Add #ifdef cfg support to XDR generator

[leighmcculloch]

What:

Lexer tokenizes #ifdef, #else, and #endif preprocessor directives. Parser tracks a cfg condition stack and attaches a CfgExpr (Feature, Not, All) to each AST definition inside a preprocessor block, including #elif/#else branches and nested #ifdef blocks; extracted nested definitions inherit the enclosing cfg. Generator propagates cfg from AST definitions to all output structs and introduces TypeEnumEntry to carry per-variant cfg alongside the type name. Templates emit #[cfg(...)] before type definitions and all generated impl blocks when a cfg expression is present. VARIANTS/VARIANTS_STR and variants() in the type enum template change from fixed-size arrays to slices to accommodate a variable number of cfg-gated variants.

Why:

XDR specification files use C preprocessor directives to conditionally include types based on compile-time features. Without this support, the generator had no way to represent or emit those conditions, making it impossible to produce correct Rust code from specs that rely on #ifdef guards.

Close #497

[Copilot]

Pull request overview

Adds Rust-based XDR parsing + code generation support for C-preprocessor-style conditional blocks (#ifdef/#ifndef/#elif/#else/#endif), propagating those conditions into emitted Rust via #[cfg(...)] so specs with conditional type inclusion can be generated correctly.

Changes:

• Implemented lexer+parser support for conditional directives and a CfgExpr model attached to AST definitions (including nested extracted types).
• Added type-resolution utilities (TypeInfo) used by the Rust generator for typedef/builtin resolution, const size resolution, and cycle detection.
• Updated Rust generator templates and output model to emit #[cfg(...)] on generated items and to carry per-type cfg into the type-variant enum.

Reviewed changes

Copilot reviewed 32 out of 36 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
xdr-generator-rust/xdr-parser/src/lexer.rs	Tokenizes preprocessor conditionals and keeps %... directives skipped.
xdr-generator-rust/xdr-parser/src/parser.rs	Tracks a cfg stack and attaches CfgExpr to definitions; includes tests for directive behavior.
xdr-generator-rust/xdr-parser/src/ast.rs	Introduces CfgExpr and adds optional cfg to all definition nodes.
xdr-generator-rust/xdr-parser/src/types.rs	Adds TypeInfo + helpers for builtin/array classification and cycle detection.
xdr-generator-rust/generator/src/generator.rs	Propagates cfg into generated outputs and into type enum entries.
xdr-generator-rust/generator/templates/type_enum.rs.jinja	Emits cfg-gated type variant enums/arrays (now as slices).
Makefile / CONTRIBUTING.md	Runs both Ruby and Rust generators and diffs outputs to validate migration.

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[leighmcculloch]

Because this PR has been approved, I'm merging it and split off next changes into:

• Remove curr and next and build off xdr main with features #519