Deny final not followed by item

compiler/rustc_parse/src/errors.rs

@@ -2203,6 +2203,15 @@ pub(crate) struct DefaultNotFollowedByItem {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag("`final` is not followed by an item")]
+#[note("only associated functions in traits may be prefixed by `final`")]
+pub(crate) struct FinalNotFollowedByItem {
+    #[primary_span]
+    #[label("the `final` qualifier")]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 pub(crate) enum MissingKeywordForItemDefinition {
     #[diag("missing `enum` for enum definition")]

compiler/rustc_parse/src/parser/item.rs

@@ -197,6 +197,8 @@ impl<'a> Parser<'a> {
 
             if let Defaultness::Default(span) = def {
                 this.dcx().emit_err(errors::DefaultNotFollowedByItem { span });
+            } else if let Defaultness::Final(span) = def {
+                this.dcx().emit_err(errors::FinalNotFollowedByItem { span });
             }
 
             if !attrs_allowed {

tests/ui/traits/final/bad-positions.rs

@@ -0,0 +1,7 @@
+#![feature(final_associated_functions)]
+
+fn main() {
+    final; //~ ERROR `final` is not followed by an item
+    final 1 + 1; //~ ERROR `final` is not followed by an item
+    final { println!("text"); }; //~ ERROR `final` is not followed by an item
+}

tests/ui/traits/final/bad-positions.stderr

@@ -0,0 +1,26 @@
+error: `final` is not followed by an item
+  --> $DIR/bad-positions.rs:4:5
+   |
+LL |     final;
+   |     ^^^^^ the `final` qualifier
+   |
+   = note: only associated functions in traits may be prefixed by `final`
+
+error: `final` is not followed by an item
+  --> $DIR/bad-positions.rs:5:5
+   |
+LL |     final 1 + 1;
+   |     ^^^^^ the `final` qualifier
+   |
+   = note: only associated functions in traits may be prefixed by `final`
+
+error: `final` is not followed by an item
+  --> $DIR/bad-positions.rs:6:5
+   |
+LL |     final { println!("text"); };
+   |     ^^^^^ the `final` qualifier
+   |
+   = note: only associated functions in traits may be prefixed by `final`
+
+error: aborting due to 3 previous errors
+