How to initialize a static array with most values the same but some values different?

Question

I would like to use a static or const array, but initialize it using something other than the [T; N] syntax. I need to define specific elements but all other values can default to 0 or some other value.

In C, you can do the following:

byte ARRAY[256] = {
    [0x1F] = (1 << 4),
    // Or even simply just this
    [0x46] '\n'
};

I've tried something along the lines of:

static ARRAY: [u8; 256] = {
    // x is some arbitrary number of elements
    let mut array = [0, x];
    array[i] = 'b',
    array[j] = 'a',
    array[k] = 'd',
    array
};

This was merely trial and error based on syntax I know to work for local array declarations. This throws a compiler error saying that blocks in const and static are limited to items and tail expressions. I know that if I enclose an array definition in brackets, then the last line or last expression must be the implicit return.

Additionally, I don't have access to the std library, but I don't think a complex structure would be necessary for something this simple - to index and obtain a value.

I've looked at the Rust macro rules and think that could be a solution, but all the examples I have seen are iterative and incremental.

Answer 1

没有与您的 C 代码片段等效的 Rust。文档显示只允许使用 3 种简单的模式：

1. 空的
2. 价值、价值、价值等等……
3. 价值; 尺寸

所以，目前使用数组语法，你无法做到这一点。

关于 const 函数的RFC现在允许：

static ARRAY: [u8; 256] = {
    let mut array = [0; 256];
    array[0] = b'b';
    array[1] = b'a';
    array[2] = b'd';
    array
};

现在，让我们看一下声明性宏解决方案。没有办法“计数”，有一些技巧，但不会走得太远。proc 宏可以工作。

您还可以在编译之前使用其他工具生成文件。例如，您可以在编译之前使用 Cargo生成文件。