How to format strings in Zig using a runtime format string

Zig's printing functions expect format strings that are known at compile time. You can use C's printf if you need a dynamic format string.

Here's something I got running on version 0.12.0-dev.2296+46d592e48.

const c = @cImport({
    @cInclude("stdio.h");
});

const std = @import("std");

// Returns a null-terminated heap-allocated string, usable in C, containing the phrase "hi " repeated numHis type.
const hi = "hi ";
fn getHis(allocator: std.mem.Allocator, numHis: usize) ![*:0]u8 {
    const total_len = numHis * hi.len;
    // allocSentinel ensures it ends with a \0.
    const s = try allocator.allocSentinel(u8, total_len, 0);
    var i: usize = 0;
    while (i < total_len) : (i += hi.len) {
        // repeatedly copy the phrase into the allocated memory.
        std.mem.copyForwards(u8, s[i .. i + hi.len], hi);
    }
    return s;
}

// Returns a null-terminated heap-allocated string, usable in C, containing some border chars and the phrase "%s", which can be passed as a format string to c's printf.
const strInsert = "\n%s\n";
fn getBorderedFormatString(allocator: std.mem.Allocator, borderChar: u8) ![*:0]u8 {
    const total_len: usize = 20 + strInsert.len + 2; // add 2 for leading and trailing newline
    const s = try allocator.allocSentinel(u8, total_len, 0);
    var i: usize = 0;
    s[i] = '\n';
    i += 1;
    while (i < 11) : (i += 1) {
        s[i] = borderChar;
    }
    std.mem.copyForwards(u8, s[i .. i + strInsert.len], strInsert);
    i += strInsert.len;
    while (i < total_len - 1) : (i += 1) {
        s[i] = borderChar;
    }
    s[i] = '\n';
    return s;
}

test "dynamic_phrase" {
    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
    defer arena.deinit();
    const allocator = arena.allocator();
    const s = try getHis(allocator, 30);
    const f = try getBorderedFormatString(allocator, '+');
    _ = c.printf(f, s);
}

I'm new at Zig, so maybe there is a way to use dynamic format strings in Zig directly. If so, I didn't easily discover it.