First, thanks a ton for creating and maintaining this library (and apologies for the mouthful of a issue title).

I'm reporting this based on user behavior observed in zizmorcore/zizmor#1065 -- we use annotate-snippets to render static analysis findings, and a user observed a crash on an input that appears to be well-formed and is only accessed via valid UTF-8 spans within the application code itself.

The conditions for the panic are pretty subtle:

1. The snippet's source must contain multiple lines, but the final line shouldn't have a trailing linefeed;
2. The snippet must have multiple annotations;
3. The last character in the snippet should be multibyte (e.g. an emoji).

When put together, these conditions cause a panic when rendering multiple annotations that span to the end of the snippet's source. This panic doesn't happen when the snippet's source ends in a final linefeed.

Here's my attempt at a minified reproducer for the above, with the latest release of annotated-snippets (0.11.5):

use annotate_snippets::{Level, Renderer, Snippet};
fn main() {
 let good = r#"foobar

 foobar 🚀
"#;
 let snippet = Snippet::source(good)
 .fold(true)
 .line_start(1)
 .origin("whatever")
 .annotation(Level::Warning.span(0..good.len()).label("blah"))
 .annotation(Level::Warning.span(0..good.len()).label("blah"));
 let message = Level::Warning.title("whatever").snippet(snippet);
 let renderer = Renderer::styled();
 println!("{}", renderer.render(message));
 let bad = r#"foobar

 foobar 🚀"#;
 let snippet = Snippet::source(bad)
 .fold(true)
 .line_start(1)
 .origin("whatever")
 .annotation(Level::Warning.span(0..bad.len()).label("blah"))
 .annotation(Level::Warning.span(0..bad.len()).label("blah"));
 let message = Level::Warning.title("whatever").snippet(snippet);
 let renderer = Renderer::styled();
 println!("{}", renderer.render(message));
}

Observe that the only difference between good and bad is that good has a trailing linefeed, while bad does not. In both cases, the annotation spans are valid UTF-8 slices.

Here's what the panic looks like (after a successful render of good first):

$ ./target/debug/repro 
warning: whatever
 --> whatever:1:1
 |
1 | / foobar
2 | |
3 | | foobar 🚀
 | | -
 | |______________________|
 | blah
 | blah
 |
thread 'main' panicked at /home/william/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/annotate-snippets-0.11.5/src/renderer/display_list.rs:1440:29:
byte index 22 is not a char boundary; it is inside '🚀' (bytes 19..23) of ` foobar 🚀`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Specifically, it looks like the panicking slice happens here:

It looks like this file no longer exists on main, so it's possible this bug has been addressed in a refactor that hasn't been put in a release yet. If so, I apologize for the noise!