Possibly missing pointer cast RLS inference?

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1

It seems that @ptrCast doesn’t see through reading thus requires the use of @as() in this case.

test {
    var x: [4]u8 = "allo".*;
    const y: u32 = @ptrCast(x).*;
    _ = y;
}

Or is it intentionally omitted from inference?

2

I don’t believe this is possible as there is an ambiguity. This @ptrCast could be interpreted in 2 ways:

  1. Cast x to *u8, read the 'a' character and enlarge it implicitly to u32;
  2. Cast x to *u32*, read the 4 bytes as an u32 integer.

For both of these, there are better ways to do it:

  1. Simply read x[0];
  2. Use std.mem.bytesAsValue(u32, x) (with maybe std.mem.bigToNative to convert endianness if necessary).
3 Likes
3

In general, there’s also possible alignment issues here too.

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4

There’s std.mem.readIntNative/readIntLittle/readIntBig for this.

const y = std.mem.readIntNative(u32, &x);
1 Like
5

Yep, and as you can see in the implementation:

pub fn readIntNative(comptime T: type, bytes: *const [@divExact(@typeInfo(T).Int.bits, 8)]u8) T {
    return @as(*align(1) const T, @ptrCast(bytes)).*;
}

It’s just an aligned cast.