`create` does not work for UncheckedArray, as sizeof(UncheckedArray[T])==0

[shirleyquirk]

Example

proc mutate(x:var float) = x = 3.9

proc a(degree:int) =
  echo "a: ",degree
  var x = create(UncheckedArray[float],degree)      #second call returns the same pointer,
  #var x = cast[ptr UncheckedArray[float]](alloc0(sizeof(float)*degree)) #works
  echo x.repr

  doAssert(x[0]==0.0)
  mutate(x[0])
  x.dealloc()

proc main() =
  a(3)
  a(7)
  a(3)

main()

Current Output

a: 3
ptr 0x7fe7c52e5048 --> [...]

a: 7
ptr 0x7fe7c52e5048 --> [...]

/usercode/in.nim(21)     in
/usercode/in.nim(18)     main
/usercode/in.nim(12)     a
/playground/nim/lib/system/assertions.nim(29) failedAssertImpl
/playground/nim/lib/system/assertions.nim(22) raiseAssert
/playground/nim/lib/system/fatal.nim(49) sysFatal
Error: unhandled exception: /usercode/in.nim(12, 11) `x[0] == 0.0` 3.9 [AssertionError]

Expected Output

a: 3
ptr 0x7fad8198f070 --> [...]

a: 7
ptr 0x7fad81990120 --> [...]

a: 3
ptr 0x7fad8198f160 --> [...]

with --gc:arc the above work

Nim Compiler Version 1.5.1 [Linux: amd64]
Compiled at 2021-06-20
Copyright (c) 2006-2021 by Andreas Rumpf

git hash: ad5063aed101f0102fb867f0b4e518ede7fa188b
active boot switches: -d:release

[shirleyquirk]

silly me. sizeof(UncheckedArray[T]) == 0

[shirleyquirk]

Solution:

proc create[T](x:typedesc[UncheckedArray[T]],size:int):ptr UncheckedArray[T]{.inline,benign,raises:[].} = 
  cast[ptr UncheckedArray[T]](alloc0(sizeof(T)*size))

or make sizeof(UncheckedArray[T]) == sizeof(T) ??? don't know what that would break

[demotomohiro]

Documentation comment of create says:

Allocates a new memory block with at least T.sizeof * size bytes.

So I think when create is called with T such that sizeof(T) == 0, it should be compile time error. Because T.sizeof * size is always 0 and create cannot allocate a new memory block that you actually needs.

Making sizeof(UncheckedArray[T]) != 0 can be a breaking change. For example, payloadCheck() proc in tests/misc/tsizeof.nim become failed.

From https://nim-lang.org/docs/manual.html#types-unchecked-arrays

Additionally, an unchecked array is translated into a C array of undetermined size:

In C, C array of undetermined size in a struct is count as 0 bytes.

#include <stdio.h>
#include <stdlib.h>

typedef struct {
  int len;
  int cap;
  int data[];
} MySeq;

int main(void)
{
    printf("sizeof(int): %ld, sizeof(MySeq):%ld\n", sizeof(int), sizeof(MySeq));
    return EXIT_SUCCESS;
}

Output:

sizeof(int): 4, sizeof(MySeq): 8

I think you should create a new proc like proc createUncheckedArray(T: typedesc; size = 1.Positive): ptr UncheckedArray[T] if you really need such a proc.

[shirleyquirk]

Aha, put like that it's very clear that sizeof(UncheckedArray[T]) should be 0

Having just shot myself in the foot, I'm pro a static assert that create isn't returning a footgun, but I guess that would be a breaking change too. Maybe a warning?

If a proc returning a ptr UncheckedArray were to exist in the stdlib, my bike shedding stance is that an overload on create could never be ambiguous, and would be intuitive.

[metagn]

cast[ptr UncheckedArray[float]](create(float, degree)) should be another workaround, maybe even the intended way? Maybe create with a size parameter should return ptr UncheckedArray[T] instead?

[juancarlospaco]

Should have at least 1 test.