在 MiniTest 中检查断言/必须断言中的异常消息的预期语法是什么？

You can use the assert_raises assertion, or the must_raise expectation.

it "must raise" do
assert_raises RuntimeError do
bar.do_it
end
->     { bar.do_it }.must_raise RuntimeError
lambda { bar.do_it }.must_raise RuntimeError
proc   { bar.do_it }.must_raise RuntimeError
end

If you need to test something on the error object, you can get it from the assertion or expectation like so:

describe "testing the error object" do
it "as an assertion" do
err = assert_raises RuntimeError { bar.do_it }
assert_match /Foo/, err.message
end


it "as an exception" do
err = ->{ bar.do_it }.must_raise RuntimeError
err.message.must_match /Foo/
end
end

Minitest does not provide (yet) you a way to check the actual exception message. But you could add a helper method that does it and extend ActiveSupport::TestCase class to use everywhere in your rails test suite, e.g.: in test_helper.rb

class ActiveSupport::TestCase
def assert_raises_with_message(exception, msg, &block)
block.call
rescue exception => e
assert_match msg, e.message
else
raise "Expected to raise #{exception} w/ message #{msg}, none raised"
end
end

and use it in your tests like:

assert_raises_with_message RuntimeError, 'Foo' do
code_that_raises_RuntimeError_with_Foo_message
end

To assert exception:

assert_raises FooError do
bar.do_it
end

To assert exception message:

As per API doc, assert_raises returns the exception matched so you can check the message, attributes, etc.

exception = assert_raises FooError do
bar.do_it
end
assert_equal('Foo', exception.message)

To add some more recent developments, there have been some discussions on adding assert_raises_with_message to minitest in the past without much luck.

Currently, there is a promising pull request waiting to be merged. If and when it gets merged, we will be able to use assert_raises_with_message without having to define it ourselves.

In the meanwhile, there is this handy little gem named minitest-bonus-assertions which defines exactly that method, along with few others, so that you can use it out of the box. See the docs for more information.