Fix linting when trailing macro expands to a trailing semi

When a macro is used in the trailing expression position of a block
(e.g. `fn foo() { my_macro!() }`), we currently parse it as an
expression, rather than a statement. As a result, we ended up
using the `NodeId` of the containing statement as our `lint_node_id`,
even though we don't normally do this for macro calls.

If such a macro expands to an expression with a `#[cfg]` attribute,
then the trailing statement can get removed entirely. This lead to
an ICE, since we were usng the `NodeId` of the expression to emit
a lint.

Ths commit makes us skip updating `lint_node_id` when handling
a macro in trailing expression position. This will cause us to
lint at the closest parent of the macro call.

compiler/rustc_expand/src/expand.rs

@@ -1386,14 +1386,17 @@ impl<'a, 'b> MutVisitor for InvocationCollector<'a, 'b> {
         // `SEMICOLON_IN_EXPRESSIONS_FROM_MACROS` lint if needed.
         // See #78991 for an investigation of treating macros in this position
         // as statements, rather than expressions, during parsing.
-        if let StmtKind::Expr(expr) = &stmt.kind {
-            if matches!(**expr, ast::Expr { kind: ast::ExprKind::MacCall(..), .. }) {
+        let res = match &stmt.kind {
+            StmtKind::Expr(expr)
+                if matches!(**expr, ast::Expr { kind: ast::ExprKind::MacCall(..), .. }) =>
+            {
                 self.cx.current_expansion.is_trailing_mac = true;
+                // Don't use `assign_id` for this statement - it may get removed
+                // entirely due to a `#[cfg]` on the contained expression
+                noop_flat_map_stmt(stmt, self)
             }
-        }
-
-        let res = assign_id!(self, &mut stmt.id, || noop_flat_map_stmt(stmt, self));
-
+            _ => assign_id!(self, &mut stmt.id, || noop_flat_map_stmt(stmt, self)),
+        };
         self.cx.current_expansion.is_trailing_mac = false;
         res
     }