Comptime dereference bugs tracking issue

[mlugg]

The Sema logic for accessing pointers at comptime is incredibly flawed and riddled with bugs. Recent changes in the frontend have exposed more of these bugs, such as those encountered in #19092.

I am actively working on a rewrite of this logic which will hopefully fix all of these issues, but in the meantime, workarounds are necessary.

This issue serves to track these bugs. Feel free to share workarounds here, and please link any PRs/commits - in the Zig project or otherwise - which work around the issue. When my rewrite is complete, I will test it on everything linked here to ensure there are no failing edge cases.

Resolution of this issue should likely block 0.12.0.

[paperclover]

Here is a simplified version of something I hit while trying to get bun to compile on the latest version of zig.

pub const hi = brk: {
    var buf: [4][]const u8 = undefined;

    for (0..2) |i| {
        buf[i] = "hello";
    }

    // error: comptime dereference requires '[2][]const u8' to have a well-defined layout, but it does not.
    const names = buf[0..2].*;
    break :brk names;
};

test {
    _ = &hi;
}

I think I can get a workaround by generating this code during zig build, like inlined &.{ "string 1", "string 2", ... }.

actually, while typing this, someone told me that the workaround is to ensure the definition buf has the correct size. so if line 2 is changed to include a 2, the code compiles.

Besides this one, I've been pretty happy about the recent changes, as they've caught a lot of const/non-const issues across the codebase.

[Inve1951]

regarding above post: i've hit this in my build.zig where you can't move the logic to the build system. buffer size needs to be computed in my case. it was previously set to the known upper bound.

[akhildevelops]

I too faced similar issue while building enum from type information that slices 2 enum fields from an array:

const std = @import("std");
const Type = std.builtin.Type;
pub fn main() !void {
    const enum_fields: [3]Type.EnumField = .{ .{ .name = "field1", .value = 1 }, .{ .name = "field2", .value = 2 }, .{ .name = "field3", .value = 3 } };
    const custom_enum = @Type(Type{ .Enum = .{ .tag_type = u8, .is_exhaustive = true, .decls = &.{}, .fields = enum_fields[0..2] } });
    std.debug.print("{}", .{custom_enum.field1});
}

[mlugg]

Thanks for the examples, everyone. I'm getting my branch into a working state now, and will be testing the given examples shortly.