Rollup merge of #151783 - mu001999-contrib:impl/final-method, r=fee1-dead

Implement RFC 3678: Final trait methods

Tracking: https://github.com/rust-lang/rust/issues/131179

This PR is based on rust-lang/rust#130802, with some minor changes and conflict resolution.

Futhermore, this PR excludes final methods from the vtable of a dyn Trait.

And some excerpt from the original PR description:
> Implements the surface part of https://github.com/rust-lang/rfcs/pull/3678.
>
> I'm using the word "method" in the title, but in the diagnostics and the feature gate I used "associated function", since that's more accurate.

cc @joshtriplett

compiler/rustc_ast/src/ast.rs

@@ -3131,8 +3131,16 @@ pub enum Const {
 /// For details see the [RFC #2532](https://github.com/rust-lang/rfcs/pull/2532).
 #[derive(Copy, Clone, PartialEq, Encodable, Decodable, Debug, HashStable_Generic, Walkable)]
 pub enum Defaultness {
+    /// Item is unmarked. Implicitly determined based off of position.
+    /// For impls, this is `final`; for traits, this is `default`.
+    ///
+    /// If you're expanding an item in a built-in macro or parsing an item
+    /// by hand, you probably want to use this.
+    Implicit,
+    /// `default`
     Default(Span),
-    Final,
+    /// `final`; per RFC 3678, only trait items may be *explicitly* marked final.
+    Final(Span),
 }
 
 #[derive(Copy, Clone, PartialEq, Encodable, Decodable, HashStable_Generic, Walkable)]
@@ -4140,7 +4148,7 @@ impl AssocItemKind {
             | Self::Fn(box Fn { defaultness, .. })
             | Self::Type(box TyAlias { defaultness, .. }) => defaultness,
             Self::MacCall(..) | Self::Delegation(..) | Self::DelegationMac(..) => {
-                Defaultness::Final
+                Defaultness::Implicit
             }
         }
     }

compiler/rustc_ast_lowering/src/item.rs

@@ -939,7 +939,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         );
         let trait_item_def_id = hir_id.expect_owner();
 
-        let (ident, generics, kind, has_default) = match &i.kind {
+        let (ident, generics, kind, has_value) = match &i.kind {
             AssocItemKind::Const(box ConstItem {
                 ident,
                 generics,
@@ -1088,13 +1088,17 @@ impl<'hir> LoweringContext<'_, 'hir> {
             }
         };
 
+        let (defaultness, _) = self.lower_defaultness(i.kind.defaultness(), has_value, || {
+            hir::Defaultness::Default { has_value }
+        });
+
         let item = hir::TraitItem {
             owner_id: trait_item_def_id,
             ident: self.lower_ident(ident),
             generics,
             kind,
             span: self.lower_span(i.span),
-            defaultness: hir::Defaultness::Default { has_value: has_default },
+            defaultness,
             has_delayed_lints: !self.delayed_lints.is_empty(),
         };
         self.arena.alloc(item)
@@ -1122,7 +1126,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
         // `defaultness.has_value()` is never called for an `impl`, always `true` in order
         // to not cause an assertion failure inside the `lower_defaultness` function.
         let has_val = true;
-        let (defaultness, defaultness_span) = self.lower_defaultness(defaultness, has_val);
+        let (defaultness, defaultness_span) =
+            self.lower_defaultness(defaultness, has_val, || hir::Defaultness::Final);
         let modifiers = TraitBoundModifiers {
             constness: BoundConstness::Never,
             asyncness: BoundAsyncness::Normal,
@@ -1151,7 +1156,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
     ) -> &'hir hir::ImplItem<'hir> {
         // Since `default impl` is not yet implemented, this is always true in impls.
         let has_value = true;
-        let (defaultness, _) = self.lower_defaultness(i.kind.defaultness(), has_value);
+        let (defaultness, _) =
+            self.lower_defaultness(i.kind.defaultness(), has_value, || hir::Defaultness::Final);
         let hir_id = hir::HirId::make_owner(self.current_hir_id_owner.def_id);
         let attrs = self.lower_attrs(
             hir_id,
@@ -1304,15 +1310,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
         &self,
         d: Defaultness,
         has_value: bool,
+        implicit: impl FnOnce() -> hir::Defaultness,
     ) -> (hir::Defaultness, Option<Span>) {
         match d {
+            Defaultness::Implicit => (implicit(), None),
             Defaultness::Default(sp) => {
                 (hir::Defaultness::Default { has_value }, Some(self.lower_span(sp)))
             }
-            Defaultness::Final => {
-                assert!(has_value);
-                (hir::Defaultness::Final, None)
-            }
+            Defaultness::Final(sp) => (hir::Defaultness::Final, Some(self.lower_span(sp))),
         }
     }
 

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -65,6 +65,28 @@ impl TraitOrImpl {
     }
 }
 
+enum AllowDefault {
+    Yes,
+    No,
+}
+
+impl AllowDefault {
+    fn when(b: bool) -> Self {
+        if b { Self::Yes } else { Self::No }
+    }
+}
+
+enum AllowFinal {
+    Yes,
+    No,
+}
+
+impl AllowFinal {
+    fn when(b: bool) -> Self {
+        if b { Self::Yes } else { Self::No }
+    }
+}
+
 struct AstValidator<'a> {
     sess: &'a Session,
     features: &'a Features,
@@ -563,10 +585,32 @@ impl<'a> AstValidator<'a> {
         }
     }
 
-    fn check_defaultness(&self, span: Span, defaultness: Defaultness) {
-        if let Defaultness::Default(def_span) = defaultness {
-            let span = self.sess.source_map().guess_head_span(span);
-            self.dcx().emit_err(errors::ForbiddenDefault { span, def_span });
+    fn check_defaultness(
+        &self,
+        span: Span,
+        defaultness: Defaultness,
+        allow_default: AllowDefault,
+        allow_final: AllowFinal,
+    ) {
+        match defaultness {
+            Defaultness::Default(def_span) if matches!(allow_default, AllowDefault::No) => {
+                let span = self.sess.source_map().guess_head_span(span);
+                self.dcx().emit_err(errors::ForbiddenDefault { span, def_span });
+            }
+            Defaultness::Final(def_span) if matches!(allow_final, AllowFinal::No) => {
+                let span = self.sess.source_map().guess_head_span(span);
+                self.dcx().emit_err(errors::ForbiddenFinal { span, def_span });
+            }
+            _ => (),
+        }
+    }
+
+    fn check_final_has_body(&self, item: &Item<AssocItemKind>, defaultness: Defaultness) {
+        if let AssocItemKind::Fn(box Fn { body: None, .. }) = &item.kind
+            && let Defaultness::Final(def_span) = defaultness
+        {
+            let span = self.sess.source_map().guess_head_span(item.span);
+            self.dcx().emit_err(errors::ForbiddenFinalWithoutBody { span, def_span });
         }
     }
 
@@ -1190,7 +1234,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 },
             ) => {
                 self.visit_attrs_vis_ident(&item.attrs, &item.vis, ident);
-                self.check_defaultness(item.span, *defaultness);
+                self.check_defaultness(item.span, *defaultness, AllowDefault::No, AllowFinal::No);
 
                 for EiiImpl { eii_macro_path, .. } in eii_impls {
                     self.visit_path(eii_macro_path);
@@ -1360,7 +1404,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 });
             }
             ItemKind::Const(box ConstItem { defaultness, ident, rhs_kind, .. }) => {
-                self.check_defaultness(item.span, *defaultness);
+                self.check_defaultness(item.span, *defaultness, AllowDefault::No, AllowFinal::No);
                 if !rhs_kind.has_expr() {
                     self.dcx().emit_err(errors::ConstWithoutBody {
                         span: item.span,
@@ -1398,7 +1442,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             ItemKind::TyAlias(
                 ty_alias @ box TyAlias { defaultness, bounds, after_where_clause, ty, .. },
             ) => {
-                self.check_defaultness(item.span, *defaultness);
+                self.check_defaultness(item.span, *defaultness, AllowDefault::No, AllowFinal::No);
                 if ty.is_none() {
                     self.dcx().emit_err(errors::TyAliasWithoutBody {
                         span: item.span,
@@ -1428,7 +1472,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
     fn visit_foreign_item(&mut self, fi: &'a ForeignItem) {
         match &fi.kind {
             ForeignItemKind::Fn(box Fn { defaultness, ident, sig, body, .. }) => {
-                self.check_defaultness(fi.span, *defaultness);
+                self.check_defaultness(fi.span, *defaultness, AllowDefault::No, AllowFinal::No);
                 self.check_foreign_fn_bodyless(*ident, body.as_deref());
                 self.check_foreign_fn_headerless(sig.header);
                 self.check_foreign_item_ascii_only(*ident);
@@ -1448,7 +1492,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 ty,
                 ..
             }) => {
-                self.check_defaultness(fi.span, *defaultness);
+                self.check_defaultness(fi.span, *defaultness, AllowDefault::No, AllowFinal::No);
                 self.check_foreign_kind_bodyless(*ident, "type", ty.as_ref().map(|b| b.span));
                 self.check_type_no_bounds(bounds, "`extern` blocks");
                 self.check_foreign_ty_genericless(generics, after_where_clause);
@@ -1707,9 +1751,19 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             self.check_nomangle_item_asciionly(ident, item.span);
         }
 
-        if ctxt == AssocCtxt::Trait || self.outer_trait_or_trait_impl.is_none() {
-            self.check_defaultness(item.span, item.kind.defaultness());
-        }
+        let defaultness = item.kind.defaultness();
+        self.check_defaultness(
+            item.span,
+            defaultness,
+            // `default` is allowed on all associated items in impls.
+            AllowDefault::when(matches!(ctxt, AssocCtxt::Impl { .. })),
+            // `final` is allowed on all associated *functions* in traits.
+            AllowFinal::when(
+                ctxt == AssocCtxt::Trait && matches!(item.kind, AssocItemKind::Fn(..)),
+            ),
+        );
+
+        self.check_final_has_body(item, defaultness);
 
         if let AssocCtxt::Impl { .. } = ctxt {
             match &item.kind {

compiler/rustc_ast_passes/src/errors.rs

@@ -159,6 +159,24 @@ pub(crate) struct ForbiddenDefault {
     pub def_span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag("`final` is only allowed on associated functions in traits")]
+pub(crate) struct ForbiddenFinal {
+    #[primary_span]
+    pub span: Span,
+    #[label("`final` because of this")]
+    pub def_span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag("`final` is only allowed on associated functions if they have a body")]
+pub(crate) struct ForbiddenFinalWithoutBody {
+    #[primary_span]
+    pub span: Span,
+    #[label("`final` because of this")]
+    pub def_span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag("associated constant in `impl` without body")]
 pub(crate) struct AssocConstWithoutBody {

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -580,6 +580,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all!(frontmatter, "frontmatters are experimental");
     gate_all!(coroutines, "coroutine syntax is experimental");
     gate_all!(const_block_items, "const block items are experimental");
+    gate_all!(final_associated_functions, "`final` on trait functions is experimental");
 
     if !visitor.features.never_patterns() {
         if let Some(spans) = spans.get(&sym::never_patterns) {

compiler/rustc_ast_pretty/src/pprust/state/item.rs

@@ -51,7 +51,7 @@ impl<'a> State<'a> {
                 expr.as_deref(),
                 vis,
                 *safety,
-                ast::Defaultness::Final,
+                ast::Defaultness::Implicit,
                 define_opaque.as_deref(),
             ),
             ast::ForeignItemKind::TyAlias(box ast::TyAlias {
@@ -201,7 +201,7 @@ impl<'a> State<'a> {
                     body.as_deref(),
                     &item.vis,
                     ast::Safety::Default,
-                    ast::Defaultness::Final,
+                    ast::Defaultness::Implicit,
                     define_opaque.as_deref(),
                 );
             }

compiler/rustc_builtin_macros/src/alloc_error_handler.rs

@@ -83,7 +83,7 @@ fn generate_handler(cx: &ExtCtxt<'_>, handler: Ident, span: Span, sig_span: Span
 
     let body = Some(cx.block_expr(call));
     let kind = ItemKind::Fn(Box::new(Fn {
-        defaultness: ast::Defaultness::Final,
+        defaultness: ast::Defaultness::Implicit,
         sig,
         ident: Ident::from_str_and_span(&global_fn_name(ALLOC_ERROR_HANDLER), span),
         generics: Generics::default(),

compiler/rustc_builtin_macros/src/autodiff.rs

@@ -334,7 +334,7 @@ mod llvm_enzyme {
 
         // The first element of it is the name of the function to be generated
         let d_fn = Box::new(ast::Fn {
-            defaultness: ast::Defaultness::Final,
+            defaultness: ast::Defaultness::Implicit,
             sig: d_sig,
             ident: first_ident(&meta_item_vec[0]),
             generics,

compiler/rustc_builtin_macros/src/deriving/coerce_pointee.rs

@@ -136,7 +136,7 @@ pub(crate) fn expand_deriving_coerce_pointee(
                     of_trait: Some(Box::new(ast::TraitImplHeader {
                         safety: ast::Safety::Default,
                         polarity: ast::ImplPolarity::Positive,
-                        defaultness: ast::Defaultness::Final,
+                        defaultness: ast::Defaultness::Implicit,
                         trait_ref,
                     })),
                     constness: ast::Const::No,
@@ -159,7 +159,7 @@ pub(crate) fn expand_deriving_coerce_pointee(
                 of_trait: Some(Box::new(ast::TraitImplHeader {
                     safety: ast::Safety::Default,
                     polarity: ast::ImplPolarity::Positive,
-                    defaultness: ast::Defaultness::Final,
+                    defaultness: ast::Defaultness::Implicit,
                     trait_ref,
                 })),
                 constness: ast::Const::No,

compiler/rustc_builtin_macros/src/deriving/generic/mod.rs

@@ -614,7 +614,7 @@ impl<'a> TraitDef<'a> {
                 },
                 attrs: ast::AttrVec::new(),
                 kind: ast::AssocItemKind::Type(Box::new(ast::TyAlias {
-                    defaultness: ast::Defaultness::Final,
+                    defaultness: ast::Defaultness::Implicit,
                     ident,
                     generics: Generics::default(),
                     after_where_clause: ast::WhereClause::default(),
@@ -851,7 +851,7 @@ impl<'a> TraitDef<'a> {
                 of_trait: Some(Box::new(ast::TraitImplHeader {
                     safety: self.safety,
                     polarity: ast::ImplPolarity::Positive,
-                    defaultness: ast::Defaultness::Final,
+                    defaultness: ast::Defaultness::Implicit,
                     trait_ref,
                 })),
                 constness: if self.is_const { ast::Const::Yes(DUMMY_SP) } else { ast::Const::No },
@@ -1073,7 +1073,7 @@ impl<'a> MethodDef<'a> {
         let trait_lo_sp = span.shrink_to_lo();
 
         let sig = ast::FnSig { header: ast::FnHeader::default(), decl: fn_decl, span };
-        let defaultness = ast::Defaultness::Final;
+        let defaultness = ast::Defaultness::Implicit;
 
         // Create the method.
         Box::new(ast::AssocItem {

compiler/rustc_builtin_macros/src/global_allocator.rs

@@ -77,7 +77,7 @@ impl AllocFnFactory<'_, '_> {
         let sig = FnSig { decl, header, span: self.span };
         let body = Some(self.cx.block_expr(result));
         let kind = ItemKind::Fn(Box::new(Fn {
-            defaultness: ast::Defaultness::Final,
+            defaultness: ast::Defaultness::Implicit,
             sig,
             ident: Ident::from_str_and_span(&global_fn_name(method.name), self.span),
             generics: Generics::default(),

compiler/rustc_builtin_macros/src/test.rs

@@ -283,7 +283,7 @@ pub(crate) fn expand_test_or_bench(
             // const $ident: test::TestDescAndFn =
             ast::ItemKind::Const(
                 ast::ConstItem {
-                    defaultness: ast::Defaultness::Final,
+                    defaultness: ast::Defaultness::Implicit,
                     ident: Ident::new(fn_.ident.name, sp),
                     generics: ast::Generics::default(),
                     ty: cx.ty(sp, ast::TyKind::Path(None, test_path("TestDescAndFn"))),

compiler/rustc_builtin_macros/src/test_harness.rs

@@ -330,7 +330,7 @@ fn mk_main(cx: &mut TestCtxt<'_>) -> Box<ast::Item> {
 
     let decl = ecx.fn_decl(ThinVec::new(), ast::FnRetTy::Ty(main_ret_ty));
     let sig = ast::FnSig { decl, header: ast::FnHeader::default(), span: sp };
-    let defaultness = ast::Defaultness::Final;
+    let defaultness = ast::Defaultness::Implicit;
 
     // Honor the reexport_test_harness_main attribute
     let main_ident = match cx.reexport_test_harness_main {

compiler/rustc_expand/src/build.rs

@@ -729,7 +729,7 @@ impl<'a> ExtCtxt<'a> {
         ty: Box<ast::Ty>,
         rhs_kind: ast::ConstItemRhsKind,
     ) -> Box<ast::Item> {
-        let defaultness = ast::Defaultness::Final;
+        let defaultness = ast::Defaultness::Implicit;
         self.item(
             span,
             AttrVec::new(),

compiler/rustc_feature/src/unstable.rs

@@ -494,6 +494,8 @@ declare_features! (
     (unstable, ffi_const, "1.45.0", Some(58328)),
     /// Allows the use of `#[ffi_pure]` on foreign functions.
     (unstable, ffi_pure, "1.45.0", Some(58329)),
+    /// Allows marking trait functions as `final` to prevent overriding impls
+    (unstable, final_associated_functions, "CURRENT_RUSTC_VERSION", Some(1)),
     /// Controlling the behavior of fmt::Debug
     (unstable, fmt_debug, "1.82.0", Some(129709)),
     /// Allows using `#[align(...)]` on function items

compiler/rustc_hir_analysis/src/check/check.rs

@@ -1175,13 +1175,35 @@ pub(super) fn check_specialization_validity<'tcx>(
 
     if let Err(parent_impl) = result {
         if !tcx.is_impl_trait_in_trait(impl_item) {
-            report_forbidden_specialization(tcx, impl_item, parent_impl);
+            let span = tcx.def_span(impl_item);
+            let ident = tcx.item_ident(impl_item);
+
+            let err = match tcx.span_of_impl(parent_impl) {
+                Ok(sp) => errors::ImplNotMarkedDefault::Ok { span, ident, ok_label: sp },
+                Err(cname) => errors::ImplNotMarkedDefault::Err { span, ident, cname },
+            };
+
+            tcx.dcx().emit_err(err);
         } else {
             tcx.dcx().delayed_bug(format!("parent item: {parent_impl:?} not marked as default"));
         }
     }
 }
 
+fn check_overriding_final_trait_item<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    trait_item: ty::AssocItem,
+    impl_item: ty::AssocItem,
+) {
+    if trait_item.defaultness(tcx).is_final() {
+        tcx.dcx().emit_err(errors::OverridingFinalTraitFunction {
+            impl_span: tcx.def_span(impl_item.def_id),
+            trait_span: tcx.def_span(trait_item.def_id),
+            ident: tcx.item_ident(impl_item.def_id),
+        });
+    }
+}
+
 fn check_impl_items_against_trait<'tcx>(
     tcx: TyCtxt<'tcx>,
     impl_id: LocalDefId,
@@ -1259,6 +1281,8 @@ fn check_impl_items_against_trait<'tcx>(
             impl_id.to_def_id(),
             impl_item,
         );
+
+        check_overriding_final_trait_item(tcx, ty_trait_item, ty_impl_item);
     }
 
     if let Ok(ancestors) = trait_def.ancestors(tcx, impl_id.to_def_id()) {

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -197,18 +197,6 @@ pub(super) fn maybe_check_static_with_link_section(tcx: TyCtxt<'_>, id: LocalDef
     }
 }
 
-fn report_forbidden_specialization(tcx: TyCtxt<'_>, impl_item: DefId, parent_impl: DefId) {
-    let span = tcx.def_span(impl_item);
-    let ident = tcx.item_ident(impl_item);
-
-    let err = match tcx.span_of_impl(parent_impl) {
-        Ok(sp) => errors::ImplNotMarkedDefault::Ok { span, ident, ok_label: sp },
-        Err(cname) => errors::ImplNotMarkedDefault::Err { span, ident, cname },
-    };
-
-    tcx.dcx().emit_err(err);
-}
-
 fn missing_items_err(
     tcx: TyCtxt<'_>,
     impl_def_id: LocalDefId,

compiler/rustc_hir_analysis/src/errors.rs

@@ -892,6 +892,16 @@ pub(crate) enum ImplNotMarkedDefault {
 #[diag("this item cannot be used as its where bounds are not satisfied for the `Self` type")]
 pub(crate) struct UselessImplItem;
 
+#[derive(Diagnostic)]
+#[diag("cannot override `{$ident}` because it already has a `final` definition in the trait")]
+pub(crate) struct OverridingFinalTraitFunction {
+    #[primary_span]
+    pub impl_span: Span,
+    #[note("`{$ident}` is marked final here")]
+    pub trait_span: Span,
+    pub ident: Ident,
+}
+
 #[derive(Diagnostic)]
 #[diag("not all trait items implemented, missing: `{$missing_items_msg}`", code = E0046)]
 pub(crate) struct MissingTraitItem {

compiler/rustc_parse/src/errors.rs

@@ -3787,6 +3787,17 @@ pub(crate) struct RecoverImportAsUse {
     pub token_name: String,
 }
 
+#[derive(Diagnostic)]
+#[diag("{$article} {$descr} cannot be `final`")]
+#[note("only associated functions in traits can be `final`")]
+pub(crate) struct InappropriateFinal {
+    #[primary_span]
+    #[label("`final` because of this")]
+    pub span: Span,
+    pub article: &'static str,
+    pub descr: &'static str,
+}
+
 #[derive(Diagnostic)]
 #[diag("expected `::`, found `:`")]
 #[note("import paths are delimited using `::`")]

compiler/rustc_parse/src/parser/item.rs

@@ -206,14 +206,24 @@ impl<'a> Parser<'a> {
         })
     }
 
-    /// Error in-case `default` was parsed in an in-appropriate context.
+    /// Error in-case `default`/`final` was parsed in an in-appropriate context.
     fn error_on_unconsumed_default(&self, def: Defaultness, kind: &ItemKind) {
-        if let Defaultness::Default(span) = def {
-            self.dcx().emit_err(errors::InappropriateDefault {
-                span,
-                article: kind.article(),
-                descr: kind.descr(),
-            });
+        match def {
+            Defaultness::Default(span) => {
+                self.dcx().emit_err(errors::InappropriateDefault {
+                    span,
+                    article: kind.article(),
+                    descr: kind.descr(),
+                });
+            }
+            Defaultness::Final(span) => {
+                self.dcx().emit_err(errors::InappropriateFinal {
+                    span,
+                    article: kind.article(),
+                    descr: kind.descr(),
+                });
+            }
+            Defaultness::Implicit => (),
         }
     }
 
@@ -229,8 +239,8 @@ impl<'a> Parser<'a> {
         fn_parse_mode: FnParseMode,
         case: Case,
     ) -> PResult<'a, Option<ItemKind>> {
-        let check_pub = def == &Defaultness::Final;
-        let mut def_ = || mem::replace(def, Defaultness::Final);
+        let check_pub = def == &Defaultness::Implicit;
+        let mut def_ = || mem::replace(def, Defaultness::Implicit);
 
         let info = if !self.is_use_closure() && self.eat_keyword_case(exp!(Use), case) {
             self.parse_use_item()?
@@ -1005,8 +1015,11 @@ impl<'a> Parser<'a> {
         {
             self.bump(); // `default`
             Defaultness::Default(self.prev_token_uninterpolated_span())
+        } else if self.eat_keyword(exp!(Final)) {
+            self.psess.gated_spans.gate(sym::final_associated_functions, self.prev_token.span);
+            Defaultness::Final(self.prev_token_uninterpolated_span())
         } else {
-            Defaultness::Final
+            Defaultness::Implicit
         }
     }
 
@@ -1130,7 +1143,7 @@ impl<'a> Parser<'a> {
                         }) => {
                             self.dcx().emit_err(errors::AssociatedStaticItemNotAllowed { span });
                             AssocItemKind::Const(Box::new(ConstItem {
-                                defaultness: Defaultness::Final,
+                                defaultness: Defaultness::Implicit,
                                 ident,
                                 generics: Generics::default(),
                                 ty,

compiler/rustc_parse/src/parser/token_type.rs

@@ -91,6 +91,7 @@ pub enum TokenType {
     KwElse,
     KwEnum,
     KwExtern,
+    KwFinal,
     KwFn,
     KwFor,
     KwGen,
@@ -233,6 +234,7 @@ impl TokenType {
             KwExtern,
             KwFn,
             KwFor,
+            KwFinal,
             KwGen,
             KwIf,
             KwImpl,
@@ -309,6 +311,7 @@ impl TokenType {
             TokenType::KwExtern => Some(kw::Extern),
             TokenType::KwFn => Some(kw::Fn),
             TokenType::KwFor => Some(kw::For),
+            TokenType::KwFinal => Some(kw::Final),
             TokenType::KwGen => Some(kw::Gen),
             TokenType::KwIf => Some(kw::If),
             TokenType::KwImpl => Some(kw::Impl),
@@ -524,6 +527,7 @@ macro_rules! exp {
     (Extern)         => { exp!(@kw, Extern,     KwExtern) };
     (Fn)             => { exp!(@kw, Fn,         KwFn) };
     (For)            => { exp!(@kw, For,        KwFor) };
+    (Final)          => { exp!(@kw, Final,      KwFinal) };
     (Gen)            => { exp!(@kw, Gen,        KwGen) };
     (If)             => { exp!(@kw, If,         KwIf) };
     (Impl)           => { exp!(@kw, Impl,       KwImpl) };

compiler/rustc_span/src/symbol.rs

@@ -1096,6 +1096,7 @@ symbols! {
         fields,
         file,
         file_options,
+        final_associated_functions,
         flags,
         float,
         float_to_int_unchecked,

compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs

@@ -314,6 +314,11 @@ pub fn dyn_compatibility_violations_for_assoc_item(
     trait_def_id: DefId,
     item: ty::AssocItem,
 ) -> Vec<DynCompatibilityViolation> {
+    // `final` assoc functions don't prevent a trait from being dyn-compatible
+    if tcx.defaultness(item.def_id).is_final() {
+        return Vec::new();
+    }
+
     // Any item that has a `Self: Sized` requisite is otherwise exempt from the regulations.
     if tcx.generics_require_sized_self(item.def_id) {
         return Vec::new();

compiler/rustc_trait_selection/src/traits/vtable.rs

@@ -210,6 +210,11 @@ fn own_existential_vtable_entries_iter(
         debug!("own_existential_vtable_entry: trait_method={:?}", trait_method);
         let def_id = trait_method.def_id;
 
+        // Final methods should not be included in the vtable.
+        if trait_method.defaultness(tcx).is_final() {
+            return None;
+        }
+
         // Some methods cannot be called on an object; skip those.
         if !is_vtable_safe_method(tcx, trait_def_id, trait_method) {
             debug!("own_existential_vtable_entry: not vtable safe");

compiler/rustc_ty_utils/src/instance.rs

@@ -237,6 +237,12 @@ fn resolve_associated_item<'tcx>(
         }
         traits::ImplSource::Builtin(BuiltinImplSource::Object(_), _) => {
             let trait_ref = ty::TraitRef::from_assoc(tcx, trait_id, rcvr_args);
+
+            // `final` methods should be called directly.
+            if tcx.defaultness(trait_item_id).is_final() {
+                return Ok(Some(ty::Instance::new_raw(trait_item_id, rcvr_args)));
+            }
+
             if trait_ref.has_non_region_infer() || trait_ref.has_non_region_param() {
                 // We only resolve totally substituted vtable entries.
                 None