[Feature] WGSL Shader Support with Hot-Reload

[vmarcella]

Overview

Extend the shader compilation pipeline to support WGSL alongside GLSL by adding a shader language field to VirtualShader

Current State

VirtualShader currently assumes GLSL and compiles to SPIR-V:

// crates/lambda-rs-platform/src/shader/types.rs
pub enum VirtualShader {
  File {
    path: String,
    kind: ShaderKind,
    name: String,
    entry_point: String,
  },
  Source {
    source: String,
    kind: ShaderKind,
    name: String,
    entry_point: String,
  },

There is no way to specify the shader language. WGSL is wgpu's native shader language and can be passed directly without SPIR-V conversion.

Proposed Design

Add ShaderLanguage enum:

// crates/lambda-rs-platform/src/shader/types.rs

/// Shader source language.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
pub enum ShaderLanguage {
  /// GLSL source, compiled to SPIR-V via naga or shaderc.
  #[default]
  Glsl,
  /// WGSL source, passed directly to wgpu.
  Wgsl,
  /// Pre-compiled SPIR-V binary.
  SpirV,
}

Extend VritualShader variants with a language field:

pub enum VirtualShader {
  File {
    path: String,
    kind: ShaderKind,
    name: String,
    entry_point: String,
    language: ShaderLanguage,  // NEW
  },
  Source {
    source: String,
    kind: ShaderKind,
    name: String,
    entry_point: String,
    language: ShaderLanguage,  // NEW
  },
}

Update ShaderCompiler to compile based on language:

impl ShaderCompiler {
  pub fn compile(&self, shader: &VirtualShader) -> CompiledShader {
    match shader.language() {
      ShaderLanguage::Glsl => self.compile_glsl_to_spirv(shader),
      ShaderLanguage::Wgsl => self.pass_through_wgsl(shader),
      ShaderLanguage::SpirV => self.load_spirv_binary(shader),
    }
  }
}

Add new builder methods in ShaderBuilder:

// crates/lambda-rs/src/render/shader.rs
impl ShaderBuilder {
  /// Build from a GLSL file (default behavior, backward compatible).
  pub fn from_glsl_file(path: &str, kind: ShaderKind) -> VirtualShader;
  
  /// Build from a WGSL file.
  pub fn from_wgsl_file(path: &str, kind: ShaderKind) -> VirtualShader;
  
  /// Build from inline WGSL source.
  pub fn from_wgsl_source(source: &str, kind: ShaderKind) -> VirtualShader;
}

Hot-reload

• Implement file timestamp watching for shader source files
• Queue pipeline recreation on the next frame when a shader file changes
• Provide safe pipeline swap that does not interrupt in-flight rendering
• Gate behind a Cargo feature shader-hot-reloading (or something similarly named)

Acceptance criteria

• ShaderLanguage enum with Glsl, Wgsl, and SpirV variants
• language field added to VirtualShader::File and VirtualShader::Source
• ShaderCompiler dispatches compilation based on language
• WGSL sources passed directly to wgpu without SPIR-V conversion
• Convenience builder methods for GLSL and WGSL in lambda-rs
• Backward compatibility maintained (GLSL default)
• File watcher detects shader modifications (timestamp-based)
• Safe pipeline swap on next frame without render interruption
• shader-hot-reload feature flag documented in features.md
• Example demonstrating WGSL shader usage
• Example or documentation for hot-reload workflow

Notes

• Hot-reload SHOULD be disabled by default in release builds
• Consider using notify crate or simple timestamp polling for file watching
• Pipeline recreation errors SHOULD be logged but not crash the application
• ShaderKind remains unchanged (Vertex, Fragment, Compute)