Coercion to pointer to vector segfaults at runtime rather than catching the alignment issue at comptime

[hmusgrave]

Zig Version

0.12.0-dev.86+197d9a9eb

Steps to Reproduce and Observed Behavior

It looks like coercion involving pointer-of-vector types doesn't have the correct alignment in its calculations. Zig normally catches potentially unsafe coercion at comptime, so it's weird that this sort of code could segfault.

test "segfaults more often than not" {
    var x: [16]f32 = undefined;
    var y: *[1]@Vector(16, f32) = &x;
    for (y) |*v|
        v.* = @splat(0);
}

test "fails at comptime, as it should" {
    var x: [16]f32 = undefined;
    // cast increases pointer alignment
    var y: *[1]@Vector(16, f32) = @ptrCast(&x);
    _ = y;
}

test "also fails at comptime since the coercion doesn't involve vectors" {
    var x: u128 = 0;
    var y: *align(64) u128 = &x;
    _ = y;
}

Expected Behavior

All three of those tests should fail to compile because there's a potentially unsafe coercion from a less aligned type to a more aligned type.

[Vexu]

The array to vector coercion was removed in #25186 and the other two cases now require @alignCast:

a.zig:35:35: error: expected type '*[1]@Vector(16, f32)', found '*[16]f32'
    var y: *[1]@Vector(16, f32) = &x;
                                  ^~
a.zig:35:35: note: pointer type child '[16]f32' cannot cast into pointer type child '[1]@Vector(16, f32)'
a.zig:35:35: note: array of length 16 cannot cast into an array of length 1
a.zig:44:35: error: @ptrCast increases pointer alignment
    var y: *[1]@Vector(16, f32) = @ptrCast(&x);
                                  ^~~~~~~~~~~~
a.zig:44:44: note: '*[16]f32' has alignment '4'
    var y: *[1]@Vector(16, f32) = @ptrCast(&x);
                                           ^~
a.zig:44:35: note: '*[1]@Vector(16, f32)' has alignment '32'
a.zig:44:35: note: use @alignCast to assert pointer alignment
a.zig:50:30: error: expected type '*align(64) u128', found '*u128'
    var y: *align(64) u128 = &x;
                             ^~
a.zig:50:30: note: pointer alignment '16' cannot cast into pointer alignment '64'