In #3121, there are tests for which we find coverage checks pointing to a non-existing coverage region out of the line boundaries. This results in an out-of-bounds error with the standard highlighting algorithm used by the report subcommand:

thread 'main' panicked at /rustc/8d6b88b168e45ee1624699c19443c49665322a91/library/alloc/src/string.rs:1699:9:
assertion failed: self.is_char_boundary(idx)
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

The issue appears in at least these two tests in the coverage-based suite:

    [coverage-based] coverage/assert/test.rs
    [coverage-based] coverage/known_issues/assert_uncovered_end/test.rs

The coverage regions in these checks are always single unit and appear right after the line boundaries. In other words, they cover a single column after the string's length. These region might represent terminators not being covered, but this is unclear and more investigation is needed.

For the time being (i.e., in #3121), we're adding a space to extend the line boundaries so we can represent the code region using the report command¹. More investigation is needed in order to determine what these regions represent, and how we can help users understand such results and act on them (note #2639 and related issues). This will be a necessary condition for the stability of the feature.