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andyleiserson merged 2 commits into from
Mar 18, 2026
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Fixes for dual-source blending validation #9200

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a82d627
refactor(naga): Update `IO_SHAREABLE` definition
andyleiserson Mar 11, 2026
fb1e694
fix: dual-source blending validation
andyleiserson Mar 11, 2026
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4 changes: 4 additions & 0 deletions CHANGELOG.md
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Expand Up @@ -258,6 +258,10 @@ By @kpreid in [#9042](https://github.com/gfx-rs/wgpu/pull/9042).
- Tracing support has been restored. By @andyleiserson in [#8429](https://github.com/gfx-rs/wgpu/pull/8429).
- Pipelines using passthrough shaders now correctly require explicit pipeline layout. By @inner-daemons in #8881.
- Allow using a shader that defines I/O for dual-source blending in a pipeline that does not make use of it. By @andyleiserson in [#8856](https://github.com/gfx-rs/wgpu/pull/8856).
- Improve validation of dual-source blending, by @andyleiserson in [#9200](https://github.com/gfx-rs/wgpu/pull/9200):
- Validate structs with `@blend_src` members whether or not they are used by an entry point.
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@ErichDonGubler ErichDonGubler Mar 16, 2026

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thought: This is/these are technically a breaking change, which we normally advertise, but this is a bug fix for spec. validation. My other validation fix PRs have advertised these as BREAKING, so maybe we should talk about some consistent criteria for doing that.

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@andyleiserson andyleiserson Mar 18, 2026

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It's a borderline case. Somewhat recently I added a "validation" section in the changelog which lists a bunch of validation changes, at least some of which probably make it stricter, and none of which are noted as breaking.

That seems right to me... I think it's worth reserving the "breaking" label for API changes that require users to update their code, or for validation changes that we think carry a high risk of breakage. Breaking code that wouldn't work in Chrome/Dawn (although I can't say I specifically checked that here), that was never legal under the spec, and where the problem will be obvious (shader compilation error rather than e.g. a runtime behavior change), doesn't seem like it needs to be called out at the same level and dilute the communication of the truly breaking changes.

- Dual-source blending is not supported when there are multiple color attachments.
- `TypeFlags::IO_SHAREABLE` is not set for structs other than `@blend_src` structs.
- Validate `strip_index_format` isn't None and equals index buffer format for indexed drawing with strip topology. By @beicause in [#8850](https://github.com/gfx-rs/wgpu/pull/8850).
- Renamed `EXPERIMENTAL_PASSTHROUGH_SHADERS` to `PASSTHROUGH_SHADERS` and made this no longer an experimental feature. by @inner-daemons in [#9054](https://github.com/gfx-rs/wgpu/pull/9054).
- BREAKING: End offsets in trace and `player` commands are now represented using `offset` + `size` instead. By @ErichDonGubler in [9073](https://github.com/gfx-rs/wgpu/pull/9073).
Expand Down
7 changes: 2 additions & 5 deletions cts_runner/test.lst
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Expand Up @@ -247,7 +247,7 @@ webgpu:api,validation,render_pass,resolve:resolve_attachment:*
webgpu:api,validation,render_pipeline,depth_stencil_state:*
webgpu:api,validation,render_pipeline,float32_blendable:*
webgpu:api,validation,render_pipeline,fragment_state:color_target_exists:*
webgpu:api,validation,render_pipeline,fragment_state:dual_source_blending,use_blend_src:*
webgpu:api,validation,render_pipeline,fragment_state:dual_source_blending,*
webgpu:api,validation,render_pipeline,fragment_state:limits,*
webgpu:api,validation,render_pipeline,fragment_state:targets_blend:*
webgpu:api,validation,render_pipeline,fragment_state:targets_format_filterable:*
Expand Down Expand Up @@ -415,10 +415,7 @@ webgpu:shader,validation,expression,call,builtin,value_constructor:*
webgpu:shader,validation,expression,call,builtin,workgroupUniformLoad:*
webgpu:shader,validation,expression,overload_resolution:*
webgpu:shader,validation,extension,clip_distances:*
webgpu:shader,validation,extension,dual_source_blending:blend_src_same_type:*
webgpu:shader,validation,extension,dual_source_blending:blend_src_stage_input_output:*
webgpu:shader,validation,extension,dual_source_blending:blend_src_syntax_validation:*
webgpu:shader,validation,extension,dual_source_blending:use_blend_src_requires_extension_enabled:*
webgpu:shader,validation,extension,dual_source_blending:*
webgpu:shader,validation,extension,pointer_composite_access:*
webgpu:shader,validation,functions,restrictions:param_type_can_be_alias:*
webgpu:shader,validation,parse,blankspace:blankspace:*
Expand Down
134 changes: 63 additions & 71 deletions naga/src/valid/interface.rs
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Expand Up @@ -60,7 +60,10 @@ pub enum GlobalVariableError {
pub enum VaryingError {
#[error("The type {0:?} does not match the varying")]
InvalidType(Handle<crate::Type>),
#[error("The type {0:?} cannot be used for user-defined entry point inputs or outputs")]
#[error(
"The type {0:?} cannot be used for user-defined entry point inputs or outputs. \
Only numeric scalars and vectors are allowed."
)]
NotIOShareableType(Handle<crate::Type>),
#[error("Interpolation is not valid")]
InvalidInterpolation,
Expand Down Expand Up @@ -93,15 +96,23 @@ pub enum VaryingError {
InvalidInputAttributeInStage(&'static str, crate::ShaderStage),
#[error("The attribute {0:?} is not valid for stage {1:?}")]
InvalidAttributeInStage(&'static str, crate::ShaderStage),
#[error("The `blend_src` attribute can only be used on location 0, only indices 0 and 1 are valid. Location was {location}, index was {blend_src}.")]
#[error("`@blend_src` can only be used at location 0, indices 0 and 1. Found `@location({location}) @blend_src({blend_src})`.")]
InvalidBlendSrcIndex { location: u32, blend_src: u32 },
#[error("If `blend_src` is used, there must be exactly two outputs both with location 0, one with `blend_src(0)` and the other with `blend_src(1)`.")]
IncompleteBlendSrcUsage,
#[error("If `blend_src` is used, both outputs must have the same type. `blend_src(0)` has type {blend_src_0_type:?} and `blend_src(1)` has type {blend_src_1_type:?}.")]
#[error(
"`@blend_src` structure must specify two sources. \
Found `@blend_src({present_blend_src})` but not `@blend_src({absent_blend_src})`.",
absent_blend_src = if *present_blend_src == 0 { 1 } else { 0 },
)]
IncompleteBlendSrcUsage { present_blend_src: u32 },
#[error("Structure using `@blend_src` may not specify `@location` on any other members. Found a binding at `@location({location})`.")]
InvalidBlendSrcWithOtherBindings { location: u32 },
#[error("Both `@blend_src` structure members must have the same type. `blend_src(0)` has type {blend_src_0_type:?} and `blend_src(1)` has type {blend_src_1_type:?}.")]
BlendSrcOutputTypeMismatch {
blend_src_0_type: Handle<crate::Type>,
blend_src_1_type: Handle<crate::Type>,
},
#[error("`@blend_src` can only be used on struct members, not directly on entry point I/O")]
BlendSrcNotOnStructMember,
#[error("Workgroup size is multi dimensional, `@builtin(subgroup_id)` and `@builtin(subgroup_invocation_id)` are not supported.")]
InvalidMultiDimensionalSubgroupBuiltIn,
#[error("The `@per_primitive` attribute can only be used in fragment shader inputs or mesh shader primitive outputs")]
Expand Down Expand Up @@ -209,7 +220,7 @@ struct VaryingContext<'a> {
types: &'a UniqueArena<crate::Type>,
type_info: &'a Vec<super::r#type::TypeInfo>,
location_mask: &'a mut BitSet,
blend_src_mask: &'a mut BitSet,
dual_source_blending: Option<&'a mut bool>,
built_ins: &'a mut crate::FastHashSet<crate::BuiltIn>,
capabilities: Capabilities,
flags: super::ValidationFlags,
Expand Down Expand Up @@ -721,38 +732,8 @@ impl VaryingContext<'_> {
return Err(VaryingError::InvalidPerPrimitive);
}

if let Some(blend_src) = blend_src {
// `blend_src` is only valid if dual source blending was explicitly enabled,
// see https://www.w3.org/TR/WGSL/#extension-dual_source_blending
if !self
.capabilities
.contains(Capabilities::DUAL_SOURCE_BLENDING)
{
return Err(VaryingError::UnsupportedCapability(
Capabilities::DUAL_SOURCE_BLENDING,
));
}
if self.stage != crate::ShaderStage::Fragment {
return Err(VaryingError::InvalidAttributeInStage(
"blend_src",
self.stage,
));
}
if !self.output {
return Err(VaryingError::InvalidInputAttributeInStage(
"blend_src",
self.stage,
));
}
if (blend_src != 0 && blend_src != 1) || location != 0 {
return Err(VaryingError::InvalidBlendSrcIndex {
location,
blend_src,
});
}
if !self.blend_src_mask.insert(blend_src as usize) {
return Err(VaryingError::BindingCollisionBlendSrc { blend_src });
}
if blend_src.is_some() {
return Err(VaryingError::BlendSrcNotOnStructMember);
} else if !self.location_mask.insert(location as usize)
&& self.flags.contains(super::ValidationFlags::BINDINGS)
{
Expand Down Expand Up @@ -852,37 +833,51 @@ impl VaryingContext<'_> {
}
};

for (index, member) in members.iter().enumerate() {
let span_context = self.types.get_span_context(ty);
match member.binding {
None => {
if self.flags.contains(super::ValidationFlags::BINDINGS) {
return Err(VaryingError::MemberMissingBinding(index as u32)
.with_span_context(span_context));
}
}
Some(ref binding) => self
.validate_impl(ep, member.ty, binding)
.map_err(|e| e.with_span_context(span_context))?,
}
}

if !self.blend_src_mask.is_empty() {
let span_context = self.types.get_span_context(ty);

// If there's any blend_src usage, it must apply to all members of which there must be exactly 2.
if members.len() != 2 || self.blend_src_mask.count() != 2 {
if self.type_info[ty.index()]
.flags
.contains(super::TypeFlags::IO_SHAREABLE)
{
// `@blend_src` is the only case where `IO_SHAREABLE` is set on a struct (as
// opposed to members of a struct). The struct definition is validated during
// type validation.
if self.stage != crate::ShaderStage::Fragment {
return Err(
VaryingError::IncompleteBlendSrcUsage.with_span_context(span_context)
VaryingError::InvalidAttributeInStage("blend_src", self.stage)
.with_span(),
);
}
// Also, all members must have the same type.
if members[0].ty != members[1].ty {
return Err(VaryingError::BlendSrcOutputTypeMismatch {
blend_src_0_type: members[0].ty,
blend_src_1_type: members[1].ty,
if !self.output {
return Err(VaryingError::InvalidInputAttributeInStage(
"blend_src",
self.stage,
)
.with_span());
}
// Dual blend sources must always be at location 0.
if !self.location_mask.insert(0)
&& self.flags.contains(super::ValidationFlags::BINDINGS)
{
return Err(VaryingError::BindingCollision { location: 0 }.with_span());
}

**self
.dual_source_blending
.as_mut()
.expect("unexpected dual source blending") = true;
} else {
for (index, member) in members.iter().enumerate() {
let span_context = self.types.get_span_context(ty);
match member.binding {
None => {
if self.flags.contains(super::ValidationFlags::BINDINGS) {
return Err(VaryingError::MemberMissingBinding(index as u32)
.with_span_context(span_context));
}
}
Some(ref binding) => self
.validate_impl(ep, member.ty, binding)
.map_err(|e| e.with_span_context(span_context))?,
}
.with_span_context(span_context));
}
}
Ok(())
Expand Down Expand Up @@ -1198,7 +1193,7 @@ impl super::Validator {
types: &module.types,
type_info: &self.types,
location_mask: &mut self.location_mask,
blend_src_mask: &mut self.blend_src_mask,
dual_source_blending: None,
built_ins: &mut result_built_ins,
capabilities: self.capabilities,
flags: self.flags,
Expand Down Expand Up @@ -1370,7 +1365,7 @@ impl super::Validator {
types: &module.types,
type_info: &self.types,
location_mask: &mut self.location_mask,
blend_src_mask: &mut self.blend_src_mask,
dual_source_blending: Some(&mut info.dual_source_blending),
built_ins: &mut argument_built_ins,
capabilities: self.capabilities,
flags: self.flags,
Expand All @@ -1390,7 +1385,7 @@ impl super::Validator {
types: &module.types,
type_info: &self.types,
location_mask: &mut self.location_mask,
blend_src_mask: &mut self.blend_src_mask,
dual_source_blending: Some(&mut info.dual_source_blending),
built_ins: &mut result_built_ins,
capabilities: self.capabilities,
flags: self.flags,
Expand Down Expand Up @@ -1418,9 +1413,6 @@ impl super::Validator {
return Err(EntryPointError::WrongTaskShaderEntryResult.with_span());
}
}
if !self.blend_src_mask.is_empty() {
info.dual_source_blending = true;
}
} else if ep.stage == crate::ShaderStage::Vertex {
return Err(EntryPointError::MissingVertexOutputPosition.with_span());
} else if ep.stage == crate::ShaderStage::Task {
Expand Down
3 changes: 0 additions & 3 deletions naga/src/valid/mod.rs
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Expand Up @@ -361,7 +361,6 @@ pub struct Validator {
types: Vec<r#type::TypeInfo>,
layouter: Layouter,
location_mask: BitSet,
blend_src_mask: BitSet,
ep_resource_bindings: FastHashSet<crate::ResourceBinding>,
switch_values: FastHashSet<crate::SwitchValue>,
valid_expression_list: Vec<Handle<crate::Expression>>,
Expand Down Expand Up @@ -608,7 +607,6 @@ impl Validator {
types: Vec::new(),
layouter: Layouter::default(),
location_mask: BitSet::new(),
blend_src_mask: BitSet::new(),
ep_resource_bindings: FastHashSet::default(),
switch_values: FastHashSet::default(),
valid_expression_list: Vec::new(),
Expand Down Expand Up @@ -638,7 +636,6 @@ impl Validator {
self.types.clear();
self.layouter.clear();
self.location_mask.make_empty();
self.blend_src_mask.make_empty();
self.ep_resource_bindings.clear();
self.switch_values.clear();
self.valid_expression_list.clear();
Expand Down
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