The UI tests are intended to capture the compiler‘s complete output, so that we can test all aspects of the presentation. They work by compiling a file (e.g., hello_world/main.rs), capturing the output, and then applying some normalization (see below). This normalized result is then compared against reference files named hello_world/main.stderr and hello_world/main.stdout. If either of those files doesn’t exist, the output must be empty. If the test run fails, we will print out the current output, but it is also saved in build/<target-triple>/test/ui/hello_world/main.stdout (this path is printed as part of the test failure mesage), so you can run diff and so forth.
If you have changed the compiler's output intentionally, or you are making a new test, you can use the script update-references.sh to update the references. When you run the test framework, it will report various errors: in those errors is a command you can use to run the update-references.sh script, which will then copy over the files from the build directory and use them as the new reference. You can also just run update-all-references.sh. In both cases, you can run the script with --help to get a help message.
The normalization applied is aimed at filenames:
$DIR