alternative, much simpler algorithm for strict func checking (#21066)

* alternative, much simpler algorithm for strict func checking

* forgot to git add new compiler module

* new spec is incredibly simple to describe

* fixes bigints regression

* typos

* closes #16305; closes #17387; closes #20863

tests/effects/tfuncs_cannot_mutate.nim

@@ -1,9 +1,6 @@
 discard """
-  errormsg: "'mutate' can have side effects"
-  nimout: '''an object reachable from 'n' is potentially mutated
-tfuncs_cannot_mutate.nim(39, 15) the mutation is here
-tfuncs_cannot_mutate.nim(37, 7) is the statement that connected the mutation to the parameter
-'''
+  errormsg: "cannot mutate location select(x, z).data within a strict func"
+  line: 35
 """
 
 {.experimental: "strictFuncs".}
@@ -25,8 +22,7 @@ func len(n: Node): int =
     it = it.ri
 
 func doNotDistract(n: Node) =
-  var m = Node()
-  m.data = "abc"
+  var m = Node(data: "abc")
 
 func select(a, b: Node): Node = b
 

tests/effects/tfuncs_cannot_mutate2.nim

@@ -1,9 +1,6 @@
 discard """
-  errormsg: "'copy' can have side effects"
-  nimout: '''an object reachable from 'y' is potentially mutated
-tfuncs_cannot_mutate2.nim(15, 7) the mutation is here
-tfuncs_cannot_mutate2.nim(13, 10) is the statement that connected the mutation to the parameter
-'''
+  errormsg: "cannot mutate location x[0].a within a strict func"
+  line: 12
 """
 
 {.experimental: "strictFuncs".}

tests/effects/tfuncs_cannot_mutate_simple.nim

@@ -1,7 +1,6 @@
 discard """
-  errormsg: "'edit' can have side effects"
-  nimout: '''an object reachable from 'x' is potentially mutated
-tfuncs_cannot_mutate_simple.nim(16, 4) the mutation is here'''
+  errormsg: '''cannot mutate location x.data within a strict func'''
+  line: 15
 """
 
 {.experimental: "strictFuncs".}

tests/effects/tstrictfuncs_misc.nim

@@ -0,0 +1,65 @@
+discard """
+  action: compile
+"""
+
+{.experimental: "strictFuncs".}
+
+func sortedFake1[T](a: openArray[T]): seq[T] =
+  for i in 0 .. a.high: result.add a[i]
+func sortedFake2[T](a: openArray[T]): seq[T] =
+  result = newSeq[T](a.len)
+  for i in 0 .. a.high: result[i] = a[i]
+type Foo1 = object
+type Foo2 = ref object
+block:
+  let a1 = sortedFake1([Foo1()]) # ok
+  let a2 = sortedFake1([Foo2()]) # ok
+block:
+  let a1 = sortedFake2([Foo1()]) # ok
+  let a2 = sortedFake2([Foo2()]) # error: Error: 'sortedFake2' can have side effects
+
+
+import std/sequtils
+type Foob = ref object
+  x: int
+let a1 = zip(@[1,2], @[1,2]) # ok
+let a2 = zip(@[Foob(x: 1)], @[Foob(x: 2)]) # error in 1.6.0 RC2, but not 1.4.x
+
+
+# bug #20863
+type
+  Fooc = ref object
+
+func twice(foo: Fooc) =
+  var a = newSeq[Fooc](2)
+  a[0] = foo # No error.
+  a[1] = foo # Error: 'twice' can have side effects.
+
+let foo = Fooc()
+twice(foo)
+
+# bug #17387
+import json
+
+func parseColumn(columnNode: JsonNode) =
+  let columnName = columnNode["name"].str
+
+parseColumn(%*{"a": "b"})
+
+type
+  MyTable = object
+    data: seq[int]
+
+  JsonNode3 = ref object
+    fields: MyTable
+
+proc `[]`(t: var MyTable, key: string): var int =
+  result = t.data[0]
+
+proc `[]`(x: JsonNode3, key: string): int =
+  result = x.fields[key]
+
+func parseColumn(columnNode: JsonNode3) =
+  var columnName = columnNode["test"]
+
+parseColumn(JsonNode3())