Parse and lower final for methods

Co-authored-by: Michael Goulet <michael@errs.io>

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 => {
+            Defaultness::Final(sp) => (hir::Defaultness::Final, Some(self.lower_span(sp))),
-                assert!(has_value);
-                (hir::Defaultness::Final, None)
-            }
         }
     }
 

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -583,6 +583,7 @@ 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

@@ -329,7 +329,7 @@ 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_parse/src/errors.rs

@@ -3795,6 +3795,17 @@ 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 {
+        match def {
-            self.dcx().emit_err(errors::InappropriateDefault {
+            Defaultness::Default(span) => {
-                span,
+                self.dcx().emit_err(errors::InappropriateDefault {
-                article: kind.article(),
+                    span,
-                descr: kind.descr(),
+                    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 check_pub = def == &Defaultness::Implicit;
-        let mut def_ = || mem::replace(def, Defaultness::Final);
+        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

@@ -1094,6 +1094,7 @@ symbols! {
         fields,
         file,
         file_options,
+        final_associated_functions,
         flags,
         float,
         float_to_int_unchecked,

tests/ui/coroutine/gen_fn.none.stderr

@@ -1,8 +1,8 @@
-error: expected one of `#`, `async`, `const`, `default`, `extern`, `fn`, `pub`, `safe`, `unsafe`, or `use`, found `gen`
+error: expected one of `#`, `async`, `const`, `default`, `extern`, `final`, `fn`, `pub`, `safe`, `unsafe`, or `use`, found `gen`
   --> $DIR/gen_fn.rs:4:1
    |
 LL | gen fn foo() {}
-   | ^^^ expected one of 10 possible tokens
+   | ^^^ expected one of 11 possible tokens
 
 error: aborting due to 1 previous error
 

tests/ui/coroutine/gen_fn.rs

@@ -2,7 +2,7 @@
 //@[e2024] edition: 2024
 
 gen fn foo() {}
-//[none]~^ ERROR: expected one of `#`, `async`, `const`, `default`, `extern`, `fn`, `pub`, `safe`, `unsafe`, or `use`, found `gen`
+//[none]~^ ERROR: expected one of `#`, `async`, `const`, `default`, `extern`, `final`, `fn`, `pub`, `safe`, `unsafe`, or `use`, found `gen`
 //[e2024]~^^ ERROR: gen blocks are experimental
 
 fn main() {}

tests/ui/feature-gates/feature-gate-final-associated-functions.rs

@@ -0,0 +1,6 @@
+trait Foo {
+    final fn bar() {}
+    //~^ ERROR `final` on trait functions is experimental
+}
+
+fn main() {}

tests/ui/feature-gates/feature-gate-final-associated-functions.stderr

@@ -0,0 +1,13 @@
+error[E0658]: `final` on trait functions is experimental
+  --> $DIR/feature-gate-final-associated-functions.rs:2:5
+   |
+LL |     final fn bar() {}
+   |     ^^^^^
+   |
+   = note: see issue #1 <https://github.com/rust-lang/rust/issues/1> for more information
+   = help: add `#![feature(final_associated_functions)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/parser/duplicate-visibility.rs

@@ -2,8 +2,8 @@ fn main() {}
 
 extern "C" { //~ NOTE while parsing this item list starting here
     pub pub fn foo();
-    //~^ ERROR expected one of `(`, `async`, `const`, `default`, `extern`, `fn`, `safe`, `unsafe`, or `use`, found keyword `pub`
+    //~^ ERROR expected one of `(`, `async`, `const`, `default`, `extern`, `final`, `fn`, `safe`, `unsafe`, or `use`, found keyword `pub`
-    //~| NOTE expected one of 9 possible tokens
+    //~| NOTE expected one of 10 possible tokens
     //~| HELP there is already a visibility modifier, remove one
     //~| NOTE explicit visibility first seen here
 } //~ NOTE the item list ends here

tests/ui/parser/duplicate-visibility.stderr

@@ -1,4 +1,4 @@
-error: expected one of `(`, `async`, `const`, `default`, `extern`, `fn`, `safe`, `unsafe`, or `use`, found keyword `pub`
+error: expected one of `(`, `async`, `const`, `default`, `extern`, `final`, `fn`, `safe`, `unsafe`, or `use`, found keyword `pub`
   --> $DIR/duplicate-visibility.rs:4:9
    |
 LL | extern "C" {
@@ -6,7 +6,7 @@ LL | extern "C" {
 LL |     pub pub fn foo();
    |         ^^^
    |         |
-   |         expected one of 9 possible tokens
+   |         expected one of 10 possible tokens
    |         help: there is already a visibility modifier, remove one
 ...
 LL | }

tests/ui/parser/misspelled-keywords/pub-fn.stderr

@@ -1,8 +1,8 @@
-error: expected one of `#`, `async`, `auto`, `const`, `default`, `enum`, `extern`, `fn`, `gen`, `impl`, `macro_rules`, `macro`, `mod`, `pub`, `safe`, `static`, `struct`, `trait`, `type`, `unsafe`, or `use`, found `puB`
+error: expected one of `#`, `async`, `auto`, `const`, `default`, `enum`, `extern`, `final`, `fn`, `gen`, `impl`, `macro_rules`, `macro`, `mod`, `pub`, `safe`, `static`, `struct`, `trait`, `type`, `unsafe`, or `use`, found `puB`
   --> $DIR/pub-fn.rs:1:1
    |
 LL | puB fn code() {}
-   | ^^^ expected one of 21 possible tokens
+   | ^^^ expected one of 22 possible tokens
    |
 help: write keyword `pub` in lowercase
    |

tests/ui/traits/final/final-kw.gated.stderr

@@ -0,0 +1,13 @@
+error[E0658]: `final` on trait functions is experimental
+  --> $DIR/final-kw.rs:5:5
+   |
+LL |     final fn foo() {}
+   |     ^^^^^
+   |
+   = note: see issue #1 <https://github.com/rust-lang/rust/issues/1> for more information
+   = help: add `#![feature(final_associated_functions)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/traits/final/final-kw.rs

@@ -0,0 +1,9 @@
+//@ revisions: ungated gated
+
+#[cfg(ungated)]
+trait Trait {
+    final fn foo() {}
+    //~^ ERROR `final` on trait functions is experimental
+}
+
+fn main() {}

tests/ui/traits/final/final-kw.ungated.stderr

@@ -0,0 +1,13 @@
+error[E0658]: `final` on trait functions is experimental
+  --> $DIR/final-kw.rs:5:5
+   |
+LL |     final fn foo() {}
+   |     ^^^^^
+   |
+   = note: see issue #1 <https://github.com/rust-lang/rust/issues/1> for more information
+   = help: add `#![feature(final_associated_functions)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.