compiler crash ptrCast'ing optional slice to allowzero-many-pointer about zig HOT 4 CLOSED

update: no allowzero also crashed

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this variation crashes the same way on master but passes on 0.11. the original code snippet also segfaults on 0.11.

const std = @import("std");

test {
    try foo(null);
}

const Color = struct {
    r: u8,
    g: u8,
    b: u8,
    a: u8,
};

fn foo(pixels: ?[]const Color) !void {
    const data: [*]allowzero const u8 = @ptrCast(pixels);
    const int = @intFromPtr(data);
    try std.testing.expect(int == 0);
}

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Here's what I've managed to figure out:

1. In ptrCastFull, pointer type information about the source and destination types is retrieved through Type.ptrInfo
2. Type.ptrInfo checks for pointer types wrapped in optional types, and retrieves the underlying InternPool.Key.PtrType
3. When checking if the source type is a slice and the dest type is a many, C or one pointer, this is done by using the PtrType which does not know whether either type is an option
4. Finally, analyzeSlicePtr is called and assumes its receiving a slice that isn't optional, resulting in a crash

Working on a PR...

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#18981

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