[naga wgsl] New TypeContext method write_unnamed_struct.

When asked to generate WGSL for `TypeInner::Struct`, rather than
unconditionally calling `unreachable!`, defer to a new `TypeContext`
method, `write_unnamed_struct`.

Provide appropriate `write_unnamed_struct` implementations:

- In the WGSL backend, implement this as `unreachable!`, since the WGSL
  backend should always know the proper name to use for a struct.

- For diagnostic messages, generate something human-readable that
  indicates that some struct type was encountered.

- For logging and debugging, defer to `TypeInner`'s `Debug`
  implementation.

naga/src/back/wgsl/writer.rs

@@ -1764,6 +1764,10 @@ impl TypeContext for WriterTypeContext<'_> {
         self.names[&NameKey::Type(handle)].as_str()
     }
 
+    fn write_unnamed_struct<W: Write>(&self, _: &TypeInner, _: &mut W) -> core::fmt::Result {
+        unreachable!("the WGSL back end should always provide type handles");
+    }
+
     fn write_override<W: Write>(&self, _: Handle<crate::Override>, _: &mut W) -> core::fmt::Result {
         unreachable!("overrides should be validated out");
     }

naga/src/common/wgsl/types.rs

@@ -39,6 +39,14 @@ pub trait TypeContext {
         out: &mut W,
     ) -> core::fmt::Result;
 
+    /// Write a [`TypeInner::Struct`] for which we are unable to find a name.
+    ///
+    /// The names of struct types are only available if we have `Handle<Type>`,
+    /// not from [`TypeInner`]. For logging and debugging, it's fine to just
+    /// write something helpful to the developer, but for generating WGSL,
+    /// this should be unreachable.
+    fn write_unnamed_struct<W: Write>(&self, inner: &TypeInner, out: &mut W) -> core::fmt::Result;
+
     /// Write a [`TypeInner`] that has no representation as WGSL source,
     /// even including Naga extensions.
     ///
@@ -363,7 +371,7 @@ where
             write!(out, "acceleration_structure{}", caps)?
         }
         TypeInner::Struct { .. } => {
-            unreachable!("structs can only be referenced by name in WGSL");
+            ctx.write_unnamed_struct(inner, out)?;
         }
         TypeInner::RayQuery { vertex_return } => {
             let caps = if vertex_return { "<vertex_return>" } else { "" };
@@ -414,6 +422,10 @@ impl TypeContext for crate::proc::GlobalCtx<'_> {
             .unwrap_or("{anonymous type}")
     }
 
+    fn write_unnamed_struct<W: Write>(&self, _: &TypeInner, out: &mut W) -> core::fmt::Result {
+        write!(out, "{{unnamed struct}}")
+    }
+
     fn write_override<W: Write>(
         &self,
         handle: Handle<crate::Override>,
@@ -446,6 +458,10 @@ impl TypeContext for crate::UniqueArena<crate::Type> {
         self[handle].name.as_deref().unwrap_or("{anonymous type}")
     }
 
+    fn write_unnamed_struct<W: Write>(&self, inner: &TypeInner, out: &mut W) -> core::fmt::Result {
+        write!(out, "{{unnamed struct {inner:?}}}")
+    }
+
     fn write_override<W: Write>(
         &self,
         handle: Handle<crate::Override>,

naga/src/front/wgsl/lower/mod.rs

@@ -412,6 +412,14 @@ impl TypeContext for ExpressionContext<'_, '_, '_> {
             None => write!(out, "{{anonymous override {handle:?}}}"),
         }
     }
+
+    fn write_unnamed_struct<W: core::fmt::Write>(
+        &self,
+        _: &crate::TypeInner,
+        _: &mut W,
+    ) -> core::fmt::Result {
+        unreachable!("the WGSL front end should always know the type name");
+    }
 }
 
 impl<'source, 'temp, 'out> ExpressionContext<'source, 'temp, 'out> {