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

tests/vm/tconststaticvar.nim

@@ -0,0 +1,79 @@
+block: # issue #8758
+  template baz() =
+    var i = 0
+
+  proc foo() =
+    static:
+      var i = 0
+      baz()
+
+block: # issue #10828
+  proc test(i: byte): bool =  
+    const SET = block: # No issues when defined outside proc
+      var s: set[byte]
+      for i in 0u8 .. 255u8: incl(s, i)
+      s
+    return i in SET
+  doAssert test(0)
+  doAssert test(127)
+  doAssert test(255)
+
+block: # issue #12172
+  const TEST = block:
+    var test: array[5, string]
+    for i in low(test)..high(test):
+      test[i] = $i
+    test
+  proc test =
+    const TEST2 = block:
+      var test: array[5, string] # Error here
+      for i in low(test)..high(test):
+        test[i] = $i
+      test
+    doAssert TEST == TEST2
+    doAssert TEST == @["0", "1", "2", "3", "4"]
+    doAssert TEST2 == @["0", "1", "2", "3", "4"]
+  test()
+
+block: # issue #21610
+  func stuff(): int =
+    const r = block:
+      var r = 1 # Error: cannot evaluate at compile time: r
+      for i in 2..10:
+        r *= i
+      r
+    r
+  doAssert stuff() == 3628800
+
+block: # issue #23803
+  func foo1(c: int): int {.inline.} =
+    const arr = block:
+      var res: array[0..99, int]
+      res[42] = 43
+      res
+    arr[c]
+  doAssert foo1(41) == 0
+  doAssert foo1(42) == 43
+  doAssert foo1(43) == 0
+
+  # works
+  func foo2(c: int): int {.inline.} =
+    func initArr(): auto =
+      var res: array[0..99, int]
+      res[42] = 43
+      res
+    const arr = initArr()
+    arr[c]
+  doAssert foo2(41) == 0
+  doAssert foo2(42) == 43
+  doAssert foo2(43) == 0
+
+  # also works
+  const globalArr = block:
+    var res: array[0..99, int]
+    res[42] = 43
+    res
+  func foo3(c: int): int {.inline.} = globalArr[c]
+  doAssert foo3(41) == 0
+  doAssert foo3(42) == 43
+  doAssert foo3(43) == 0

tests/vm/tconststaticvar2.nim

@@ -0,0 +1,13 @@
+# issue #24634
+
+type J = object
+
+template m(u: J): int =
+  let v = u
+  0
+
+proc g() =
+  const x = J()
+  const _ = m(x)
+
+g()

tests/vm/tconststaticvar3.nim

@@ -0,0 +1,9 @@
+# issue #24633
+
+import std/sequtils
+
+proc f(a: static openArray[int]) =
+  const s1 = a.mapIt(it)
+  const s2 = a.toSeq()
+
+f([1,2,3])

tests/vm/tconststaticvar_wrong.nim

@@ -0,0 +1,6 @@
+proc test =
+  const TEST = block:
+    let i = 1
+    const j = i + 1 #[tt.Error
+              ^ cannot evaluate at compile time: i]#
+    j