let changes value after initialization when initialized from loop iterator

[etan-status]

Description

var xs: seq[proc()]

for mutableVal in 1 .. 5:
  let immutableVal = mutableVal
  xs.add proc() = echo $immutableVal

for x in xs:
  x()

Nim Version

% nim -v
Nim Compiler Version 2.0.14 [MacOSX: arm64]
Compiled at 2025-01-06
Copyright (c) 2006-2023 by Andreas Rumpf

git hash: bf4de6a
active boot switches: -d:release

Current Output

5
5
5
5
5

Expected Output

1
2
3
4
5

Known Workarounds

Use closureScope around the let immutableVal = mutableVal to ensure that let contents don't change after initialization.

Additional Information

Both in --mm:refc and --mm:orc.

[etan-status]

Reason being that immutableVal shares the memory location across all loop iterations, as evident in

  echo $repr(cast[pointer](mutableVal.unsafeAddr))
  echo $repr(cast[pointer](immutableVal.unsafeAddr))

It's a bit like a use-after-free issue where the inner procs point to memory that has become invalid and now points somewhere else (to the new value), and we are just lucky that other code doesn't end up in those variables.

Maybe compiler should check lifetime analysis and automatically inject closureScope in that case or put error / warning if user tries to use a variable in an escaping way.