Fix linting when trailing macro expands to a trailing semi

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
     }

src/test/ui/macros/lint-trailing-macro-call.rs

@@ -0,0 +1,16 @@
+// check-pass
+//
+// Ensures that we properly lint
+// a removed 'expression' resulting from a macro
+// in trailing expression position
+
+macro_rules! expand_it {
+    () => {
+        #[cfg(FALSE)] 25; //~  WARN trailing semicolon in macro
+                          //~| WARN this was previously
+    }
+}
+
+fn main() {
+    expand_it!()
+}

src/test/ui/macros/lint-trailing-macro-call.stderr

@@ -0,0 +1,18 @@
+warning: trailing semicolon in macro used in expression position
+  --> $DIR/lint-trailing-macro-call.rs:9:25
+   |
+LL |         #[cfg(FALSE)] 25;
+   |                         ^
+...
+LL |     expand_it!()
+   |     ------------ in this macro invocation
+   |
+   = note: `#[warn(semicolon_in_expressions_from_macros)]` on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #79813 <https://github.com/rust-lang/rust/issues/79813>
+   = note: macro invocations at the end of a block are treated as expressions
+   = note: to ignore the value produced by the macro, add a semicolon after the invocation of `expand_it`
+   = note: this warning originates in the macro `expand_it` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+warning: 1 warning emitted
+