Sema: implement @splat for arrays

[mlugg]

Resolves: #20433

[Rexicon226]

these need to be @intCasted, prob just cast the const len =

[Rexicon226]

these two will also need to be int casted. ditto above.

[andrewrk]

Release notes please!

[mlugg]

Release Notes

Zig 0.14.0 expands the @splat builtin to apply not only to vectors, but to arrays. This is useful when default-initializing an array to a constant value. For instance, in conjunction with [Decl Literals}(link to relevent release notes section), we can elegantly initialize an array of "color" values:

const Rgba = struct {
    r: u8,
    b: u8,
    g: u8,
    a: u8,
    pub const black: Rgba = .{ .r = 0, .g = 0, .b = 0, .a = 255 };
};
var pixels: [width][height]Rgba = @splat(@splat(.black));

The operand may be comptime-known or runtime-known. In addition, this builtin can also be used to initialize sentinel-terminated arrays.

const std = @import("std");
const assert = std.debug.assert;
const expect = std.testing.expect;
test "initialize sentinel-terminated array" {
    // the sentinel does not need to match the value 
    const arr: [2:0]u8 = @splat(10);
    try expect(arr[0] == 10);
    try expect(arr[1] == 10);
    try expect(arr[2] == 0);
}
test "initialize runtime array" {
    var runtime_known: u8 = undefined;
    runtime_known = 123;
    // the operand can be runtime-known, giving a runtime-known array
    const arr: [2]u8 = @splat(runtime_known);
    try expect(arr[0] == 123);
    try expect(arr[1] == 123);
}
test "initialize zero-length sentinel-terminated array" {
    var runtime_known: u8 = undefined;
    runtime_known = 123;
    const arr: [0:10]u8 = @splat(runtime_known);
    // the operand was runtime-known, but since the array length was zero, the result is comptime-known
    comptime assert(arr[0] == 10);
}