函数
同样的规则可以应用于函数：当类型 T 前面加上 <T> 时，它就变成了泛型。
使用泛型函数有时需要显式指定类型参数。 如果函数在返回类型是泛型的情况下被调用，或者编译器没有足够的信息来推断必要的类型参数，就可能会出现这种情况。
带有显式指定类型参数的函数调用看起来像：fun::<A, B, ...>()。
struct A; // Concrete type `A`. struct S(A); // Concrete type `S`. struct SGen<T>(T); // Generic type `SGen`. // The following functions all take ownership of the variable passed into // them and immediately go out of scope, freeing the variable. // Define a function `reg_fn` that takes an argument `_s` of type `S`. // This has no `<T>` so this is not a generic function. fn reg_fn(_s: S) {} // Define a function `gen_spec_t` that takes an argument `_s` of type `SGen<T>`. // It has been explicitly given the type parameter `A`, but because `A` has not // been specified as a generic type parameter for `gen_spec_t`, it is not generic. fn gen_spec_t(_s: SGen<A>) {} // Define a function `gen_spec_i32` that takes an argument `_s` of type `SGen<i32>`. // It has been explicitly given the type parameter `i32`, which is a specific type. // Because `i32` is not a generic type, this function is also not generic. fn gen_spec_i32(_s: SGen<i32>) {} // Define a function `generic` that takes an argument `_s` of type `SGen<T>`. // Because `SGen<T>` is preceded by `<T>`, this function is generic over `T`. fn generic<T>(_s: SGen<T>) {} fn main() { // Using the non-generic functions reg_fn(S(A)); // Concrete type. gen_spec_t(SGen(A)); // Implicitly specified type parameter `A`. gen_spec_i32(SGen(6)); // Implicitly specified type parameter `i32`. // Explicitly specified type parameter `char` to `generic()`. generic::<char>(SGen('a')); // Implicitly specified type parameter `char` to `generic()`. generic(SGen('c')); }