Pointer (even if it's const) can be reassigned to non-pointer

[ghost]

This nonsense code successfully compiles:

const Struct = struct { field: u32 };
pub fn main() void {
    var val = Struct { .field = 1 };
    const ptr = &val;
    ptr = Struct { .field = 2 };

    @import("std").debug.assert(ptr == &val);
}

It will correctly fail if you change the struct to a plain u32, or if you try to reassign ptr to a pre-existing variable instead of a literal.

A variation - this segfaults the compiler:

const Struct = struct { field: u32 };
pub fn main() void {
    // val is now const, ptr is now var
    const val = Struct { .field = 1 };
    var ptr = &val;
    ptr = Struct { .field = 2 };
}   

[momumi]

Another variant using slices:

const warn = @import("std").debug.warn;

pub fn main() anyerror!void {
    var odd_nums = [3]u8 {1,3,5};
    var even_nums = [3]u8 {2,4,6};
    const slice: []u8 = even_nums[0..];
    slice[0] = 0;
    slice = odd_nums[0..]; // should be invalid, slice is const
    slice[0] = 0;

    warn("odds : {x}\n", .{ odd_nums }); // prints  000305
    warn("evens: {x}\n", .{ even_nums }); // prints 000406
}

Edit:

I tried reproducing the versions with struct above, but it seems to be fixed now in master. However, the bug still seems to effect slices.

Edit2:

Another example with structs that shouldn't work:

const Struct = struct { field: u32 };
pub fn main() void {
    const val = Struct { .field = 1 };
    const ptr: *const Struct = &val;
    ptr.* = Struct { .field = 2 }; // should be invalid, since ptr points to const data
    // ptr.field = 2; // this fails to compile (correct behavior)

    warn("{}\n", .{ val.field }); // prints 2
}

Edit3:

This code also ignores const without using pointers:

const U = union {
    A: u32,
    B: u32,
};

pub fn main() void {
    const a: U = U{ .A = 0 };

    a = U{ .B = 1 }; // invalid, a is const

    warn("warn: {}\n", .{a.B});
}

The root cause of this bug might be related to copy elision bugs: #4021

Some other bugs where const gets ignored: (#4015, #3800)

[Vexu]

Fixed by #4975.

./build/a.zig:266:18: error: cannot assign to constant
    ptr = Struct { .field = 2 };
                 ^
./build/a.zig:267:18: error: type '*const Struct' does not support struct initialization syntax
    ptr = Struct { .field = 2 };
                 ^
./build/a.zig:267:21: error: cannot assign to constant
    slice = odd_nums[0..]; // should be invalid, slice is const
                    ^
./build/a.zig:297:20: error: cannot assign to constant
    ptr.* = Struct { .field = 2 }; // should be invalid, since ptr points to const data
                   ^
./build/a.zig:270:10: error: cannot assign to constant
    a = U{ .B = 1 }; // invalid, a is const
         ^