Prevent =copy if return value is the last use

[derekdai]

Related to this forum post.

Example

from strformat import `&`

type X = distinct int

proc `=sink`(p1: var X; p2: X) = echo &"=sink {cast[int](p2)}"
proc `=copy`(p1: var X; p2: X) = echo &"=copy {cast[int](p2)}"
proc `=destroy`(p: var X) = echo &"=destroy {cast[int](p)}"

proc main =
  echo "~~~~~~~~~~"
  block:
    proc byLet(v: int): owned X =
      let x = X(v)
      x
    discard byLet(1)
  echo "~~~~~~~~~~"
  
  block:
    proc byVar(v: int): owned X =
      var x = X(v)
      x
    discard byVar(2)
  echo "~~~~~~~~~~"
  
  block:
    proc byCast(v: int): owned X = cast[X](v)
    discard byCast(3)
  echo "~~~~~~~~~~"
  
  block:
    proc byConv(v: int): owned X = (X)(v)
    discard byConv(4)
  echo "~~~~~~~~~~"
  
  block:
    proc byVarMove(v: int): owned X =
      var x = X(v)
      move(x)
    discard byVarMove(5)
  echo "~~~~~~~~~~"
  
  block:
    proc byVarWasMoved(v: int): owned X =
      var x = X(v)
      result = x
      wasMoved(x)
    discard byVarWasMoved(6)
  echo "~~~~~~~~~~"

main()

Current Output

~~~~~~~~~~
=copy 1
=destroy 0
~~~~~~~~~~
=copy 2
=destroy 0
~~~~~~~~~~
=copy 3
=destroy 0
~~~~~~~~~~
=destroy 4
~~~~~~~~~~
=destroy 0
=destroy 5
~~~~~~~~~~
=copy 6
=destroy 0
~~~~~~~~~~

Expected Output

~~~~~~~~~~
=destroy 0
~~~~~~~~~~
=destroy 0
~~~~~~~~~~
=destroy 0
~~~~~~~~~~
=destroy 4
~~~~~~~~~~
=destroy 0
=destroy 5
~~~~~~~~~~
=destroy 0
~~~~~~~~~~

Possible Solution

Additional Information

Nim Compiler Version 1.6.0 [Linux: amd64]
Compiled at 2021-10-19
Copyright (c) 2006-2021 by Andreas Rumpf

git hash: 727c6378d2464090564dbcd9bc8b9ac648467e38
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