What is the expected syntax for checking exception messages in MiniTest’s assert_raises/must_raise?

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