Validate use of alternate blend source in shader validator

src/valid/interface.rs

@@ -98,6 +98,10 @@ pub enum EntryPointError {
     InvalidIntegerInterpolation { location: u32 },
     #[error(transparent)]
     Function(#[from] FunctionError),
+    #[error(
+        "Invalid locations {location_mask:?} are set for blend_src_1. Only the first location may be set."
+    )]
+    InvalidLocationMaskForAlternateBlendSource { location_mask: BitSet },
 }
 
 #[cfg(feature = "validate")]
@@ -115,6 +119,7 @@ fn storage_usage(access: crate::StorageAccess) -> GlobalUse {
 struct VaryingContext<'a> {
     stage: crate::ShaderStage,
     output: bool,
+    alternate_blend_source: bool,
     types: &'a UniqueArena<crate::Type>,
     type_info: &'a Vec<super::r#type::TypeInfo>,
     location_mask: &'a mut BitSet,
@@ -319,6 +324,7 @@ impl VaryingContext<'_> {
                         }
                     }
                 }
+                self.alternate_blend_source = alternate_blend_source;
 
                 let needs_interpolation = match self.stage {
                     crate::ShaderStage::Vertex => self.output,
@@ -628,6 +634,7 @@ impl super::Validator {
             let mut ctx = VaryingContext {
                 stage: ep.stage,
                 output: false,
+                alternate_blend_source: false,
                 types: &module.types,
                 type_info: &self.types,
                 location_mask: &mut self.location_mask,
@@ -647,6 +654,7 @@ impl super::Validator {
             let mut ctx = VaryingContext {
                 stage: ep.stage,
                 output: true,
+                alternate_blend_source: false,
                 types: &module.types,
                 type_info: &self.types,
                 location_mask: &mut self.location_mask,
@@ -658,6 +666,27 @@ impl super::Validator {
             };
             ctx.validate(fr.ty, fr.binding.as_ref())
                 .map_err_inner(|e| EntryPointError::Result(e).with_span())?;
+            #[cfg(feature = "validate")]
+            if ctx.alternate_blend_source {
+                /* Check if only the first bit in the location mask is set */
+                if !self
+                    .location_mask
+                    .get_ref()
+                    .iter()
+                    .enumerate()
+                    .all(|(idx, val)| match idx {
+                        0 => val,
+                        _ => !val,
+                    })
+                {
+                    return Err(
+                        EntryPointError::InvalidLocationMaskForAlternateBlendSource {
+                            location_mask: self.location_mask.clone(),
+                        }
+                        .with_span(),
+                    );
+                }
+            }
 
             #[cfg(feature = "validate")]
             if ep.stage == crate::ShaderStage::Vertex