Shared expression-visitor base for codegen backends (PChecker migration)

[ankushdesai]

Summary

First step of consolidating the imperative code-generator backends (PChecker, PEx, PObserve). Today each one independently walks the AST with its own parallel ~31-case WriteExpr switch. They handle the identical set of 31 expression nodes, but nothing enforces that — a missed node fails silently (wrong generated output) instead of at compile time, so the backends drift as the language evolves.

This PR introduces ExpressionGenerator<TContext> (Backend/ExpressionGenerator.cs), which owns the IPExpr dispatch once and declares one abstract Write*Expr method per expression node. Backends extend it and implement the per-node emission. Adding a new IPExpr node (plus its dispatch arm) now forces every backend to implement it or the build breaks — that compile-time exhaustiveness is the whole point.

This commit migrates PChecker onto the base; PEx and PObserve follow in separate PRs. PVerifier stays separate by design — it's functional (ExprToString returns UCLID5 terms) and handles a different, verification-specific node set, so forcing it under this base would be a leaky abstraction.

What changed

• New ExpressionGenerator<TContext> base (dispatch + 31 abstract methods).
• PCheckerCodeGenerator's WriteExpr switch replaced by 31 override methods, bodies moved verbatim. Class changed public → internal (it's only referenced from TargetLanguage, same assembly) to satisfy accessibility for the protected overrides over the internal CompilationContext.

Verification

• Build: CompilerCore builds, 0 errors (only the pre-existing unrelated TransformASTPass.cs:796 CS0162 warning).
• Output is byte-identical: compiled tutorials 1–6 (ClientServer, TwoPhaseCommit, EspressoMachine, FailureDetector, Paxos, Raft — together exercising the full expression set) with the master compiler and with this branch, then diff -rq on the generated C#: identical, no differences.

Notes

• No behavior change; pure structural refactor.
• The CI full test suite will exercise the broader Samples/Regression corpus.

Context: implements the "shared visitor base for the imperative backends" simplification (PVerifier kept separate, phased rollout) discussed for reducing multi-backend maintenance cost. Follow-ups: PEx expressions, then statements, then PObserve.

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Generated by Claude Code

[Copilot]

Pull request overview

Introduces a shared ExpressionGenerator<TContext> base class to centralize IPExpr dispatch for imperative codegen backends, and migrates the PChecker backend to implement per-expression emission via overrides (eliminating the local WriteExpr switch).

Changes:

• Added ExpressionGenerator<TContext> with a single IPExpr dispatch switch and one abstract Write*Expr method per supported expression node.
• Refactored PCheckerCodeGenerator to inherit from ExpressionGenerator<CompilationContext> and replace its expression switch with protected override Write*Expr implementations.
• Adjusted PCheckerCodeGenerator visibility from public to internal to align accessibility with the internal base/context usage.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Src/PCompiler/CompilerCore/Backend/PChecker/PCheckerCodeGenerator.cs	Migrates PChecker expression emission to ExpressionGenerator overrides and updates generator visibility.
Src/PCompiler/CompilerCore/Backend/ExpressionGenerator.cs	Adds shared expression dispatch base class to enforce backend exhaustiveness at compile time.

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