More optimizations and snapshot integration tests

rust_compiler/libs/integration_tests/README.md

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+# Integration Tests for Slang Compiler with Optimizer
+
+This crate contains end-to-end integration tests for the Slang compiler that verify the complete compilation pipeline including all optimization passes.
+
+## Snapshot Testing with Insta
+
+These tests use [insta](https://insta.rs/) for snapshot testing, which captures the entire compiled output and stores it in snapshot files for comparison.
+
+### Running Tests
+
+```bash
+# Run all integration tests
+cargo test --package integration_tests
+
+# Run a specific test
+cargo test --package integration_tests test_simple_leaf_function
+```
+
+### Updating Snapshots
+
+When you make changes to the compiler or optimizer that affect the output:
+
+```bash
+# Update all snapshots automatically
+INSTA_UPDATE=always cargo test --package integration_tests
+
+# Or use cargo-insta for interactive review (install first: cargo install cargo-insta)
+cargo insta test --package integration_tests
+cargo insta review --package integration_tests
+```
+
+### Understanding Snapshots
+
+Snapshot files are stored in `src/snapshots/` and contain:
+
+- The full IC10 assembly output from compiling Slang source code
+- Metadata about which test generated them
+- The expression that produced the output
+
+Example snapshot structure:
+
+```
+---
+source: libs/integration_tests/src/lib.rs
+expression: output
+---
+j main
+move r8 10
+j ra
+```
+
+### What We Test
+
+1. **Leaf Function Optimization** - Removal of unnecessary `push sp/ra` and `pop ra/sp`
+2. **Function Calls** - Preservation of stack frame when calling functions
+3. **Constant Folding** - Compile-time evaluation of constant expressions
+4. **Algebraic Simplification** - Identity operations like `x * 1` → `x`
+5. **Strength Reduction** - Converting expensive operations like `x * 2` → `x + x`
+6. **Dead Code Elimination** - Removal of unused variables
+7. **Peephole Comparison Fusion** - Combining comparison + branch instructions
+8. **Select Optimization** - Converting if/else to single `select` instruction
+9. **Complex Arithmetic** - Multiple optimizations working together
+10. **Nested Function Calls** - Full program optimization
+
+### Adding New Tests
+
+To add a new integration test:
+
+1. Add a new `#[test]` function in `src/lib.rs`
+2. Call `compile_optimized()` with your Slang source code
+3. Use `insta::assert_snapshot!(output)` to capture the output
+4. Run with `INSTA_UPDATE=always` to create the initial snapshot
+5. Review the snapshot file to ensure it looks correct
+
+Example:
+
+```rust
+#[test]
+fn test_my_optimization() {
+    let source = "fn foo(x) { return x + 1; }";
+    let output = compile_optimized(source);
+    insta::assert_snapshot!(output);
+}
+```
+
+### Benefits of Snapshot Testing
+
+- **Full Output Verification**: Tests the entire compiled output, not just snippets
+- **Easy to Review**: Visual diffs show exactly what changed in the output
+- **Regression Detection**: Any change to output is immediately visible
+- **Living Documentation**: Snapshots serve as examples of compiler output
+- **Less Brittle**: No need to manually update expected strings when making intentional changes