Nil type check implementation (#15287)

* Nil checking
* Enable current older not nil checking again, run new checking only under flag, skip our test
* Enable tests, work on try/except and bugs, fix notnil tests

* Enable strictNotNil tests (currently with lowercase category) and add some expected output
* Work on try/except/finally: still some things unclear and a lot of code can raise out of try
* Fix the notnil build by going back to the old version of a test which I shouldn't have changed

* Fix test : use action compile
* Work on mutation and aliasing: not finished
* Render var parititions graph, try to understand it, fix a nilcheck if bug
* Rebase, progress on working with partitions
* Improve time logic
* Fix some bugs, use graph indices instead of symbol in nil map
* Fix bugs, test simpler ident aliasing for now, support two mutation levels
* Support ContentMutation and ReAssignment: for now just detect possible re assignment for var parameters of calls
* Enable several simple passing tests
* Cleanup a bit, fix condition/branch infix-related bug
* Remove some files, address some comments by Araq
* Use internalError and no quit for now
* Separate tests with expected warnings and with expected ok, fix a bug with if with a single branch related to copyMap
* Fix new data structures, bugs: make tests pass, disable some for now
* Work on fixing errors with non-sym nodes, aliasing: tests fail
* Work on alias support: simple set-based logic, todo more tests and ref sets?
* Use ref sets: TODO can we think of handle seq-s similar to varpartitions' Araq ones
* Handle defers in one place, stop raising in reverse to make an async test compile with strictNotNil, add a commented out test
* Dot expressions: call/reassignment. Other refactorings and distinct, SeqOfDistinct support. Checkout an older varpartitions
* Work on field tracking
* Backup : trying to fix bugs when running some stdlib stuff for running an async test
* Start a section about strict not nil checking in experimental manual
* Fix experimental strict not nil manual section and move it to another file based on Araq feedback
* Fix unstructured flow and double warning problems, fix manual, cleanup
* Fix if/elif/else : take in account structure according to Araq feedback
* Refactor a bit
* Work on bracket expr support, re-enable tests, clarify in manual/tests/implementation static index support for now
* Work on compiling stdlib and compiler with strictNotNil
* Small fixes to the manual for strictNotNil
* Fix idgen for strict check nil rebase
* Enable some simple tests, remove old stuff, comment out code/print
* Copy the original varpartitions source instead of my changes
* Remove some files

compiler/ast.nim

@@ -1004,7 +1004,7 @@ const
   ConstantDataTypes*: TTypeKinds = {tyArray, tySet,
                                     tyTuple, tySequence}
   NilableTypes*: TTypeKinds = {tyPointer, tyCString, tyRef, tyPtr,
-    tyProc, tyError}
+    tyProc, tyError} # TODO
   PtrLikeKinds*: TTypeKinds = {tyPointer, tyPtr} # for VM
   ExportableSymKinds* = {skVar, skConst, skProc, skFunc, skMethod, skType,
     skIterator,
@@ -1387,6 +1387,7 @@ proc newType*(kind: TTypeKind, id: ItemId; owner: PSym): PType =
       echo "KNID ", kind
       writeStackTrace()
 
+
 proc mergeLoc(a: var TLoc, b: TLoc) =
   if a.k == low(typeof(a.k)): a.k = b.k
   if a.storage == low(typeof(a.storage)): a.storage = b.storage
@@ -1952,9 +1953,13 @@ proc canRaise*(fn: PNode): bool =
   elif fn.kind == nkSym and fn.sym.magic == mEcho:
     result = true
   else:
-    result = fn.typ != nil and fn.typ.n != nil and ((fn.typ.n[0].len < effectListLen) or
-      (fn.typ.n[0][exceptionEffects] != nil and
-      fn.typ.n[0][exceptionEffects].safeLen > 0))
+    # TODO check for n having sons? or just return false for now if not
+    if fn.typ != nil and fn.typ.n != nil and fn.typ.n[0].kind == nkSym:
+      result = false
+    else:
+      result = fn.typ != nil and fn.typ.n != nil and ((fn.typ.n[0].len < effectListLen) or
+        (fn.typ.n[0][exceptionEffects] != nil and
+        fn.typ.n[0][exceptionEffects].safeLen > 0))
 
 proc toHumanStrImpl[T](kind: T, num: static int): string =
   result = $kind