trait_ref(
         &self,
         span: Span,
+        hir_id: hir::HirId,
         hir_trait_bounds: &[hir::PolyTraitRef<'_>],
         lifetime: &hir::Lifetime,
         borrowed: bool,
@@ -1125,9 +1127,18 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         // So every `Projection` clause is an `Assoc = Foo` bound. `associated_types` contains all associated
         // types's `DefId`, so the following loop removes all the `DefIds` of the associated types that have a
         // corresponding `Projection` clause
-        for (projection_bound, _) in &projection_bounds {
+        for (projection_bound, span) in &projection_bounds {
             for def_ids in associated_types.values_mut() {
-                def_ids.remove(&projection_bound.projection_def_id());
+                let def_id = projection_bound.projection_def_id();
+                def_ids.remove(&def_id);
+                if tcx.generics_require_sized_self(def_id) {
+                    tcx.emit_spanned_lint(
+                        UNUSED_ASSOCIATED_TYPE_BOUNDS,
+                        hir_id,
+                        *span,
+                        crate::errors::UnusedAssociatedTypeBounds { span: *span },
+                    );
+                }
             }
         }
 
@@ -2812,7 +2823,14 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     TraitObjectSyntax::DynStar => ty::DynStar,
                 };
 
-                self.conv_object_ty_poly_trait_ref(ast_ty.span, bounds, lifetime, borrowed, repr)
+                self.conv_object_ty_poly_trait_ref(
+                    ast_ty.span,
+                    ast_ty.hir_id,
+                    bounds,
+                    lifetime,
+                    borrowed,
+                    repr,
+                )
             }
             hir::TyKind::Path(hir::QPath::Resolved(maybe_qself, path)) => {
                 debug!(?maybe_qself, ?path);

compiler/rustc_hir_analysis/src/errors.rs

@@ -5,7 +5,7 @@ use rustc_errors::{
     error_code, Applicability, DiagnosticBuilder, ErrorGuaranteed, Handler, IntoDiagnostic,
     MultiSpan,
 };
-use rustc_macros::{Diagnostic, Subdiagnostic};
+use rustc_macros::{Diagnostic, LintDiagnostic, Subdiagnostic};
 use rustc_middle::ty::{self, print::TraitRefPrintOnlyTraitPath, Ty};
 use rustc_span::{symbol::Ident, Span, Symbol};
 
@@ -900,3 +900,11 @@ pub(crate) enum LateBoundInApit {
         param_span: Span,
     },
 }
+
+#[derive(LintDiagnostic)]
+#[diag(hir_analysis_unused_associated_type_bounds)]
+#[note]
+pub struct UnusedAssociatedTypeBounds {
+    #[suggestion(code = "")]
+    pub span: Span,
+}

compiler/rustc_lint_defs/src/builtin.rs

@@ -3468,6 +3468,32 @@ declare_lint! {
     report_in_external_macro
 }
 
+declare_lint! {
+    /// The `unused_associated_type_bounds` lint is emitted when an
+    /// associated type bound is added to a trait object, but the associated
+    /// type has a `where Self: Sized` bound, and is thus unavailable on the
+    /// trait object anyway.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// trait Foo {
+    ///     type Bar where Self: Sized;
+    /// }
+    /// type Mop = dyn Foo<Bar = ()>;
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Just like methods with `Self: Sized` bounds are unavailable on trait
+    /// objects, associated types can be removed from the trait object.
+    pub UNUSED_ASSOCIATED_TYPE_BOUNDS,
+    Warn,
+    "detects unused `Foo = Bar` bounds in `dyn Trait<Foo = Bar>`"
+}
+
 declare_lint! {
     /// The `unused_doc_comments` lint detects doc comments that aren't used
     /// by `rustdoc`.

tests/ui/object-safety/assoc_type_bounds_sized_unnecessary.rs

@@ -0,0 +1,14 @@
+// check-pass
+
+trait Foo {
+    type Bar
+    where
+        Self: Sized;
+}
+
+fn foo(_: &dyn Foo<Bar = ()>) {}
+//~^ WARN: unnecessary associated type bound for not object safe associated type
+//~| WARN: unnecessary associated type bound for not object safe associated type
+//~| WARN: unnecessary associated type bound for not object safe associated type
+
+fn main() {}

tests/ui/object-safety/assoc_type_bounds_sized_unnecessary.stderr

@@ -0,0 +1,27 @@
+warning: unnecessary associated type bound for not object safe associated type
+  --> $DIR/assoc_type_bounds_sized_unnecessary.rs:9:20
+   |
+LL | fn foo(_: &dyn Foo<Bar = ()>) {}
+   |                    ^^^^^^^^ help: remove this bound
+   |
+   = note: this associated type has a `where Self: Sized` bound. Thus, while the associated type can be specified, it cannot be used in any way, because trait objects are not `Sized`.
+   = note: `#[warn(unused_associated_type_bounds)]` on by default
+
+warning: unnecessary associated type bound for not object safe associated type
+  --> $DIR/assoc_type_bounds_sized_unnecessary.rs:9:20
+   |
+LL | fn foo(_: &dyn Foo<Bar = ()>) {}
+   |                    ^^^^^^^^ help: remove this bound
+   |
+   = note: this associated type has a `where Self: Sized` bound. Thus, while the associated type can be specified, it cannot be used in any way, because trait objects are not `Sized`.
+
+warning: unnecessary associated type bound for not object safe associated type
+  --> $DIR/assoc_type_bounds_sized_unnecessary.rs:9:20
+   |
+LL | fn foo(_: &dyn Foo<Bar = ()>) {}
+   |                    ^^^^^^^^ help: remove this bound
+   |
+   = note: this associated type has a `where Self: Sized` bound. Thus, while the associated type can be specified, it cannot be used in any way, because trait objects are not `Sized`.
+
+warning: 3 warnings emitted
+