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.)