Validate no override impl definitions

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

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 {

tests/ui/traits/final/overriding.rs

@@ -0,0 +1,12 @@
+#![feature(final_associated_functions)]
+
+trait Foo {
+    final fn method() {}
+}
+
+impl Foo for () {
+    fn method() {}
+    //~^ ERROR cannot override `method` because it already has a `final` definition in the trait
+}
+
+fn main() {}

tests/ui/traits/final/overriding.stderr

@@ -0,0 +1,14 @@
+error: cannot override `method` because it already has a `final` definition in the trait
+  --> $DIR/overriding.rs:8:5
+   |
+LL |     fn method() {}
+   |     ^^^^^^^^^^^
+   |
+note: `method` is marked final here
+  --> $DIR/overriding.rs:4:5
+   |
+LL |     final fn method() {}
+   |     ^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+

tests/ui/traits/final/positions.rs

@@ -34,12 +34,15 @@
 final impl Trait for Foo {
     final fn method() {}
     //~^ ERROR `final` is only allowed on associated functions in traits
+    //~^^ ERROR cannot override `method` because it already has a `final` definition in the trait
 
     final type Foo = ();
     //~^ ERROR `final` is only allowed on associated functions in traits
+    //~^^ ERROR cannot override `Foo` because it already has a `final` definition in the trait
 
     final const FOO: usize = 1;
     //~^ ERROR `final` is only allowed on associated functions in traits
+    //~^^ ERROR cannot override `FOO` because it already has a `final` definition in the trait
 }
 
 

tests/ui/traits/final/positions.stderr

@@ -15,7 +15,7 @@ LL | final trait Trait {
    = note: only associated functions in traits can be `final`
 
 error: a static item cannot be `final`
-  --> $DIR/positions.rs:64:5
+  --> $DIR/positions.rs:67:5
    |
 LL |     final static FOO_EXTERN: usize = 0;
    |     ^^^^^ `final` because of this
@@ -23,7 +23,7 @@ LL |     final static FOO_EXTERN: usize = 0;
    = note: only associated functions in traits can be `final`
 
 error: an extern block cannot be `final`
-  --> $DIR/positions.rs:55:1
+  --> $DIR/positions.rs:58:1
    |
 LL | final unsafe extern "C" {
    | ^^^^^ `final` because of this
@@ -79,7 +79,7 @@ LL |     final fn method() {}
    |     `final` because of this
 
 error: `final` is only allowed on associated functions in traits
-  --> $DIR/positions.rs:38:5
+  --> $DIR/positions.rs:39:5
    |
 LL |     final type Foo = ();
    |     -----^^^^^^^^^^^^^^^
@@ -87,7 +87,7 @@ LL |     final type Foo = ();
    |     `final` because of this
 
 error: `final` is only allowed on associated functions in traits
-  --> $DIR/positions.rs:41:5
+  --> $DIR/positions.rs:43:5
    |
 LL |     final const FOO: usize = 1;
    |     -----^^^^^^^^^^^^^^^^^^^^^^
@@ -95,7 +95,7 @@ LL |     final const FOO: usize = 1;
    |     `final` because of this
 
 error: `final` is only allowed on associated functions in traits
-  --> $DIR/positions.rs:46:1
+  --> $DIR/positions.rs:49:1
    |
 LL | final fn foo() {}
    | -----^^^^^^^^^
@@ -103,7 +103,7 @@ LL | final fn foo() {}
    | `final` because of this
 
 error: `final` is only allowed on associated functions in traits
-  --> $DIR/positions.rs:49:1
+  --> $DIR/positions.rs:52:1
    |
 LL | final type FooTy = ();
    | -----^^^^^^^^^^^^^^^^^
@@ -111,7 +111,7 @@ LL | final type FooTy = ();
    | `final` because of this
 
 error: `final` is only allowed on associated functions in traits
-  --> $DIR/positions.rs:52:1
+  --> $DIR/positions.rs:55:1
    |
 LL | final const FOO: usize = 0;
    | -----^^^^^^^^^^^^^^^^^^^^^^
@@ -119,7 +119,7 @@ LL | final const FOO: usize = 0;
    | `final` because of this
 
 error: `final` is only allowed on associated functions in traits
-  --> $DIR/positions.rs:58:5
+  --> $DIR/positions.rs:61:5
    |
 LL |     final fn foo_extern();
    |     -----^^^^^^^^^^^^^^^^^
@@ -127,7 +127,7 @@ LL |     final fn foo_extern();
    |     `final` because of this
 
 error: `final` is only allowed on associated functions in traits
-  --> $DIR/positions.rs:61:5
+  --> $DIR/positions.rs:64:5
    |
 LL |     final type FooExtern;
    |     -----^^^^^^^^^^^^^^^^
@@ -135,7 +135,7 @@ LL |     final type FooExtern;
    |     `final` because of this
 
 error: incorrect `static` inside `extern` block
-  --> $DIR/positions.rs:64:18
+  --> $DIR/positions.rs:67:18
    |
 LL | final unsafe extern "C" {
    | ----------------------- `extern` blocks define existing foreign statics and statics inside of them cannot have a body
@@ -147,5 +147,41 @@ LL |     final static FOO_EXTERN: usize = 0;
    |
    = note: for more information, visit https://doc.rust-lang.org/std/keyword.extern.html
 
-error: aborting due to 18 previous errors
+error: cannot override `method` because it already has a `final` definition in the trait
+  --> $DIR/positions.rs:35:5
+   |
+LL |     final fn method() {}
+   |     ^^^^^^^^^^^^^^^^^
+   |
+note: `method` is marked final here
+  --> $DIR/positions.rs:13:5
+   |
+LL |     final fn method() {}
+   |     ^^^^^^^^^^^^^^^^^
+
+error: cannot override `Foo` because it already has a `final` definition in the trait
+  --> $DIR/positions.rs:39:5
+   |
+LL |     final type Foo = ();
+   |     ^^^^^^^^^^^^^^
+   |
+note: `Foo` is marked final here
+  --> $DIR/positions.rs:16:5
+   |
+LL |     final type Foo = ();
+   |     ^^^^^^^^^^^^^^
+
+error: cannot override `FOO` because it already has a `final` definition in the trait
+  --> $DIR/positions.rs:43:5
+   |
+LL |     final const FOO: usize = 1;
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: `FOO` is marked final here
+  --> $DIR/positions.rs:19:5
+   |
+LL |     final const FOO: usize = 1;
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 21 previous errors
 

tests/ui/traits/final/works.rs

@@ -0,0 +1,13 @@
+//@ check-pass
+
+#![feature(final_associated_functions)]
+
+trait Foo {
+    final fn bar(&self) {}
+}
+
+impl Foo for () {}
+
+fn main() {
+    ().bar();
+}