assign2: compile-time assignment support (#234)

stew/assign2.nim

@@ -24,16 +24,29 @@ func assign*[T](tgt: var openArray[T], src: openArray[T]) =
     raiseAssert "Target and source lengths don't match: " &
       $tgt.len & " vs " & $src.len
 
+  when nimvm:
+    for i in 0..<tgt.len:
+      tgt[i] = src[i]
+  else:
     assignImpl(tgt, src)
 
 func assign*[T](tgt: var seq[T], src: openArray[T]) =
   mixin assign
 
   tgt.setLen(src.len)
+
+  when nimvm:
+    for i in 0..<tgt.len:
+      tgt[i] = src[i]
+  else:
     assignImpl(tgt.toOpenArray(0, tgt.high), src)
 
 func assign*(tgt: var string, src: string) =
   tgt.setLen(src.len)
+  when nimvm:
+    for i in 0..<tgt.len:
+      tgt[i] = src[i]
+  else:
     assignImpl(tgt.toOpenArrayByte(0, tgt.high), src.toOpenArrayByte(0, tgt.high))
 
 macro unsupported(T: typed): untyped =
@@ -44,7 +57,9 @@ func assign*[T](tgt: var T, src: T) =
   # is ridiculously slow. When syncing, the application was spending 50%+ CPU
   # time in it - `assign`, in the same test, doesn't even show in the perf trace
   mixin assign
-
+  when nimvm:
+    tgt = src
+  else:
     when supportsCopyMem(T):
       when sizeof(src) <= sizeof(int):
         tgt = src

tests/test_assign2.nim

@@ -10,6 +10,9 @@ import
   unittest2,
   ../stew/assign2
 
+proc makeCopy(a: array[2, byte]): array[2, byte] =
+  assign(result, a)
+
 suite "assign2":
   test "basic":
     type X = distinct int
@@ -39,3 +42,7 @@ suite "assign2":
     assign(dis, X(55))
 
     check: int(dis) == 55
+
+    const x = makeCopy([byte 0, 2]) # compile-time evaluation
+    check x[1] == 2
+