initial changes

src/expr/builtin_fn.rs

@@ -83,22 +83,38 @@ pub fn eval_builtin_assert(
     query: &mut expr::EvalFunctionQuery)
     -> Result<expr::Value, ()>
 {
-    query.ensure_arg_number(1)?;
+    query.ensure_min_max_arg_number(1, 2)?;
 
-    let condition = query.args[0].value.expect_bool(
-        query.report,
-        query.args[0].span)?;
+    let condition = query.args[0]
+        .value
+        .expect_bool(
+            query.report,
+            query.args[0].span)?;
+
+    let msg = if query.args.len() >= 2
+    {
+        diagn::Message::error_span(
+            format!(
+                "assertion failed: {}",
+                query.args[1]
+                    .value
+                    .expect_string(query.report, query.args[1].span)?
+                    .utf8_contents
+                    .clone()
+            ),
+            query.span,
+        )
+    }
+    else {
+        diagn::Message::error_span("assertion failed", query.span)
+    };
 
     if condition
     {
         Ok(expr::Value::Void)
     }
     else
     {
-        let msg = diagn::Message::error_span(
-            "assertion failed",
-            query.span);
-
         Ok(expr::Value::FailedConstraint(
             query.report.wrap_in_parents_capped(msg)))
     }

src/expr/eval.rs

@@ -219,6 +219,29 @@ impl<'a> EvalFunctionQuery<'a>
 			Ok(())
 		}
 	}
+
+	pub fn ensure_min_max_arg_number(
+		&mut self,
+		minimum_expected_arg_number: usize,
+		maximum_expected_arg_number: usize,
+	) -> Result<(), ()> {
+		if !((self.args.len() >= minimum_expected_arg_number)
+			&& (self.args.len() <= maximum_expected_arg_number))
+		{
+			self.report.error_span(
+				format!(
+					"function expected {} to {} arguments (but got {})",
+					minimum_expected_arg_number,
+					maximum_expected_arg_number,
+					self.args.len()
+				),
+				self.span,
+			);
+			Err(())
+		} else {
+			Ok(())
+		}
+	}
 }