destructors: optimize more assignments into moves; also fixes #9294

tests/destructor/tdestructor3.nim

@@ -19,9 +19,12 @@ proc `=`(lhs: var T, rhs: T) =
 proc `=destroy`(v: var T) =
     echo "destroy"
 
+proc use(x: T) = discard
+
 proc usedToBeBlock =
     var v1 : T
     var v2 : T = v1
+    use v1
 
 usedToBeBlock()
 

tests/destructor/tmatrix.nim

@@ -1,6 +1,7 @@
 discard """
-  output: '''after 3 3
-after 3 3
+  output: '''after 2 2
+after 2 2
+after 2 2
 after 2 2'''
 """
 # bug #9263
@@ -107,6 +108,11 @@ proc test3 =
   #    a = a - b
   b = -b + a
 
+proc test4 =
+  # bug #9294
+  var a = matrix(5, 5, 1.0)
+  a = -a + a
+
 test1()
 info()
 
@@ -115,3 +121,6 @@ info()
 
 test3()
 info()
+
+test4()
+info()

tests/destructor/topttree.nim

@@ -90,6 +90,8 @@ proc write(t: opt[Tree]) =
     write stdout, it.data, "\n"
     write(it.ri)
 
+proc use(t: opt[Tree]) = discard
+
 proc main =
   var t: opt[Tree]
   insert t, 60.0
@@ -99,6 +101,7 @@ proc main =
   write t
   let copy = t
   write copy
+  use t
 
 main()
 echo allocCount, " ", deallocCount

tests/destructor/tprevent_assign2.nim

@@ -0,0 +1,48 @@
+discard """
+  errormsg: "'=' is not available for type <Foo>; requires a copy because it's not the last read of 'otherTree'"
+  line: 44
+"""
+
+type
+  Foo = object
+    x: int
+
+proc `=destroy`(f: var Foo) = f.x = 0
+proc `=`(a: var Foo; b: Foo) {.error.} # = a.x = b.x
+proc `=sink`(a: var Foo; b: Foo) = a.x = b.x
+
+proc createTree(x: int): Foo =
+  Foo(x: x)
+
+proc take2(a, b: sink Foo) =
+  echo a.x, " ", b.x
+
+proc allowThis() =
+  var otherTree: Foo
+  for i in 0..3:
+    while true:
+      #if i == 0:
+      otherTree = createTree(44)
+      case i
+      of 0:
+        echo otherTree
+        take2(createTree(34), otherTree)
+      of 1:
+        take2(createTree(34), otherTree)
+      else:
+        discard
+
+proc preventThis() =
+  var otherTree: Foo
+  for i in 0..3:
+    while true:
+      if i == 0:
+        otherTree = createTree(44)
+      case i
+      of 0:
+        echo otherTree
+        take2(createTree(34), otherTree)
+      of 1:
+        take2(createTree(34), otherTree)
+      else:
+        discard