An example of returning a dynamic error message in Zig

dynamic_error_msg.zig

const std = @import("std");

const Context = struct {
    canceled: bool = false,
    err: [4096]u8 = undefined,

    pub fn formatErr(self: *Context, comptime fmt: []const u8, args: anytype) error{NoSpaceLeft}!void {
        var fbs = std.io.fixedBufferStream(self.err[0..]);
        var writer = fbs.writer();
        try std.fmt.format(writer, fmt, args);
    }
};

const ContextError = error{InternalError, NoSpaceLeft};

fn do_something(ctx: *Context, input: i32) ContextError!void {
    if (input < 10) {
        ctx.canceled = true;
        try ctx.formatErr("less than 10: was {}", .{input});
        return error.InternalError;
    }
}

pub fn main() !void {
    var ctx = Context{};
    do_something(&ctx, 9) catch |err| {
        std.log.err("{s}", .{ctx.err});
        return err;
    };
}

run.log

$ zig run dynamic_error_msg.zig 
error: less than 10: was 9
error: InternalError
/home/hnakamur/ghq/github.com/hnakamur/learn-zig/dynamic_error_msg.zig:20:9: 0x22a49e in do_something (dynamic_error_msg)
        return error.InternalError;
        ^
/home/hnakamur/ghq/github.com/hnakamur/learn-zig/dynamic_error_msg.zig:28:9: 0x228224 in main (dynamic_error_msg)
        return err;
        ^