Lint on redundant trailing semicolon after item

We now lint on code like this:

```rust
fn main() {
    fn foo() {};
    struct Bar {};
}
```

Previously, this caused warnings in Cargo, so it was disabled.

compiler/rustc_lint/src/redundant_semicolon.rs

@@ -28,40 +28,26 @@ declare_lint_pass!(RedundantSemicolons => [REDUNDANT_SEMICOLONS]);
 
 impl EarlyLintPass for RedundantSemicolons {
     fn check_block(&mut self, cx: &EarlyContext<'_>, block: &Block) {
-        let mut after_item_stmt = false;
         let mut seq = None;
         for stmt in block.stmts.iter() {
             match (&stmt.kind, &mut seq) {
                 (StmtKind::Empty, None) => seq = Some((stmt.span, false)),
                 (StmtKind::Empty, Some(seq)) => *seq = (seq.0.to(stmt.span), true),
-                (_, seq) => {
-                    maybe_lint_redundant_semis(cx, seq, after_item_stmt);
-                    after_item_stmt = matches!(stmt.kind, StmtKind::Item(_));
-                }
+                (_, seq) => maybe_lint_redundant_semis(cx, seq),
             }
         }
-        maybe_lint_redundant_semis(cx, &mut seq, after_item_stmt);
+        maybe_lint_redundant_semis(cx, &mut seq);
     }
 }
 
-fn maybe_lint_redundant_semis(
-    cx: &EarlyContext<'_>,
-    seq: &mut Option<(Span, bool)>,
-    after_item_stmt: bool,
-) {
+fn maybe_lint_redundant_semis(cx: &EarlyContext<'_>, seq: &mut Option<(Span, bool)>) {
     if let Some((span, multiple)) = seq.take() {
         // FIXME: Find a better way of ignoring the trailing
         // semicolon from macro expansion
         if span == rustc_span::DUMMY_SP {
             return;
         }
 
-        // FIXME: Lint on semicolons after item statements
-        // once doing so doesn't break bootstrapping
-        if after_item_stmt {
-            return;
-        }
-
         cx.struct_span_lint(REDUNDANT_SEMICOLONS, span, |lint| {
             let (msg, rem) = if multiple {
                 ("unnecessary trailing semicolons", "remove these semicolons")

src/test/ui/lint/redundant-semicolon/item-stmt-semi.rs

@@ -1,10 +1,6 @@
-// check-pass
-// This test should stop compiling
-// we decide to enable this lint for item statements.
-
 #![deny(redundant_semicolons)]
 
 fn main() {
-    fn inner() {};
-    struct Bar {};
+    fn inner() {}; //~ ERROR unnecessary
+    struct Bar {}; //~ ERROR unnecessary
 }

src/test/ui/lint/redundant-semicolon/item-stmt-semi.stderr

@@ -0,0 +1,20 @@
+error: unnecessary trailing semicolon
+  --> $DIR/item-stmt-semi.rs:4:18
+   |
+LL |     fn inner() {};
+   |                  ^ help: remove this semicolon
+   |
+note: the lint level is defined here
+  --> $DIR/item-stmt-semi.rs:1:9
+   |
+LL | #![deny(redundant_semicolons)]
+   |         ^^^^^^^^^^^^^^^^^^^^
+
+error: unnecessary trailing semicolon
+  --> $DIR/item-stmt-semi.rs:5:18
+   |
+LL |     struct Bar {};
+   |                  ^ help: remove this semicolon
+
+error: aborting due to 2 previous errors
+