Field checks for everybody (#8957)

* Field checks for JS backend

* Clean nkCall nodes with no arguments

Generating a nkEmpty in place of no arguments makes no sense form the
AST point of view and also trips up the VM codegen.

* Field checks for VM backend

* Test case for #6612

This patchset fixes #6612

* Add test case for LHS double evaluation

* Prevent LHS double-eval for JS backend

* Prevent double evaluation in VM backend

tests/js/t6612.nim

@@ -0,0 +1,24 @@
+discard """
+  action: "run"
+"""
+
+proc fillWith(sq: var seq[int], n: int, unused: string) =
+  sq = @[n]
+
+type
+  Object = object of RootObj
+    case hasNums: bool
+    of true:
+      numbers: seq[int]
+    of false:
+      discard
+    always: seq[int]
+
+var obj = Object(hasNums: true)
+
+obj.always.fillWith(5, "unused")
+doAssert obj.always == @[5]
+
+obj.numbers.fillWith(3, "unused")
+doAssert obj.numbers == @[3]
+doAssert obj.always == @[5]

tests/js/tfieldchecks.nim

@@ -0,0 +1,48 @@
+discard """
+  output: '''
+foo
+C
+3.14
+foo
+3.14
+3.14
+'''
+"""
+
+type
+  V = enum
+    A, B, C
+  X = object
+    f0: string
+    case f1: V
+    of A: f2: string
+    of B: discard
+    of C: f3: float
+
+var obj = X(f0: "foo", f1: C, f3: 3.14)
+
+block:
+  echo obj.f0
+  echo obj.f1
+  doAssertRaises(FieldError): echo obj.f2
+  echo obj.f3
+
+block:
+  let a0 = addr(obj.f0)
+  echo a0[]
+  # let a1 = unsafeAddr(obj.f1)
+  # echo a1[]
+  doAssertRaises(FieldError):
+    let a2 = addr(obj.f2)
+    echo a2[]
+  let a3 = addr(obj.f3)
+  echo a3[]
+
+# Prevent double evaluation of LHS
+block:
+  var flag = false
+  proc wrap(x: X): X =
+    doAssert flag == false
+    flag = true
+    result = x
+  echo wrap(obj).f3