assert(): Make message argument optional

docs/markdown/Reference-manual.md

@@ -144,6 +144,9 @@ build target (e.g. return value of [executable()](#executable), custom_target(),
 
 Abort with an error message if `condition` evaluates to `false`.
 
+*Since 0.53.0* `message` argument is optional and defaults to print the condition
+statement instead.
+
 ### benchmark()
 
 ``` meson

mesonbuild/ast/printer.py

@@ -100,7 +100,7 @@ class AstPrinter(AstVisitor):
 
     def visit_ComparisonNode(self, node: mparser.ComparisonNode):
         node.left.accept(self)
-        self.append_padded(mparser.comparison_map[node.ctype], node)
+        self.append_padded(node.ctype, node)
         node.right.accept(self)
 
     def visit_ArithmeticNode(self, node: mparser.ArithmeticNode):
@@ -192,7 +192,10 @@ class AstPrinter(AstVisitor):
             if break_args:
                 self.newline()
         for key, val in node.kwargs.items():
-            self.append(key, node)
+            if isinstance(key, str):
+                self.append(key, node)
+            else:
+                key.accept(self)
             self.append_padded(':', node)
             val.accept(self)
             self.append(', ', node)

mesonbuild/interpreter.py

@@ -2366,14 +2366,24 @@ external dependencies (including libraries) must go to "dependencies".''')
 
     @noKwargs
     def func_assert(self, node, args, kwargs):
-        if len(args) != 2:
-            raise InterpreterException('Assert takes exactly two arguments')
-        value, message = args
+        if len(args) == 1:
+            FeatureNew('assert function without message argument', '0.53.0').use(self.subproject)
+            value = args[0]
+            message = None
+        elif len(args) == 2:
+            value, message = args
+            if not isinstance(message, str):
+                raise InterpreterException('Assert message not a string.')
+        else:
+            raise InterpreterException('Assert takes between one and two arguments')
         if not isinstance(value, bool):
             raise InterpreterException('Assert value not bool.')
-        if not isinstance(message, str):
-            raise InterpreterException('Assert message not a string.')
         if not value:
+            if message is None:
+                from .ast import AstPrinter
+                printer = AstPrinter()
+                node.args.arguments[0].accept(printer)
+                message = printer.result
             raise InterpreterException('Assert failed: ' + message)
 
     def validate_arguments(self, args, argcount, arg_types):

run_unittests.py

@@ -4417,6 +4417,12 @@ class FailureTests(BasePlatformTests):
         match = 'Meson version is.*but project requires >=2000'
         self.assertMesonRaises("", match, meson_version='>=2000', options=options)
 
+    def test_assert_default_message(self):
+        self.assertMesonRaises("k1 = 'a'\n" +
+                               "assert({\n" +
+                               "  k1: 1,\n" +
+                               "}['a'] == 2)\n",
+                               r"Assert failed: {k1 : 1}\['a'\] == 2")
 
 @unittest.skipUnless(is_windows() or is_cygwin(), "requires Windows (or Windows via Cygwin)")
 class WindowsTests(BasePlatformTests):