244ea27d5f
In a subsequent commit, we're going to start using `annotate-snippets`'s functionality for diagnostic IDs in the rendering. As part of doing that, I wanted to remove this special casing of an empty message. I did that independently to see what, if anything, would change. (The changes look fine to me. They'll be tweaked again in the next commit along with a bunch of others.)
Markdown files within the mdtest/ subdirectory are tests of type inference and type checking;
executed by the tests/mdtest.rs integration test.
See crates/ty_test/README.md for documentation of this test format.