Function truncate_at_char_boundary 

pub(crate) fn truncate_at_char_boundary(s: &str, max_bytes: usize) -> &str

Shorten s to at most max_bytes, cutting at a character boundary.

Returns s unchanged when it already fits. When it does not, the cut moves down to the nearest boundary at or below max_bytes, so the result can be shorter than the budget but is never longer and never invalid.

Illustrative rather than executable: this helper is crate-internal, and a doctest compiles as its own crate, so no import path reaches it. The cases below are asserted for real in this module’s tests.

truncate_at_char_boundary:

truncate_at_char_boundary("hello", 10)  ==  "hello"
truncate_at_char_boundary("hello", 3)  ==  "hel"

// "α" is two bytes, so a budget of 3 fits one of them, not one and a half.
truncate_at_char_boundary("ααα", 3)  ==  "α"