> If you just encoded your string to bytes naïvely

By "naïvely" I assume you mean you would just plug in UTF-8 bytestrings for haystack & needle, without adjusting the implementation?

Wouldn't the code still need to take into account where characters (code points) begin and end, though, in order to prevent incorrect matches?

IDK what "encoded your string to bytes naively" means personally. There is only one way to correctly UTF-8 encode a sequence of Unicode scalar values.

In any case, no, this works because UTF-8 is self synchronizing. As long as both your needle and your haystack are valid UTF-8, the byte offsets returned by the search will always fall on a valid codepoint boundary.

In terms of getting "combining characters wrong," this is a reference to different Unicode normalization forms.

To be more precise... Consider a needle and a haystack, represented by a sequence of Unicode scalar values (typically represented by a sequence of unsigned 32-bit integers). Now encode them to UTF-8 (a sequence of unsigned 8-bit integers) and run a byte level search as shown by the OP here. That will behave as if you've executed the search on the sequence of Unicode scalar values.

So semantically, a "substring search" is a "sequence of Unicode scalar values search." At the semantic level, this may or may not be what you want. For example, if you always want `office` to find substrings like `office` in your haystack, then this byte level search will not do what you want.

The standard approach for performing a substring search that accounts for normalization forms is to convert both the needle and haystack to the same normal form and then execute a byte level search.

(One small caveat is when the needle is an empty string. If you want to enforce correct UTF-8 boundaries, you'll need to handle that specially.)