Incorrect length for FFI sequence return value

[qsctr]

As a follow-up to the question in #1538, the user reports:

Output parameters seem to have the same issue. For example, if you have this function:

foreign myfunc :
  {n} (fin n)
  => [n][32] -> [n + 1][32]

Cryptol will only take the first n elements of the output from the FFI function.

I am unable to reproduce this. The corresponding C function should have the signature

void myfunc(size_t n, uint32_t * in, uint32_t * out);

as generated by the :generate-foreign-header Cryptol REPL command. With the following C implementation,

void myfunc(size_t n, uint32_t * in, uint32_t * out) {
    for (size_t i = 0; i < n; ++i) {
        out[i] = in[i];
    }
    out[n] = 42;
}

it seems to return the correct result:

Cryptol> :l test.cry
Loading module Cryptol
Loading module Main
Loading dynamic library test.so
Main> :t myfunc
myfunc : {n} (fin n) => [n][32] -> [1 + n][32]
Main> myfunc [1, 2, 3, 4, 5]
[0x00000001, 0x00000002, 0x00000003, 0x00000004, 0x00000005,
 0x0000002a]

If there are more details regarding the situation in which this bug happens, that would be appreciated!

cc @eddywestbrook @weaversa