Strings, Bytes, and the Cost of Conversion

string(b) and []byte(s) are basically free — they just reinterpret the same bytes with a different type." (And its inverse, from performance folklore: "every conversion copies unless it's literally m[string(b)].")

the conversions copy by default, because a string is immutable and a []byte is mutable — otherwise a write through the slice would mutate a string, which the language forbids. But the compiler elides the copy wherever it can prove the copy is unobservable, and that proof now covers more than the old folklore says: string(b) used directly in a map index, comparison, switch, or range; a string(b) assigned to a local that provably stays transient (the two-step map lookup is now free); []byte(s) that is never written (zero-copy since Go 1.22); and any small non-escaping conversion, which lands in a 32-byte stack buffer. What always copies: a conversion whose result is retained (returned, stored in a map or struct) or whose bytes may be mutated.