Test indexes are a light-weight representation of the actual coverage data cargo-difftests gets from LLVM. Note that they are different from LLVM’s notion of an indexed .profraw file (e.g. a .profdata file).
They are typically self-contained, and by default only contain the list of file names that had at least a bit of code run during the execution of the test, when the test was run, and a bit of information about the test.
Full indexes
They can, however, be expanded to contain region information, to use, for example, with the git-diff-hunks algorithm, by passing the --full-index option to whichever cargo-difftests subcommand was used to compile the index.