emit error with span for empty asserts

Fixes #55547.

src/libsyntax_ext/assert.rs

@@ -24,6 +24,14 @@ pub fn expand_assert<'cx>(
     tts: &[TokenTree],
 ) -> Box<dyn MacResult + 'cx> {
     let mut parser = cx.new_parser_from_tts(tts);
+
+    if parser.token == token::Eof {
+        cx.struct_span_err(sp, "macro requires a boolean expression as an argument")
+            .span_label(sp, "boolean expression required")
+            .emit();
+        return DummyResult::expr(sp);
+    }
+
     let cond_expr = panictry!(parser.parse_expr());
     let custom_msg_args = if parser.eat(&token::Comma) {
         let ts = parser.parse_tokens();

src/test/ui/macros/assert.rs

@@ -0,0 +1,4 @@
+fn main() {
+    assert!();  //~ ERROR requires a boolean expression
+    debug_assert!(); //~ ERROR requires a boolean expression
+}

src/test/ui/macros/assert.stderr

@@ -0,0 +1,16 @@
+error: macro requires a boolean expression as an argument
+  --> $DIR/assert.rs:2:5
+   |
+LL |     assert!();  //~ ERROR requires a boolean expression
+   |     ^^^^^^^^^^ boolean expression required
+
+error: macro requires a boolean expression as an argument
+  --> $DIR/assert.rs:3:5
+   |
+LL |     debug_assert!(); //~ ERROR requires a boolean expression
+   |     ^^^^^^^^^^^^^^^^ boolean expression required
+   |
+   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+
+error: aborting due to 2 previous errors
+