`[]byte(string)` vs `[]byte(*string)`: When Does Go Optimize Byte Slice Conversions?

[]byte(string) vs []byte(*string): Understanding the Performance Trade-Off

While examining the Go programming language, one may question the absence of a []byte(*string) method. Given that strings are immutable, it seems counterintuitive that []byte(string) would create a copy of the input string, incurring a performance penalty.

In actuality, []byte("something") is not a method call but a type conversion. This conversion itself does not cause duplication. However, when converting a string to a []byte, a copy is necessary. This is because the resulting byte slice is mutable, and altering it would indirectly modify the immutable string value. As per the Go specification, "Strings are immutable: once created, it is impossible to change the contents of a string."

In certain optimized scenarios, however, the compiler eliminates this copying step. For instance, when retrieving a value from a map using a string key indexed with a converted []byte, the key copy is not made. Additionally, when explicitly converting a string to a byte slice for iteration over its UTF8-encoded bytes, copying is optimized away.

Therefore, while []byte(string) may necessitate copying in most cases, Go employs optimizations when possible to minimize its performance impact.

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