It is very expensive if you repeatedly measure and forget the length, this is presumably some of the price in Google's problem where some engineers wanted to use 0-terminated char* as the type of a string but others wanted C++ std::string and so the software ends up measuring how long the string is, allocating and copying, then immediately forgetting that length, only to once again measure how long it is, allocate and copy again.
That's a language design defect, C++ got its string slice reference (named std::string_view) only in 2017, years after Rust 1.0 shipped this as a core language feature, even though C++ is decades older.
On the other hand I can well believe on a 1970s computer where you'd be lucky to have 64kB of RAM the trade looks very different, I just think that by C89 it should have been fixed.
