track introduced locals in vmgen for eval check (#24674)

fixes #8758, fixes #10828, fixes #12172, fixes #21610, fixes #23803,
fixes #24633, fixes #24634, succeeds #24085

We simply track the symbol ID of every traversed `var`/`let` definition
in `vmgen`, then these symbols are always considered evaluable in the
current `vmgen` context. The set of symbols is reset before every
generation, but both tests worked properly without doing this including
the nested `const`, so maybe it's already done in some way I'm not
seeing.

(cherry picked from commit a5cc33c1d3)

compiler/vm.nim

@@ -12,7 +12,7 @@
 
 import semmacrosanity
 import
-  std/[strutils, tables, parseutils],
+  std/[strutils, tables, intsets, parseutils],
   msgs, vmdef, vmgen, nimsets, types,
   parser, vmdeps, idents, trees, renderer, options, transf,
   gorgeimpl, lineinfos, btrees, macrocacheimpl,
@@ -2425,9 +2425,12 @@ proc evalConstExprAux(module: PSym; idgen: IdGenerator;
   setupGlobalCtx(module, g, idgen)
   var c = PCtx g.vm
   let oldMode = c.mode
+  let oldLocals = c.locals
   c.mode = mode
+  c.locals = initIntSet()
   c.cannotEval = false
   let start = genExpr(c, n, requiresValue = mode!=emStaticStmt)
+  c.locals = oldLocals
   if c.cannotEval:
     return errorNode(idgen, prc, n)
   if c.code[start].opcode == opcEof: return newNodeI(nkEmpty, n.info)

compiler/vmdef.nim

@@ -10,7 +10,7 @@
 ## This module contains the type definitions for the new evaluation engine.
 ## An instruction is 1-3 int32s in memory, it is a register based VM.
 
-import std/[tables, strutils]
+import std/[tables, strutils, intsets]
 
 import ast, idents, options, modulegraphs, lineinfos
 
@@ -272,6 +272,7 @@ type
     vmstateDiff*: seq[(PSym, PNode)] # we remember the "diff" to global state here (feature for IC)
     procToCodePos*: Table[int, int]
     cannotEval*: bool
+    locals*: IntSet
 
   PStackFrame* = ref TStackFrame
   TStackFrame* {.acyclic.} = object

compiler/vmgen.nim

@@ -1583,6 +1583,7 @@ proc checkCanEval(c: PCtx; n: PNode) =
     # are in the right scope:
     if sfGenSym in s.flags and c.prc.sym == nil: discard
     elif s.kind == skParam and s.typ.kind == tyTypeDesc: discard
+    elif s.kind in {skVar, skLet} and s.id in c.locals: discard
     else: cannotEval(c, n)
   elif s.kind in {skProc, skFunc, skConverter, skMethod,
                   skIterator} and sfForward in s.flags:
@@ -1975,7 +1976,7 @@ proc genVarSection(c: PCtx; n: PNode) =
       c.gen(lowerTupleUnpacking(c.graph, a, c.idgen, c.getOwner))
     elif a[0].kind == nkSym:
       let s = a[0].sym
-      checkCanEval(c, a[0])
+      c.locals.incl(s.id)
       if s.isGlobal:
         let runtimeAccessToCompileTime = c.mode == emRepl and
               sfCompileTime in s.flags and s.position > 0