swap silently broken for objects inside functions (js target)

[salvipeter]

In the example below, foo should do the same as swap (at least that's what the C target does).

Example

type A = object
    n: int

proc foo(a, b: var A) =
  swap a, b

var a, b: A
a.n = 42
b.n = 1
doAssert a.n == 42
doAssert b.n == 1
a.swap b
doAssert a.n == 1
doAssert b.n == 42
a.foo b
doAssert a.n == 42
doAssert b.n == 1

Additional Information

The above function compiles (basically) to this JavaScript code:

function foo(a, b) {
    var tmp = a;
    a = b;
    b = tmp;
}

However this only swaps the local variables of the function, as JavaScript uses "call by sharing".

In contrast, the C target compiles to

void foo(A *a, A *b) {
    A tmp = *a;
    *a = *b;
    *b = tmp;
}

... where the whole struct is swapped.

• This caused a problem in a pull request for lists.

$ ./nim -v
Nim Compiler Version 1.5.1 [Linux: amd64]
git hash: 61fd19c7e6880d77d20be01d25266e4195e631fa

[timotheecour]

see also workaround mentioned in #16536 (comment) which can potentially serve as basis for fixing swap