hdl._dsl: Change FSM codegen to avoid mutating AST nodes.

[wanda-phi]

Fixes #1066.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.37%. Comparing base (f524dd0) to head (a7c649e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1164      +/-   ##
==========================================
+ Coverage   87.32%   87.37%   +0.04%     
==========================================
  Files          43       43              
  Lines        8629     8663      +34     
  Branches     2041     2048       +7     
==========================================
+ Hits         7535     7569      +34     
+ Misses        903      902       -1     
- Partials      191      192       +1     

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[whitequark]

Cursed but ultimately necessary.

[whitequark]

LGTM. This is pretty cursed but I see no real choice. Hopefully much of this (_top_comb_statements for example) will be replaced when we design and implement scopes.

[whitequark]

Thanks!

[bl0x]

Just a note:
This change makes it impossible to refer to FSM.state inside of the FSM context. It used to be possible before, but now any code doing this throws the error:
AttributeError: 'FSM' object has no attribute 'state'.
Referring to state is only possible after the FSM context.

[whitequark]

Yes, that's expected. It was never particularly well-defined, for example if you used len(fsm.state) within the FSM context, or Signal.like(fsm.state) your code was silently broken.

[bl0x]

Very good that this is fixed now.