Modifying slices with comptime-only types in comptime

[AMythicDev]

Zig Version

0.14.0-dev.3367+1cc388d52

Steps to Reproduce and Observed Output

This code:

const std = @import("std");

const TestStruct = struct { field1: u32, field2: f64 };

pub fn main() !void {
    const t = TestStruct{ .field1 = 10, .field2 = 3.14 };

    const tinfo = @typeInfo(@TypeOf(t));

    comptime var fields = @constCast(tinfo.@"struct".fields);
    const tmp = fields[0];
    fields[0] = fields[1];
    fields[1] = tmp;
}

gives the following error message:

test.zig:12:15: error: cannot store comptime-only type 'builtin.Type.StructField' at runtime
    fields[0] = fields[1];
    ~~~~~~~~~~^~~~~~~~~~~
test.zig:12:11: note: operation is runtime due to this pointer
    fields[0] = fields[1];
    ~~~~~~^~~
referenced by:
    posixCallMainAndExit: .zigverm/installs/zig-x86_64-linux-master/lib/std/start.zig:656:37
    _start: .zigverm/installs/zig-x86_64-linux-master/lib/std/start.zig:464:40
    3 reference(s) hidden; use '-freference-trace=5' to see all references

Expected Output

Here I am @constCasting the tinfo.@"struct".fields slice which should not be done.
Instead, the solution to this is to first copy the tinfo.@"struct".fields into a separate buffer and then do the swap operations on it.

I think it would be helpful if the error message pointed out something better than whatever vague it is giving right now.

I have discussed this issue with other Zig community members and they seem to agree that the error message should be improved for this one.