The wgpu backend identifies shader pipelines using string tuples like ("ps_text_run", "ALPHA_PASS,GLYPH_TRANSFORM,TEXTURE_2D"). This caused the DPR=2 text bug — a wildcard match arm silently selected the wrong shader variant. Replacing these stringly-typed keys with a flat enum gives us compiler exhaustiveness checks that prevent this bug class entirely.

A single enum rather than factored ShaderName + ShaderConfig enums, because the config flags are NOT freely combinable — each shader only uses specific configs. A product space would create hundreds of invalid combinations. The flat enum makes every valid variant explicit and every invalid variant unrepresentable.

The enum covers the ~35 shader variants actually generated under wgpu feature flags (DITHERING + DEBUG). No GL-only variants (TEXTURE_RECT, ADVANCED_BLEND, DUAL_SOURCE_BLENDING, ANTIALIASING, REPETITION) are included.

// Brush shaders (opaque + alpha)
BrushSolid, BrushSolidAlpha,
BrushImage, BrushImageAlpha,
BrushBlend, BrushBlendAlpha,
BrushMixBlend, BrushMixBlendAlpha,
BrushLinearGradient, BrushLinearGradientAlpha,
BrushOpacity, BrushOpacityAlpha,
BrushYuvImage, BrushYuvImageAlpha,

// Text (the two variants that matter)
PsTextRun, PsTextRunGlyphTransform,

// Quads
PsQuadTextured,
PsQuadGradient, PsQuadRadialGradient, PsQuadConicGradient,
PsQuadMask, PsQuadMaskFastPath,

// Prim
PsSplitComposite,

// Clip
CsClipRectangle, CsClipRectangleFastPath, CsClipBoxShadow,

// Cache tasks
CsBorderSolid, CsBorderSegment, CsLineDecoration,
CsFastLinearGradient,
CsLinearGradient, CsRadialGradient, CsConicGradient,
CsBlurColor,
CsScale,

// Debug
DebugColor, DebugFont,

// Utility (texture cache ops)
PsClear, PsCopy,

// Debug overdraw variants (compiled but rarely used at runtime)
BrushSolidDebugOverdraw, BrushBlendDebugOverdraw, BrushMixBlendDebugOverdraw,
BrushLinearGradientDebugOverdraw, BrushOpacityDebugOverdraw, BrushOpacityAntialiasing,
BrushOpacityAlphaAntialiasing, BrushOpacityAntialiasingDebugOverdraw,
BrushImageDebugOverdraw, BrushImageAlphaRepetition, BrushImageRepetition,
BrushImageRepetitionDebugOverdraw, BrushImageAlphaRepetitionDebugOverdraw,
BrushYuvImageDebugOverdraw,
PsTextRunDebugOverdraw, CsBlurAlpha,

This is large. Pragmatic approach: start with only the variants actually used at runtime (the first ~35 above). DEBUG_OVERDRAW and ANTIALIASING/REPETITION variants can map via from_shader_key() → None and be compiled but not indexed in the typed cache. They're only used with DebugFlags::SHOW_OVERDRAW which the wgpu path doesn't support yet.

Core variants (what we actually type): ~38 variants.

WGSL_SHADERS: HashMap<(&str, &str), WgslShaderSource> stays unchanged (no build.rs changes). The enum provides:

• shader_key(self) -> (&'static str, &'static str) — enum → string pair
• from_shader_key(name, config) -> Option<Self> — string pair → enum (None for debug-overdraw etc.)
• instance_layout(self) -> Option<&'static [InstanceField]> — replaces the string match in create_all_pipelines

• Define WgpuShaderVariant near WgpuBlendMode/WgpuDepthState (~line 51)

  • Derive Debug, Clone, Copy, PartialEq, Eq, Hash
  • shader_key(), from_shader_key(), instance_layout() methods
  • all() → const array for test coverage

• Change shaders HashMap (line 230): HashMap<WgpuShaderVariant, ShaderEntry> (was (&str, &str))

• Change pipelines HashMap (line 231-232): HashMap<(WgpuShaderVariant, WgpuBlendMode, WgpuDepthState, wgpu::TextureFormat), WgpuProgram>

• Change draw_instanced() signature (line 1551): variant: WgpuShaderVariant replaces shader_name: &str, config: &str Pipeline key: (variant, blend_mode, depth_state, target_format) Shader lookup: self.shaders.get(&variant)

• Update create_all_pipelines() (line 2352): Iterate WGSL_SHADERS, call from_shader_key(), skip None, store by variant. Replace string-match instance layout with variant.instance_layout().

• Update create_pipeline_for_blend() (line 2269): variant: WgpuShaderVariant replaces name: &str, config: &str. Label: format!("{:?}", variant).

• Add WgpuShaderVariant to public re-exports (line 39).

• Change batch_key_to_pipeline_key() (line 2199): Return WgpuShaderVariant instead of (&str, &str).

  BatchKind::TextRun(glyph_format) => match glyph_format {
      GlyphFormat::TransformedAlpha | GlyphFormat::TransformedSubpixel =>
          WgpuShaderVariant::PsTextRunGlyphTransform,
      _ => WgpuShaderVariant::PsTextRun,
  }

• Update draw_batch! macro (~line 2005): let variant = Self::batch_key_to_pipeline_key(...) → pass to draw_instanced.

• Update pattern_to_shader closure (~line 2439): Return WgpuShaderVariant instead of (&str, &str).

• Update all direct draw_instanced calls (~13 call sites): Replace string pairs with enum variants:

  • "cs_clip_rectangle", "" → CsClipRectangle
  • "cs_clip_rectangle", "FAST_PATH" → CsClipRectangleFastPath
  • "cs_clip_box_shadow", "TEXTURE_2D" → CsClipBoxShadow
  • "cs_blur", "COLOR_TARGET" → CsBlurColor
  • "cs_scale", "TEXTURE_2D" → CsScale
  • "cs_border_solid", "" → CsBorderSolid
  • "cs_border_segment", "" → CsBorderSegment
  • "cs_line_decoration", "" → CsLineDecoration
  • "cs_fast_linear_gradient", "" → CsFastLinearGradient
  • "cs_linear_gradient", "DITHERING" → CsLinearGradient
  • "cs_radial_gradient", "DITHERING" → CsRadialGradient
  • "cs_conic_gradient", "DITHERING" → CsConicGradient

• Update draw_cs! and draw_cs_blend! macros (~line 2575): Change $shader:expr, $config:expr to $variant:expr.

1. Define the enum in wgpu_device.rs with shader_key/from_shader_key/instance_layout. Re-export in device/mod.rs. Build succeeds — nothing uses it yet.

2. Migrate WgpuDevice internals: change shaders + pipelines HashMaps, draw_instanced signature, create_all_pipelines, create_pipeline_for_blend. This breaks the call sites temporarily.

3. Migrate renderer call sites: batch_key_to_pipeline_key return type, draw_batch macro, pattern_to_shader, all direct draw_instanced calls, draw_cs/draw_cs_blend macros.

4. Build and verify.

Steps 2-3 are effectively one atomic commit since changing draw_instanced's signature breaks all callers. Can do as a single commit.

• cargo build --bin servoshell from servo-graphshell (full compile)
• SERVO_WGPU_BACKEND=1 cargo run --bin servoshell — visual check that text, images, solid backgrounds, gradients all render correctly at DPR=1 and DPR=2
• No string shader names remain in renderer/mod.rs draw paths (grep check)