以下摘自The Go Programming Language：
For every type T, there is a corresponding conversion operation T(x) that converts the value x to type T. A conversion from one type to another is allowed if both have the same underlying type, or if both are unnamed pointer types that point to variables of the same underlying type; these conversions change the type but not the representation of the value. If x is assignable to T, a conversion is permitted but is usually redundant.
Conversions are also allowed between numeric types, and between string and some slice types. These conversions may change the representation of the value. For instance, converting a floating-point number to an integer discards any fractional part, and converting a string to a []byte slice allocates a copy of the string data. In any case, a conversion never fails at run time.