Add unreachable code detection (WIP)

[nae3x]

Fix #599

Hello! This pull request is still in the drafting phase. There are some decisions that still need to be made before the final solution is released.

Changes

• Add unreachable code detection function that triggers in proc_funcdef during deferred_code_proc
• Add related unit tests
• Update it_ret, it_ret_none, and it_ret_tuple in target_sys_test.coco to avoid triggering the unreachable code checker

Examples

# validate-issue-599.coco
def f():
  return 1
  f = 2

With --strict:

Compiling         validate-issue-599.coco ...
CoconutStyleError: found unreachable code after return statement (remove --strict to downgrade to a warning) (line 1 in 'validate-issue-599.coco')
  def f():
Coconut exiting with error: CoconutStyleError

Without --strict:

Compiling         validate-issue-599.coco ...
CoconutSyntaxWarning: found unreachable code after return statement (line 1 in 'validate-issue-599.coco')
  def f():
Compiled to       validate-issue-599.py .

Limitations

• Only checks for code that comes after terminator statements (return, raise) at the top level of function bodies
• Only reports the function definition's line number, not the line number of the unreachable code itself, as source line information is not available at this stage of compilation

Bugs

• Conflicts with yield def syntax, as the compiler appends unreachable code at the end of the function body, causing the checker to falsely trigger

[nae3x]

Hello @evhub. Thank you for the open-source contribution opportunity

Currently I am trying to figure out how to make the detection work with yield def syntax. The way it work right now is that it append the yield statement at the end of the fucntion and cause the checker to falsly trigger.

# validate-issue-599.coco
yield def yield_f():
  return 1

Result:

Compiling         validate-issue-599.coco ...
CoconutStyleError: found unreachable code after return statement (remove --strict to downgrade to a warning) (line 1 in 'validate-issue-599.coco')
  yield def yield_f():
Coconut exiting with error: CoconutStyleError

Complied Python file:

def yield_f():  #1 (line in Coconut source)
    raise _coconut.StopIteration((1))  #2 (line in Coconut source)

    if False:  #3 (line in Coconut source)
        yield  #3 (line in Coconut source)

There are some ideas that I can think of:

1. Fix at the yield def by update keyword_funcdef_handle to insert if False: yield before any terminator statement or after the docstring, so it never appears after a return/raise. This keeps the detector simple but changes the shape of compiled output for yield def.

2. Fix at the detector by update detect_unreachable_code to distinguish compiler-generated lines from user-written lines and skip them. I am not sure if this is possible

At this point, I am unsure if my solution is heading in the right direction, and I would appreciate some guidance if possible 😅.