rm support for obj.XGo_Enum(func(...))

[xushiwei]

• Proposal: for..range and for..in Iteration xgo#2841

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.66%. Comparing base (bce1b2b) to head (9632a5b).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #644      +/-   ##
==========================================
- Coverage   93.67%   93.66%   -0.02%     
==========================================
  Files          29       29              
  Lines        7262     7217      -45     
==========================================
- Hits         6803     6760      -43     
+ Misses        391      389       -2     
  Partials       68       68              

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[fennoai]

Review: remove callback-style XGo_Enum for-range iteration

The core deletion is correct and self-consistent: checkUdt now rejects any nonzero-parameter XGo_Enum signature (returns nil, false), which fails closed to a normal cannot range over ... compile error. The removed internal/foo fixtures (Foo..Foo4) leave no dangling references, and the affected tests pass. go build/go vet are clean.

The one gap is incomplete cleanup: the checker half of the callback feature was removed, but the emitter half is left behind as dead code. Nothing now assigns a negative p.udt or p.kvt, so the else branch in emitForRangeStmt, the kvt field, and the cantUseFlowsInForRange constant are all unreachable. Removing them completes the PR's intent and should recover the coverage drop (the deleted tests were the only ones exercising that branch). Security and performance passes found nothing.

Additional findings

• /workspace/gogen/util_gengo.go:1228: [P2] Dead code: negative-udt callback branch is now unreachable: This else branch handles p.udt < 0 (the X.XGo_Enum(func(k, v){...}) callback form, n = -n at 1199). After this PR, p.udt is only ever set to a positive value (2/3) at stmt.go:726; the sole producer of a negative udt (p.udt = -n in checkUdt case 1) was deleted. This branch and its p.kvt reads are therefore dead code. Removing it completes the feature removal and, per the PR note, should recover the coverage drop since the deleted tests were the only ones exercising it. The cantUseFlowsInForRange constant (below, ~line 1232) is referenced only from here and becomes unused once this branch is gone.
• stmt.go:487: [P3] Vestigial kvt field after callback path removal: kvt is now never assigned anywhere in the codebase; it is read only from the now-dead negative-udt branch in emitForRangeStmt. It can be removed from forRangeStmt as part of completing the removal.