XBasic 6.5.0 Rust bootstrap

This is the stage-0 Rust workspace for bootstrapping XBasic 6.5.0.

Resolved decisions:

1. Floating point: use plain f64/libm-style operations by default. Exact x87 JIT behavior is deferred unless compatibility tests prove it is required.
2. Windows: focus on Win64 first. Linux and macOS remain first-class targets.
3. Self-hosting: use Rust as the stage-0 host, keep compiler layers clean enough to port back into XBasic later, and start self-hosting from low-level utility/library surfaces before attempting a compiler-in-XBasic stage.

Workspace crates

Crate	Role
xb-frontend	Tokens and lexer for the XBasic syntax surface
xb-compiler	Typed compiler boundary and optional LLVM backend (llvm feature; default off)
xb-runtime	XxxMain entry contract, exception mappings, f64 math defaults, fault-hook skeletons
xb-link	Object-link command construction for Unix and Win64 drivers
xb-cli	xb command that parses/analyzes/lowers .x files and prints stable IR summaries
xb-gui	GDI-spirit drawing trait and deterministic framebuffer backend
xb-ide	eframe/egui IDE shell, feature-gated behind eframe-app

Local verification

Core check, no LLVM required:

cargo check --workspace --exclude xb-ide
cargo test --workspace --exclude xb-ide

Note: several integration suites (demo parity, multi-lib link, sockets) invoke a prebuilt target/release/xb; run cargo build --release first, or use ./checks/validate-all.sh, which builds and runs the whole release suite.

IDE stack check, requires Rust compatible with egui/eframe 0.36.1:

cargo check -p xb-ide --features eframe-app

LLVM backend check, requires LLVM 22 in PATH or LLVM_SYS_221_PREFIX:

cargo check -p xb-compiler --features llvm

On this machine, rustc in PATH is Homebrew Rust 1.94, while rustup stable is 1.97.1 after update. The IDE feature requires Rust ≥1.95 and was checked with explicit rustup cargo/rustc. Homebrew llvm is 22.1.8 (/opt/homebrew/opt/llvm); the LLVM backend builds and tests with LLVM_SYS_221_PREFIX=/opt/homebrew/opt/llvm but stays feature-gated off by default — ./checks/validate-all.sh covers default features only (no --features llvm, no CI LLVM job yet; see docs/17 LLVM-CI-BITROT).

Verified capabilities (2026-08-26)

• All 15 core libraries compile cc-clean (xbasic-6.4.5/src/{shared,linux}/*.x)
• All 15 link into one working binary — checks/link-core-libs.sh
• 114/114 demo programs compile through the Rust CEmitter (demo_parity: 112 match / 2 real-I/O skips) and through the true self-hosted cgen.x (cgen_x_compiles_all_demos_cc_clean)
• 80/80 positive-corpus programs emit byte-identical C (locked by cgen_cemitter_sync)
• Byte access {} on string scalars and array elements
• INC/DEC + SWAP subscripts on indexed/composite targets
• 276 tests across 33 binaries, 0 failures (./checks/validate-all.sh)
• Self-hosting: compiler.x → cgen.x → native C generator (bootstrap fixed point held)