Implement automated WIT interface compliance validation

[avrabe]

Problem

Currently, the validate_wit = True option in Rust and C++ component rules only provides manual validation by logging both the component's actual interface and the expected WIT specification side-by-side. Developers must manually compare the outputs to identify mismatches.

Current Behavior

When validate_wit = True is enabled:

• ✅ Extracts component's actual WIT interface using wasm-tools component wit
• ✅ Displays expected WIT specification from source file
• ✅ Creates validation log with side-by-side comparison
• ❌ Does NOT automatically fail build on interface mismatches
• ❌ Does NOT provide specific error messages for mismatches

Requested Feature

Implement automated interface compliance checking that:

1. Parses WIT interfaces: Parse both component exports and WIT specification
2. Compares interfaces programmatically:
   • Function signatures (names, parameters, return types)
   • Exported interfaces and worlds
   • Type definitions and compatibility
3. Fails build on mismatch: Exit with clear error code and specific mismatch details
4. Provides actionable error messages:
   • "Function 'hello' expects string parameter, got u32"
   • "Missing export: interface 'calculator' not found in component"
   • "Type mismatch: expected f64, found f32"

Implementation Approach

Option 1: WIT Parser Integration

• Use existing WIT parsing libraries (wit-parser, wit-bindgen internals)
• Implement comparison logic in Rust/Go
• Integrate with existing validation toolchain

Option 2: wasm-tools Enhancement

• Extend wasm-tools with interface comparison command
• Add wasm-tools component validate --wit <spec.wit> component.wasm
• Leverage existing WIT tooling infrastructure

Option 3: Custom Validation Tool

• Create dedicated validation binary in //tools/wit_validator
• Implement WIT parsing and comparison logic
• Integrate with Bazel action system

Example Usage

rust_wasm_component_bindgen(
    name = "my_component",
    srcs = ["src/lib.rs"],
    wit = ":my_interfaces",
    validate_wit = True,  # Now fails build on interface mismatch
)

Build failure example:

ERROR: WIT validation failed for //examples/basic:hello_component
Interface mismatch in hello:interfaces/greeting@0.1.0:
  - Function 'hello' parameter mismatch:
    Expected: hello(name: string) -> string  
    Found:    hello(name: u32) -> string

Benefits

• Catch interface bugs at build time instead of runtime
• Prevent deployment of incompatible components
• Improve developer experience with clear error messages
• Enable reliable component composition with verified interfaces
• Support CI/CD validation of WebAssembly component contracts

Related Work

• Current validation implementation in rust/rust_wasm_component.bzl:83-134
• C++ validation in cpp/defs.bzl:360-387
• Documentation in docs-site/src/content/docs/languages/cpp.mdx#component-validation

Priority

Medium - Enhances developer experience but current manual validation provides basic functionality.