varpartitions: add support for if/try/case paths

[alaviss]

Summary

This commit adds basic support for paths in conditional cases. Conditionals are considered valid paths when all branches of execution that does not terminate execution yields the same path.

Details

• info that couldn't fit into the summary
• relevant details; tricky parts
• anything else

Fixes full_issue_url

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Notes for Reviewers

• Right now this is for my own experimentation, so progress is gonna be slow as I play around with this.

Todo list:

• Write tests
• Update experimental docs

[saem]

Just some initial thoughts

[saem]

Any trailing finally should be removed from the iteration since it's not part of the expression, as in it will always be a statement, right?

[saem]

They're either noreturn or unit, the difference being that the latter doesn't "abort" the current expression like the former does. You might need to rely on the endsInNoReturn analysis.

[saem]

Should this dig into nkBlock expression and statement varieties?

[zerbina]

In my opinion, extending the path expression specification to have if cond: x else: x be considered a path expression is unnecessarily (in terms of implementation effort) and also not very useful).

A simpler yet more useful solution would be to allow block/if/case/try expressions in a view creation context. Implementation-wise, for varpartitions, this would amount to just adding the following procedure to varpartitions:

proc borrowFromComplex(c: var Partitions; dest: PSym, src: PNode) =
  ## Borrows from all relevant sub-expressions of `src`.
  case src.kind
  of nkStmtListExpr, nkBlockExpr, nkExceptBranch:
    borrowFromComplex(c, dest, src[^1])
  of nkIfExpr:
    for it in src.items:
      borrowFromComplex(c, dest, it[^1])
  of nkCaseStmt:
    for (_, it) in branches(src):
      borrowFromComplex(c, dest, it[^1])
  of nkTryStmt:
    for i in 0..<(src.len - ord(src[^1].kind == nkFinally)):
      borrowFromComplex(c, dest, src[i])
  of nkHiddenAddr:
    # currently necessary to skip the outermost hidden-addr
    borrowFromComplex(c, dest, src[0])
  elif endsInNoReturn(src):
    discard "nothing to do"
  else:
    borrowFrom(c, dest, src)

and then changing

nimskull/compiler/sem/varpartitions.nim

Line 594 in 677964c

borrowFrom(c, dest.sym, src)

to

borrowFromComplex(c, dest.sym, src)

borrowFrom is additive, so to speak, so calling it multiple times means that multiple locations are borrowed from. This nets you support for the let x: lent T = if cond: a else: b case.

You'll still get a run-time or compiler crash when doing so, since if, block, etc. are npt expected to appear as an nkHiddenAddr operand. I can quickly make a PR with the necessary preparations, if you want.