--experimental:views: path expressions constraint prevents useful code

[timotheecour]

Example

when true:
  proc bar(x: string): auto =
    x.len

  proc fn(a, b: var string): lent string =
    var ret: ptr string
    if a[0] <= b[0]: ret = a.addr
    else: ret = b.addr
    ret[]

  proc main() =
    var a = "abc"
    var b = "defgh"
    doAssert bar(fn(a, b)) == 3 # no copy is made
  main()

Current Output

nim r --experimental:views main

t12080.nim(11, 5) Error: cannot borrow from
  var ret: ptr string
  if a[0] <= b[0]:
    ret = addr(a)
  else:
    ret = addr(b)
  ret[], it is not a path expression; see https://nim-lang.github.io/Nim/manual_experimental.html#view-types-algorithm-path-expressions for details
      var ret: ptr string

Expected Output

works

Possible Solution

redesign --experimental:views from scratch, it's broken beyond repair; there's a much simpler design that doesn't have any of {.views}'s limitations and allows writing efficient code without getting in your way.

see also the many other issues plaguing {.views.} https://github.com/nim-lang/Nim/labels/view%20types, some of which are not fixable with this design.

Additional Information

1.5.1 8573160a44439ffe3fbbdd354188db9c976a2081

• this causes CI failure in https://github.com/nim-lang/Nim/pull/17500 because of this test: tests/views/tcan_compile_nim.nim which compiler compiler with --experimental:views

reduced case from code referenced in https://github.com/nim-lang/Nim/pull/17500#discussion_r601972687

• similar to https://github.com/nim-lang/Nim/issues/16132

[Araq]

What is this "much simpler design"? Your previous one wasn't it.