lent parameter: invalid C codegen

[mratsim]

While trying to work around #16897 I got the compiler to generate invalid C with lent input parameters.

Test case

type
  Fp[N: static int, T] = object
    big: array[N, T]

type
  QuadraticExt* = concept x
    ## Quadratic Extension concept (like complex)
    type BaseField = auto
    x.c0 is BaseField
    x.c1 is BaseField

{.experimental:"views".}

func prod(r: var QuadraticExt, a, b: lent QuadraticExt) =
  discard

type
  Fp2[N: static int, T] = object
    c0, c1: Fp[N, T]

# This should be passed by reference,
# but concepts do not respect the 24 bytes rule
# or `byref` pragma.
var r, a, b: Fp2[6, uint64]

prod(r, a, b)

The proc is defined with pointers:

/* section: NIM_merge_PROCS */
N_LIB_PRIVATE N_NIMCALL(void, prod__j3ijsYk5xsFR79akOCPoMqg)(tyObject_Fp2__SVgH9c6gmNsJfyYVtl0YdSQ* r, tyObject_Fp2__SVgH9c6gmNsJfyYVtl0YdSQ* a, tyObject_Fp2__SVgH9c6gmNsJfyYVtl0YdSQ* b) {
}

but is called with values

N_LIB_PRIVATE N_NIMCALL(void, NimMainModule)(void) {
{
	prod__j3ijsYk5xsFR79akOCPoMqg((&r__mSc0P1JKql3o5d2mQ9bx9cMQ), a__IakjUEvgThFFu4dR6kEN0A, b__4KxaakBDXQrcdXChd3FntQ);
}
}

Compiler error

[...]/nimcache/concept_param/@mconcept_param.nim.c: In function ‘NimMainModule’:
[...]/nimcache/concept_param/@mconcept_param.nim.c:99:64: error: incompatible type for argument 2 of ‘prod__j3ijsYk5xsFR79akOCPoMqg’
   99 |  prod__j3ijsYk5xsFR79akOCPoMqg((&r__mSc0P1JKql3o5d2mQ9bx9cMQ), a__IakjUEvgThFFu4dR6kEN0A, b__4KxaakBDXQrcdXChd3FntQ);
      |                                                                ^~~~~~~~~~~~~~~~~~~~~~~~~
      |                                                                |
      |                                                                tyObject_Fp2__SVgH9c6gmNsJfyYVtl0YdSQ
[...]/nimcache/concept_param/@mconcept_param.nim.c:56:143: note: expected ‘tyObject_Fp2__SVgH9c6gmNsJfyYVtl0YdSQ *’ but argument is of type ‘tyObject_Fp2__SVgH9c6gmNsJfyYVtl0YdSQ’
   56 | ALL(void, prod__j3ijsYk5xsFR79akOCPoMqg)(tyObject_Fp2__SVgH9c6gmNsJfyYVtl0YdSQ* r, tyObject_Fp2__SVgH9c6gmNsJfyYVtl0YdSQ* a, tyObject_Fp2__SVgH9c6gmNsJfyYVtl0YdSQ* b) {
      |                                                                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^

[...]/nimcache/concept_param/@mconcept_param.nim.c:99:91: error: incompatible type for argument 3 of ‘prod__j3ijsYk5xsFR79akOCPoMqg’
   99 |  prod__j3ijsYk5xsFR79akOCPoMqg((&r__mSc0P1JKql3o5d2mQ9bx9cMQ), a__IakjUEvgThFFu4dR6kEN0A, b__4KxaakBDXQrcdXChd3FntQ);
      |                                                                                           ^~~~~~~~~~~~~~~~~~~~~~~~~
      |                                                                                           |
      |                                                                                           tyObject_Fp2__SVgH9c6gmNsJfyYVtl0YdSQ
[...]/nimcache/concept_param/@mconcept_param.nim.c:56:185: note: expected ‘tyObject_Fp2__SVgH9c6gmNsJfyYVtl0YdSQ *’ but argument is of type ‘tyObject_Fp2__SVgH9c6gmNsJfyYVtl0YdSQ’
   56 | oMqg)(tyObject_Fp2__SVgH9c6gmNsJfyYVtl0YdSQ* r, tyObject_Fp2__SVgH9c6gmNsJfyYVtl0YdSQ* a, tyObject_Fp2__SVgH9c6gmNsJfyYVtl0YdSQ* b) {
      |                                                                                           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^

Error: execution of an external compiler program 'gcc -c  -w -fmax-errors=3 -O3 -fno-strict-aliasing -fno-ident   -I'[...]/.choosenim/toolchains/nim-#devel/lib' -I[...]/build -o [...]/nimcache/concept_param/@mconcept_param.nim.c.o [...]/nimcache/concept_param/@mconcept_param.nim.c' failed with exit code: 1

[Araq]

func prod(r: var QuadraticExt, a, b: lent QuadraticExt) is not a valid declaration. lent is for return types.

[mratsim]

In that case I think the compiler should refuse to compile.

[Araq]

Of course but it's more important for me to unblock you...

[mratsim]

I'm not blocked, I was trying a workaround just in case it worked.

[konsumlamm]

func prod(r: var QuadraticExt, a, b: lent QuadraticExt) is not a valid declaration. lent is for return types.

What is the reasoning behind not allowing lent parameters? (Fwiw, you can also have var and openArray parameters, or are these different somehow?) I imagine it to be useful for guaranteeing that the parameter is only a view of some object and thus cannot be modified (even if it's a view for a ref object). Or is there some alternative/workaround that allows something like that? (I know that func/{.noSideEffect.} disallows side effects, including writing to ref parameters, but that can't be applied to individual parameters afaict.)

[Araq]

a, b: QuadraticExt already is the immutable view, no need for lent. However, your extension wrt ref object makes some sense. But in general I don't like re-using type modifiers or keywords in an ad-hoc fasion.