Why does using reader.interface as a variable cause a segfault?

[kj4tmp]

Zig 0.15.1 reader/writer: Don't make copies of @fieldParentPtr()-based interfaces

Here is a dedicated write-up for those not familiar with @fieldParentPtr-based interfaces, which is how the new reader/writer interface is implemented. These two programs produce drastically different results: bad_program.zig: const std = @import("std"); pub fn main() !void { const my_struct: struct { hello: []const u8 } = .{ .hello = "world" }; var buffer: [1024]u8 = undefined; var file_writer = std.fs.File.stdout().writer(&buffer); var writer_instance = file_writer.interfa…